Luminescence and the Light Emitting Diode. The Basics and Technology of LEDS and the Luminescence...

Other Titles of Interest

Other Titles in the International Series in The Science of the Solid State (Editor B. R. Pamplin)

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Vol .2 . RAY: II-VI Compounds

Vol. 3. NAG: Theory of Electrical Transport in Semiconductors

Vol .4. JARZEBSKI: Oxide Semiconductors

Vol. 5. SHARMA and PUROHIT: Semiconductor Heterojunctions

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Vol. 7. SHAY and WERNICK: Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties and Applications

Vol. 8. BASSANI and PASTORI PARRAVICINI: Electronic States and Optical Transitions in Solids

Vol. 9. SUCHET: Electrical Conduction in Solid Materials (Physico-chemical Bases and Possible Applications)

Vol.10. TANNER: X-Ray Diffraction Topography

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*Now available in flexicover **Free specimen copy available on request

LUMINESCENCE AND THE LIGHT EMITTING DIODE The Basics and Technology of LEDS and the Luminescence

Properties of the Materials

by

E.W. WILLIAMS I.C.I., Corporate Laboratory, Runcorn, Cheshire

and

R. HALL Thorn Lighting Limited, Leicester

PERGAMON PRESS

OXFORD NEW YORK TORONTO SYDNEY PARIS FRANKFURT

U.K. Pergamon Press Ltd., Headington Hill Hall, Oxford OX3 OBW, England

U.S.A. Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, New York 10523, U.S.A.

CANADA Pergamon of Canada Ltd., 75 The East Mall, Toronto, Ontario, Canada

AUSTRALIA Pergamon Press (Aust.) Pty. Ltd., 19a Boundary Street, Rushcutters Bay, N.S.W. 2011, Australia

FRANCE Pergamon Press SARL, 24 rue des Ecoles, 75240 Paris, Cedex 05, France

FEDERAL REPUBLIC Pergamon Press GmbH, 6242 Kronberg-Taunus, OF GERMANY Pferdstrasse 1, Federal Republic of Germany

Copyright © 1978 E. W. Williams and R. Hall

All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the copyright holders

First edition 1978

Library of Congress Cataloging in Publication Data

Williams, E. W. Luminescence and the Light Emitting Diode

Includes bibliographical references. 1. Light emitting diodes. 2. Luminescence. I. Hall, R., joint author. II. Title. TK7871.89.L53W54 1977 621.3815'22 77-4427 ISBN 0-08-020442-2 (Hardcover) ISBN 0-08-020441-4 (Flexicover)

In order to make this volume available as economically and as rapidly as possible the authors' typescripts have been reproduced in their original forms. This method unfortunately has its typographical limitations but it is hoped that they in no way distract the reader.

Printed in Great Britain by Cox & Wyman Ltd, Fakenham

EDITORS' PREFACE

The light-emitting diode (LED) is now an electronic component in everyday use in pocket calculators and other alphanumeric displays and indicator lamps on stereo equipment and computers. This book provides a timely discussion of the basic physics and solid state science behind these devices. Both the authors have seen them develop from an idea to fulfilment but in different ways. Ted Williams has worked for many years, and on both sides of the Atlantic, on absorption and emission of light from semiconductors. Bob Hall, on the other hand, is a scientist who specialised in the production of light emitting solid state devices. They form an excellent matched pair for the production of this book which shines light on optical processes in semi-conductors and the useful devices which can be made.

When I left Cambridge in 1957 and joined a leading British electronics company, my section leader explained the working of a solar cell. He handed me a silicon slice 2 centimetres square which was the latest solar cell available and said: "This process must be reversible - try and produce the inverse effect and generate light from a pn junction." Three months later, after searching the literature for the causes of my failure, I reported that it might be possible when dislocation free silicon becomes available. It was not until the development of the III-V compounds in the early sixties that the path was clear for the first real light emitting diode.

Now as a component of more and more displays, the visible LED clearly has an established future. The exciting promise of optical or infra red communi-cations systems using LED or laser sources and photodiode detectors is going to play a part in the next phase of the electronic revolution. It is foreseen that the growth of communications will continue to expand at a rapid rate, with optical fibres soon bringing the video telephone, television, information and access to computers into the home.

This book provides a very readable account of the basic physics and technology of LEDs and pn junction lasers and of the materials behind them.

Brian Randall Pamplin, Scientific Advisers & Co., 15 Park Lane, Bath, England.

ix

AUTHORS' PREFACE

The advent of the pocket calculator and the digital watch has ensured the place of the LED display in electronic-device history. It was therefore felt timely to bring out a student monograph on the LED. Although this book was written as a course book for third-year graduate and post-graduate students it is hoped that it will also be helpful to a much wider range of readers who would like to know more about the LED and the material from which it is made.

The book begins with an introduction to the crystal structure and growth, and the optical and electrical properties of LED materials. Following this introduction in the first three chapters, the detailed fabrication of the LED is given in Chapter 4 and this should prove useful both for student projects and for non-graduate engineers with an interest in the LED. After this is Chapter 5 the luminescence of the material and the diode light emission is considered from a simplified theoretical point of view. The sixth chapter is a very brief review of solid-state lasers made from LED materials.

The last four chapters in the book should prove useful for both student projects and laboratory experiments. Chapter 7 describes the equipment used to measure luminescence, cathodoluminescence and the diode electro-optic characteristics. "Luminescence in LED materials" is the subject of Chapter 8 and this concentrates on the three most important LED materials at the present time: GaP, GaAsi-xPx and GaAs. Other LED materials such as SiC and the newer ternary semiconducting compounds are surveyed in Chapter 9. Also included in this chapter is detailed information on commercially available LEDs. The book closes with a brief chapter on "Applications" which gives a few ideas for student projects and should also give the electronic amateur a taste for some of the novel uses that this versatile device can be put to.

Finally, the authors would like to acknowledge the guidance of Dr. B.R. Pamplin and the many helpful discussions with their colleagues at Thorn Lighting and RSRE. The typing of Chris Steven and copying and drawing office facilities at ICI Corporate and Mond Research Laboratories was very much appreciated. This book would never have been written without the patience of our wives Margaret and Diane during the many hours we were absent and so the book is dedicated to them.

This book is published with the permission of Thorn Lighting and ICI Corporate Laboratory.

XI

1 INTRODUCTION

The light-emitting diode (LED) display became a success when the pocket-calculator boom began in the early 1970s. It is still the choice of the majority of calculator, digital watch, and electronic instrument makers because it is such a reliable display in comparison to the other types that are presently available.

Some of the other reasons why the LED is so popular are:

1. Long life

2. Compatible with integrated circuits

3. Small size and weight

4. Ruggedness

5. Multi-colour displays and tailored wavelength of light emission are possible

6. Good temperature stability

7. Fast switching times

8. Cold light - no heating

9. Low noise optical switches are possible when the LED is combined with a silicon photodetector

10. Low drive voltage makes solar-cell-powered displays an attractive prospect.

Behind the success story of the LED lie many hard years of research into the preparation and properties of semiconductors. One of the key factors in this research has been the use of luminescence to study the semiconducting material from which the LED is made at every stage of the process. With the assistance of luminescence, semiconducting compounds and alloys have been characterized so that the impurities and defects and their densities can be identified. As a direct result of this the materials preparation techniques were improved to such an extent that an efficient LED could be made with light emission in either the infra-red or visible.

Photoluminescence measurements also provided very strong evidence that the emission in these efficient LED devices occurred on the p-side of the junction.

1

2 Luminescence and the LED

Figure 1.1. clearly shows this for a diffused GaAs diode (Ref. 1) at 77 K. The photoluminescence of the p-region of the diode almost exactly matches the diode emission spectrum whereas the photoluminescence from the n-side is well displaced to higher energies.

2 IOO

80l·-

6 0

I 4 0 G a A s

' n p + D I O D E

EMISSION

H h H h SLIT WIDTH

PHOTO -LUMINESCENCE n-SIDE

1-42 1-4 6 1-48 I S O

P H O T O N ENERGY ( e V )

Fig. 1.1. A comparison of photoluminescence at 77 K from the n and the p + side of a np+ diode with the electro-luminescence from the LED provides strong evidence that the light emission occurs on the p-side of the forward-biased diode. (After Carr and Biard (Ref. 1)).

This meant that to get the maximum amount of light out and hence optimise the power efficiency or the ratio of light out to electrical power in, the LED had to be mounted with the p-side uppermost.

The LED materials that are judged to be the most useful fall into three classes:

(a) III-V binary compounds like GaAs and GaP.

(b) III-V ternary alloys like GaAsP formed by alloying GaAs with GaP.

(c) Ternary compounds that have similar properties to the binary compounds. For example, Cu In Se2«

This book will be mainly restricted to discussing the properties of materials

and devices that fall into these three categories. There are others which

fall outside these guidelines but their properties are much less understood

and they will only be referred to briefly.

All of the three types of semiconductor are derived from the "family tree" of

elements that is shown in Fig. 1.2. The parents in the "family tree" are the

Introduction

Group I H m W Y "SI YD.

Fig. 1.2. The family tree of elements from which semi-conductors are derived.

group IV elements of the periodic table. Silicon is now by far the most important of these. In solid silicon only the four outer electrons bound to the nucleus take part in the bonding. These electrons are referred to as the valence electrons.

Goodman (Ref. 2) showed that it was possible to use this valence rule of four valence electrons per atom to predict possible new semiconductors by the process of cross-substitution. The binary and ternary compounds of the three types discussed above, can all be derived in this way. Gallium arsenide is a cross-substitutional derivative of germanium obtained by substituting gallium and arsenic for two germanium atoms. Similarly ternaries can be derived from binaries and so on. Figure 1.3. illustrates the process of deriving the stable ternary compound CuGaTe2- Unfortunately not all ternary compounds derived in this way are stable. This technique has been more recently modified by Pamplin (Ref. 3) to include vacant lattice sites in the "four electron per site rule". Large numbers of new ternary and quaternary adamentine compound semiconductors can now be predicted with more certainty.

Before we get carried away in our speculative search for new semiconductors let us return to a brief look at the history of III-V materials. According to Hilsum and Rose-Innes (Ref. 4), the paper by Thiel and Koelsch in 1910 (Ref. 5) which reports the preparation of InP was the first to report on a III-V compound. It then took 42 years before Welker (Ref. 6) stressed the special semiconducting properties of this group of compounds in 1952. LEDs were first reported by Wolff et al. (Ref. 7) and Braunstein (Ref. 8) in 1955. However, it was not until the laser properties of these materials was discovered in 1962 (Refs. 9-12) that interest in the LED really began. It still took almost 10 more years of research before the LED became a commercial success.

3


Now red LEDs can be seen everywhere. In pocket calculators, watches, instruments of all types, displays and indicator lamps in a wide variety of applications. Green, amber and yellow lamps have been improved recently and may soon be as common as the red ones. However, blue ones have so far proved very inefficient and a lot more research will have to be done before we can have all the primary colours and the possibility of a full colour display.

TOTAL VALENCE NO ELECTRONS

VALENCE PER ELECTRONS ATOM

16 4

CROSS - SUBSTITUTION

4 Si W Si

16 2Cd 2 Te IT-YI CdTc

I 6 4 Cu Ga Te2 I - m - Y I 2 CuGaTe,

Fig. 1.3. The process of cross-substitution for deriving binary and ternary semiconducting compounds.

Introduction 5

REFERENCES

W. N. Carr and J. R. Biard, J. Appl. Phys. 35, 2777 (1964).

C. H. L. Goodman, J. Phys. Chem. Solids, 6_, 305 (1958) .

B. R. Pamplin, J. Phys. Chem. Solids, 25, 675 (1964). B. R. Pamplin, J. de Physique, C3.53 (1975).

C. Hilsum and A. C. Rose-Innes, Semiconducting III-V Compounds, Pergamon Press, p.l (1961).

A. Thiel and H. Koelsch, Z. anorg. Chem. 65-66, 288 (1910).

H. Welker, Z. Naturforsch. 11, 744 (1952).

G. A. Wolff, R. A. Herbert and J. D. Broder, Phys. Rev. 100, 1144 (1955).

R. Braunstein, Phys. Rev. 99, 1892 (1955).

D. N. Nasledov, A. A. Rogachev, S. M. Ryvkin and B. V. Tsarenkov, Soviet Phys. - Solid State, _4, 782 (1962) translated from Fiz. Tverd. Tela, 4_, 1062 (1962) .

T. M. Quist, R. J. Keyes, W. E. Krag, B. Lax, A. L. McWhorter, R. H. Rediker and H. J. Zeiger, Appl. Phys. Letters, 1_, 91 (1962).

M. I. Nathan, W. P. Dumke, G. Burns, F. H. Dill and G. Lasher, Appl. Phys. Letters, _1, 62 (1962).

R. N. Hall, G. E. Fenner, J. D. Kingsley, R. J. Soltys and R. 0. Carlson, Phys. Rev. Letters, 9_, 366 (1962) .

1 .

2 .

3 .

4 .

5.

6 .

7.

8 .

9 .

10 .

1 1 .

12 .

2 INTRODUCING LED CRYSTALS

2.1. CRYSTAL STRUCTURE

Although the LED is a cheap device the crystal structure of the majority of

LED semiconductors is very similar to diamond, one of the most expensive

materials, and they belong to the adamantine semiconductor family. The

crystal structures of this family are of many different types (Ref.· 1) but

the materials discussed in this book belong to three classes:

(a) zinc blende;

(b) wurtzite;

(c) chalcopyrite.

Just like diamond, sp hybrids are formed by all the atoms in these classes

and tetrahedral bonding occurs. (Ref. 2 ) . The basic tetrahedral bonding or

structure is illustrated for a semiconductor compound in Fig. 2.1. Each atom

of one group (say group III, Ga) is surrounded by four atoms of the other

group (say group V, P) at the corners of a regular tetrahedron. A«<111>

crystallographic direction is shown. The atom in the tetrahedron is exactly

in the centre so that all inter-atomic distances between it and its neighbours

are equal.

Fig. 2.1. The basic tetrahedral structure or bonding for

a semiconductor compound.

In the next three figures the same tetrahedral bonding is shown in all the

three basic structures: 2.2, zinc blende; 2.3, wurtzite; 2.4, chalcopyrite.

In the case of the zinc blende structure the two crystallographic directions

< l l l > a n d < 1 0 0 > are shown emanating from one corner of a face-centred cube

which is taken as the origin point (OOO). The cube diagonal is a < 1 1 1 >

direction and the cube sides are normal to the<100>directions.

6

Introducing LED Crystals

Fig. 2.2. The zinc blende structure.

?£°^h *± *4 n&

Fig . 2 . 3 . The wur t z i t e s t r u c t u r e .

• SILVER @ INDIUM O TELLURIUM

α.6·446Α-^|

Fig. 2.4. The chalcopyrite structure.

7


GaAs and GaP both have the zinc blende structure. In this case all the gallium atoms take up a face-centred cubic structure and all the group V atoms (arsenic or phosphorus) also take up a face-centred cubic structure. The two face-centred cubes are displaced from each other by one-quarter of a body diagonal (along a <lll>direction) . The lattice parameter "a" for a zinc blende compound of this type is indicated in Fig. 2.2.

The majority of LEDs have been made from zinc blende III-V compounds and alloys. There is only one wurtzite structure compound that has been made into an LED and that is SiC.

At the time of writing, CuInSe2 (Refs. 3-5) is the only ternary chalcopyrite semiconductor that has been made into an LED and this like AgInTe2 (Ref. 6) that is shown in the Fig. 2.4. is a I-III-VI2 compound. There is no doubt, however, that many more members of this and other related chalcopyrite families will also be made into light-emitting diodes. Indeed it has already been shown that several other I-III-VI2 and II-IV-V2 compounds can be made both p and n type, as we will see later on in this chapter.

In all of these LED compounds the separation of the atoms in the lattice is determined by the bonding between the atoms. In the case of diamond the bonding is purely covalent. Each tetrahedral bond is occupied by two electrons and the electron density is symmetrical with respect to both atoms. However, in semiconducting compounds where two or more elements are present, a description of the bonding is much more difficult. Three possible types of bonding have been considered.

First, in the case of a pure covalent bonding for a III-V compound, for example, the group V atom contributes five electrons and the group III contributes three towards the formation of the bonds. The electron density is symmetrical between neighbouring atoms just as in diamond. This means that there must have been some charge transfer between the atoms and that the more electronegative atom (group V) will be associated with a positive charge and is usually called the "anion". The group III atom, therefore, has a negative charge associated with it and is called the "cation". This pure covalent bond leads to only small effective charges on the atoms.

Second, in the neutral covalent bond there is no charge difference between the atoms; this means that for III-V or LI-VI compounds the electron density is not symmetrical because the electrons are associated more with the anions than with the cations.

Finally, in pure ionic crystals the bonding is the result of coulombic interaction between ions. As a result the bonding forces are not localised in any particular direction, and the structure of the ionic crystal is determined by geometric considerations and the condition of electrical neutrality.

Neutral bonding appears to be quite a good approximation for the majority of LED crystals as is borne out by the fact that the interatomic distance between atoms in III-V semiconductors (Ref. 7) is given by the sum of the tetrahedral radii of the atoms shown in Fig. 2.5. (Ref. 7,8) We will see later, however, that theoretical calculations of the band gap have shown that some ionic bonding must also be assumed. (Ref. 9)

Introducing LED Crys ta l s 9

I IT TTT Ef ¥ Yi yjj GROUP

Fig. 2.5. The tetrahedral radii of atoms in A (0.1 nm) (After Pauling, 1960) (Ref. 8).

In summary of this crystal structure section, the lattice parameters of the LED binary compounds are shown in Table 2.1 (Ref. 10) and compared to the lattice parameters of the group IV compounds. Notice how in every case the lattice parameter of the cross-substitutional III-V compound is almost identical to the group IV element when all the elements originate from the same period. In other words, compare C with BN, Si with AIP, Ge with GaAs and Sn with InSb (see Fig. 1.2). That this occurs is hardly surprising when one looks at the tetrahedral radii of the atoms shown in Fig. 2.5. The average tetrahedral radii of these isoelectronic compounds is the same as the atoms in the same period.

In Table 2.2 the lattice constants of some of the chalcopyrite Ι-ΙΙΙ-νΐ2 and II-IV-V2 compounds which have been found to show both n- and p-type conductivity are listed. All the ternary compound lattice parameters and band gaps shown in Table 2.2 are taken from Shay and Wernick (Ref. 11). This latter reference gives a very comprehensive review of ternary compound diamond-like semiconductors.


TABLE 2.1 Crystal Structure and Lattice Constants

Crystal Structure

type Lattice constant,(nm) T(°C)

c Si Ge

a-Sn SiC+

BN BP BAs A1N

AIP AlAs AlSb GaN

GaP GaAs GaSb InN

InP InAs InSb

Diamond Diamond Diamond Diamond Wurtzite

Zinc blende Zinc blende Zinc blende Wurtzite



Zinc blende Zinc blende Zinc blende

0.356683 ± 0.00001 0.5430951 ± 0.0000005 0.5656133 ± 0.0000010 0.64892 ± O.OOOOl a = 0.30865 c = 1.511738 0.3615 ± O.OOOl 0.4538 O.47778 a = 0.3111 c = 0.4978 0.5451 0.56622 0.61355 ± O.OOOOl a = 0.3189 c = 0.5185 0.545117 0.565321 ± 0.00003 0.609593 ± 0.000004 a = 0.3533 c = 0.5693 0.586875 ± 0.00001 0.60584 ± O.OOOOl 0.647937 ± 0.000003

25 25 25 20

24

25

18

25

25

18 18 25

There are numerous polymorphous modifications of SiC. The cubic structure is called 3-Sic while the others are hexagonal and are called á-SiC. The lattice constant given here is for the most commonly studied hexagonal 6H-SÏC.

TABLE 2.2 Lattice Constants and Energy Gaps of some of the Chalcopyrite I-III-VI? and II-IV-V? Ternary Compounds

Compound

CuInS2

CdGeP2

CdGeAs2

CdSnP2

CdSnAs2

CuInSeo

that show n-

Lattice

spacing,

a, nm

0.552279

0.5741

0.59427

0.5900

0.60937

0.577

and p-Type Conductivity

c/a

2.0158

1.877

1.8876

1.952

1.9559

2.00

Energy gap

1.5

1.72

0.57

1.17

0.26

I..OI

Introducing LED Crystals 11

2.2. BAND STRUCTURE

2.2.1 Direct- and Indirect-gap LED Compounds

A knowledge of the band structure of semiconducting compounds like gallium arsenide and gallium phosphide is essential if the optical and electrical properties are to be understood. This structure can be calculated by many different theoretical methods but two of the most successful has been the pseudo-potential model of Cohen et al. (Refs. 12,13) and the simplified Van Vechten (Ref. 9) method. The band structures calculated with the Cohen model for gallium arsenide and gallium phosphide are shown in Figs. 2.6 and 2.7. For simplicity only the edges of the conduction and valence bands closest to the forbidden gap have been shown. The critical points shown in this diagram refer to the location of the intersection of the major wave vectors with the first Brillouin zone of the zinc blende lattice in three dimensions. Figure 2.8 shows this reduced first Brillouin zone which has the form of a duo-decahedron and the critical points for a crystal lattice of a zinc blende structure like gallium phosphide or gallium arsenide.

L _i k J h—[llllaxis—T--[IOO]axis—·» "reduced wave vector

Fig. 2.6. The pseudo-potential theoretical band structure of GaAs. (After Cohen and Bergstresser, 1966) (Ref. 12).


Γ X K

(-[III] axis -+—IIOOÎaxis—I

reduced wave vector k-

Fig. 2.7. The pseudo-potential band structure of GaP. (After Cohen and Bergstresser, 1966) (Ref. 12).

Fig. 2.8. The first Brillouin zone and the critical points for a crystal lattice of the zinc blende structure.

These critical points Γ, X, L and K are important because electron transitions which result in the emission or absorption of photons (light) originate from or terminate at them. The reason for this is that at a critical point an electron transition is more probable than at other points in k-space because there is either a maximum in the joint density of states for the conduction and valence bands or an equal slope for the two bands.

From the band diagram in Fig. 2.6 it is obvious that the GaAs is a DIRECT GAP material, the minimum of the conduction band and the maximum of the valence band both occurring at the Γ point where k = 0. Thus for GaAs the electrons


in the conduction band will occur in available states centred at Γ-, . The general shape of the energy-band structure for GaP is similar to that for GaAs as might be expected since the arsenic atoms are replaced by phosphorus atoms from the same group, but with a lower mass. This results in a material with stronger bonding force so that the energy required to excite a valence electron into the conduction band is much greater. However, as seen from the band diagram in Fig. 2.7, the lowest energy states available for these conduction electrons occur at the X point, but the top of the valence band still occurs at the Γ point. Thus GaP is an INDIRECT GAP material and electron transitions are accompanied by both a change of energy and a change of momentum.

The energy band structure for the family of III-V compounds is similar to those of GaAs and GaP, with some members having a direct band gap and other an indirect gap. Table 2.3 compares the energy gaps of a range of III-V compounds with the group IV elemental semiconductors (Ref. 14). From this data it is evident that the direct-band gap III-V materials tend to be associated with low energy gaps that are less than 1.6 eV and materials which have a higher energy gap than this value exhibit indirect transitions.

TABLE 2.3. Experimental and Calculated Energy Gaps for the

Crystal

C(diamond) Si Ge -Sn

SiC(6H) BN BP BAs AIN AIP AlAs AlSb GaN GaP GaAs GaSb InN InP InAs InSb

Crystals Given

Type of energy gap

Indirect Indirect Indirect Direct Indirect Indirect Indirect Indirect Indirect Indirect Indirect Indirect Direct Indirect Direct Direct Direct Direct Direct Direct

in Table 1

Experimental

0 K

5.48 1.166 0.744 0.082 3.033

----

2.52 2.238 1.6 -

2.338 1.521 0.813 -

1.421 0.42 0.228

energy gap (eV)

300 K

5.47 1.120 0.663 t 2.996 5? 2.0

-5.9 2.45 2.16 1.5 3.7? 2.261 1.435 0.72 2.4* 1.351 0.35 0.180

Calculated

energy gap (eV)

5.48 1.04 0.61 0.13 4.54 9.57 1.81 0.85 8.35 2.63 1.87 2.15 4.80 2.75 1.58 1.00 2.33 1.45 0.84 0.39

Unstable at room temperature.

*These experimental values are uncertain at the presence time. Energy gaps at 78 K of 3.46 eV for GaN and 2.11 eV for InN have been reported recently. (K. Osamura, S. Naka and Y. Murakami, J. Appl. Phys. 46, 3432 (1975).)


Also shown in Table 2.3 are the theoretical band gap, E~, values of Van Vechten (Ref. 9). These theoretical values are calculated in a very simple mathematical way from the average ionic energy gap, C, and the average pure covalent energy gap, Eh,

2 2 2 E = E, + C . (2.1) g h

The calculations have been described in detail elsewhere (Refs. 9,10).

2.2.2 Alloys of III-V Compounds

The need to investigate mixed crystals for electroluminescence is directly attributed to obtaining efficient radiative recombination in the visible range, 400-700 nm. With the assumption that the radiation will be obtained from band-edge transitions this means that attention is focused on semiconductors with energy gaps in the range 1.7 - 3.1 eV. However, as we have seen in the previous section, the simple III-V compounds with this range of energy gaps are indirect. It can be shown that band-edge radiative recombination in indirect semiconductors is much less efficient than in direct-gap semiconductors (Ref. 15). Consequently one of the ways of obtaining efficient visible luminescence has been to grow alloys of III-V compounds and by tailoring the composition to achieve a direct band gap.

Fortunately most of the III-V compounds are mutually soluble in all proportions so that it is possible to make alloys over the whole composition range. The most explored alloy systems are the ternary III-V alloys, and of these, GaAs-,_xP is the most important commercially. These mixed crystals can be thought of simply as an additional group of III-V compounds which have a zinc blende crystal structure and where the energy band parameters are continuously variable with composition.

It is important to realise that in these ternary alloys the group V atoms are arranged randomly on lattice sites. Because of this random arrangement the alloys are known as disordered alloys. Provided the alloy remains random throughout and ordering effects do not occur the virtual crystal model first proposed by Nordheim (Ref. 16) makes it possible to assume that the energy gap will vary linearly across the range of solid solutions of the alloy. When III-V alloys like GaxIn^_xAs were first studied the optical data indicated that electron transitions at three different points in k-space all showed this linear variation and the virtual crystal model appeared to hold (Ref. 17). More recent data using advanced experimental techniques has shown that there is a relatively large deviation from linearity in alloy systems such as the GaxIn;L_xAs one for these three electron transitions (Ref. 18) . Thompson and Woolley have reviewed several III-V alloys and in every case the energy gap does not obey the virtual crystal model in that it deviates from linearity

(Ref. 19). The deviation from linearity or amount of curvature was found to increase as the mean band gap of the two component compounds in the alloy decreased. In GaAsi_xPx, for example, where the mean room temperature band gap for the direct Γ-^ -* Γ^ transition is 2.09 eV, the deviation from the virtual crystal model is very small and was only detected by the accurate modulated reflectivity measurements shown by the experimental points shown in Fig. 2.9 (Ref. 20). For the alloy system GaxIni_xAs, on the other hand, where the mean band gap is 0.89 eV, the deviation is larger as is shown in Fig. 2.10 (Refs. 18,21). Finally, for the G? -In^xSh (Ref. 22) (mean band gap 0.45 eV) and InAs;L_xSbx (Ref. 23) (mean band gap 0.26 eV) alloy system the deviation is


extremely large as Fig. 2.11 shows. In fact for the latter alloy the disorder in the alloy is so large that the band gap decreases below the lowest energy gap compound (InSb).

30 300°K

2-75

2-26

H2-0

GaAs 0-2 0-4 0-6 0-8 GaP

Fig. 2.9 Variation of the direct (Γ15 - Γχ) and indirect (Γ15 - Χχ) band gap of GaAs!_xPx alloys. The circles are the experimental points obtained by modulated reflectivity measurements (After Thompson et al. 1966) (Ref. 20).

To understand the electrical behaviour of ternary III-V compounds we must consider both the direct as well as the indirect transition. From Fig. 2.9 it is seen that for GaAs;L__xPx the cross-over from direct Γ1 5 -> Γ]_ to indirect transition Γ1 5 -> X occur at a mole fraction x = 0.44. This corresponds to an energy of 1.99 eV equivalent to a photon wavelength of 623 nm. Thus provided the fraction of phosphorus is maintained below this value, we have a direct band gap material with a potentially efficient radiation in the red region of the spectrum. The exact choice of composition will be discussed in Chapter 9.

16

Ι·7

1-5

Ι·3

> 1 1

^ 1·1 α < Ο > 0·9 8 ιυ ζ

0 ·7

0 · 5

1 rva 1

-

Luminescence and t h e LED

1

■

1

■

4 /J / 1

/ ■ / 1 /

/· J ·/ 1 / ■

/ I -j

o InAs

20 4 0 60 80 IOO GaAs

MOLE °/o GaAs Fig. 2.10 The direct band gap variation in the GaxIni_xAs alloy system. (After Williams and Rehn, 1968) (Ref. 18).

GaSb 0.8

> a> 0.6 *~~* a §> 0.4

g 0.2 LU

A 'S\

^ S\ s S — ys S

s S y s s s ^ >r \

y ^ r ^~--~^\

1 1 1 1

' ^y^ —--sH In As

0 20 InSb

40 60 80 100 % InAs

(or GaSb)

Fig. 2.11 A comparison of the direct band gap variation in the GaxIni_xSb (after Woolley and Evans, 1961) (Ref. 22) and InAs;L_xSbx (after Woolley and Warner, 1964) (Ref. 23) alloy systems. The dashed line indicates what a linear variation would give and the full line is the experimentally observed variation.


2.2.3 Temperature Dependence

The temperature dependence of the band gap of both GaAs and GaP have been measured by Panish and Casey (Ref. 24) who found that over a wide range of temperatures their measurements could be fitted to the expression:

E = E <°> - S T F (2.2) g g Τ+β

where Eg(0) is the energy gap at 0 K, β is approximately the Debye temperature Θ and a is an empirical constant. For GaAs the constants are Eg(0) = 1.522 eV, a = 5.8 x 10"

4 and 3 = 300 K; and for GaP, Eg(O) = 2.338 eV, a = 6.2 x lCT4 and 3 = 460 K.

2.2.4 Nitrogen Isoelectronic Trap

The effect of nitrogen doping has had a significant effect on the recombination efficiency of GaAs^_xPx indirect-band alloys and GaP. (This is discussed in further detail in Chapter 8.) To understand this more fully we must consider the effect that nitrogen has on the wave function of the electrons.

The substitution of nitrogen for a group V atom in GaAs;L_xPx or GaP establishes a short-range potential resulting from the combination of the difference in electronegativity between the N atom and the group V atom it replaces and the hydrostatic deformation of lattice around the N site. This potential allows the N atom to capture an electron which in turn can bind a hole by Coulomb attraction thus forming a bound exciton (Ref. 25). Since the electron is bound to the immediate vicinity of the N impurity, the wave function of the electron is diffuse in k-space and there is an enhancement near Γ (k = 0) which results in increased electron-hole recombination in the Γ-band region.

The effect of nitrogen on the modulus of the wave function | ψ (le) | in Gap along the Δ symmetry line of the Brillouin zone is shown in Fig. 2.12. It is seen that at k = O the effect of the nitrogen is to increase the electron probability by almost 3 orders of magnitude. Also shown in Fig. 2.12 is the dependence of the wave function on the wave vector for GaAsQ.55ρο.45· Ifc i s

found (Ref. 26) for GaAsi_xPx that the magnitude of |φ(0)|2 increases sharply as the crystal composition x decreases, and at x ^ 0.45 it exceeds the value of |φ (k) |2 at X.

Luminescence and the LED

Γ1 ^ X k<IOO>

Fig. 2.12 Effect of nitrogen on the electron

probability for GaP and G a A si_ xpx·

2.3 ABSORPTION

the absorption coefficient is probably one of the simplest methods for characterising the optical properties of a semiconductor. It can be calculated by experimentally measuring the incident light intensity IQ and the intensity of light I transmitted through the sample as the wavelength of the light is varied. The sample thickness, t, must be chosen to obtain an accurate measurement of the ratio l/lQ. The absorption coefficient, a, is then calculated from the equation

I (1-R)2 exp (-at)

o l-ir exp (-2at)

where R is the wavelength dependent reflection coefficient of the sample. Values of R for various III-V compounds have been compiled by Séraphin (Ref. 27).

2.3.1 Fundamental Absorption-Direct-gap Semiconductor

Fundamental absorption takes place when an electron is excited from the valence band into the conduction band by the absorption of a photon. However, because transitions are subject to selection rules and because of other absorption processes, it is often difficult to obtain a precise value of the energy gap from measurement of the absorption edge.

18


Consider the simple band-structure picture shown in Fig. 2.13(a) for a direct gap semiconductor. With the assumption that all momentum conserving vertical transitions are allowed the theoretical value for the absorption coefficient for photons with energy greater than the energy gap is given by (Ref. 28)

A1(hv E ) (2.4)

where A^. is a constant. For photons with energies less than that of the band gap the predicted absorption is zero as shown in Fig. 2.14(a). Also shown in this Figure is a typical experimental measurement for GaAs, a direct-gap material, for wavelengths corresponding to energies shorter than the band gap. This exponential absorption edge of GaAs has been found to correlate with transitions involving bands due to impurities. (Ref. 29).

hV ABS Eg

O

(a)

Γ

(Ι,Ο,Ο)

Fig. 2.13 Fundamental absorption in: (a) a direct and

(b) an indirect gap semiconductor.

20 Luminescence and t h e LED

DIRECT GAP INDIRECT GAP

ÉÏ'

ICr

TYPICAL [EXPERIMENTAL

(cm-')

IO:

lO1

-THEORY

Eg hv (eV)

(a)

Fig. 2.14 (a) A comparison of the theoretical and the typical experimental absorption in a direct gap semiconductor like GaAs, and (b) a plot of aS against hv the light energy for an indirect gap semiconductor.

For materials in which quantum selection rules forbid direct transitions at k = O but allow them at k / O the transition probability increase with k2. In this case the absorption coefficient has an energy dependence given by (Ref. 28)

= A2 (hv E ) 3/2

(2.5)

where A2 is a different constant from A-L in equation (2.4) above.

2.3.2 Fundamental Absorption - Indirect-gap Semiconductor

The simplified band structure diagram which illustrates the optical absorption at the band gap in indirect-gap compounds is shown in Fig. 2.13(b). In this case the transition requires a change both in energy and momentum. Since the photon cannot provide a significant change in momentum phonon interaction must occur. (Phonons are quantised lattice vibrations with energy Ep and will be described in more detail in a later section.) Position V.S. shown in Fig. 2.13(b) refers to an electron virtual state which simply shows that the excitation is a two-step process; first electron excitation without momentum change and then electron-phonon interaction to give the momentum change and vice versa. For indirect-gap semiconductors there are no quantum restrictions forbidding particular transitions between the valence and conduction bands.

The absorption coefficient for a transition with phonon absorption is given by


2 a (hv) = A_(hv - E - E ) (2.6) a 3 g p

and the absorption coefficient for a transition with phonon emission is

a (hv) = A.(hv - E + E ) 2 . (2.7) e 3 g p

Since both phonon emission and absorption are possible when hv > E + E then the total absorption coefficient is 9 P

a(hv) = a (hv) + a (hv) . (2.8) a e

The amount of phonon emission or absorption is determined by the temperature. At low temperatures phonon emission dominates because the lattice is not vibrating very much. At higher temperatures phonon absorption dominates because the lattice is now generating phonons. Dean and Thomas (Ref. 30) show this for undoped gallium phosphide. At 1.6 K only phonon emission is seen whereas at 77 K phonon absorption is easily observed in the absorption spectra.

This phonon absorption and emission in indirect-gap semiconductors means that the absorption edge is much broader than in direct-gap semiconductors and this makes the determination of the band gap much more difficult. However, by plotting a2 against hv, the light energy, as shown in Fig. 2.14(b), the energy gap can approximately be determined by extrapolating the curve to a = 0 as shown by the dashed line. This correctly assumes that at the temperature of measurement phonon absorption (or emission) is equal to zero and ignores the phonon energy since it is so much smaller than the energy gap.

2.3.3 Doped Semiconductors

We have seen how a low density of impurities can affect the absorption edge in GaAs. In LED fabrication we are involved in producing highly conducting n- and p-type material, so that the concentration dependence of the absorption coefficient is important.

Measurements of the absorption coefficient of lightly and heavily doped n-type GaAs (Ref. 31) are shown in Fig. 2.15 (a) and (b). These figures show that the absorption coefficient at high energies tends to saturate between 1-2 x 104 cm~l. As the carrier concentration exceeds 6 x 101 7 cm"^ the absorption edge shift to higher energies due to the filling of the conduction band by free electrons when the donor impurity band overlaps with the conduction band. This is generally referred to as the Burstein shift (Ref. 32) Finally, for carrier concentration of 6.7 x cm"3 and above, additional absorption below 1.38 eV is beginning to become significant. Haga and Kimiva (Ref. 33) have ascribed this to transitions between the conduction band minima.


I 0 5

E o

O

o

10 '

Ώ

<

10 1.30

High purity 2 x l 0 1 7

n = 5x I 0 l 6 c m " 3

- L

1.40 1.50 Energy , E (eV)

(a)

1.60

. I04

I 0 2

n = 5.9xlOKcm

2x l0 1 8 -

1.30 1.40 1.50 Energy, E ( e V )

I.6C

Fig . 2.15 (a) Absorption coe f f i c i en t for l i g h t l y doped n-type GaAs a t 297 K and (b) absorpt ion c o e f f i c i e n t for heavi ly doped n-type GaAs a t 297 K. (After Casey, J r . e t a l . , 1975) (Ref. 31) .


A similar increase in lower energy (1.35 eV) absorption is also observed for p-type GaAs and is shown in Fig. 2.16. Although in this case it has been (Ref. 34) partially ascribed to transitions between the heavy hole band and the band split by spin orbit interaction. It is also noted from Fig. 2.16 that the absorption shifts to lower energy for increasing hole concentration. This is due to the increase in band tailing of the valence band as the acceptor impurity band merges with it.

1.30 1.40 1.50 1-60

Energy, E (eV)

Fig. 2.16 Absorption coefficient for heavily doped p-type GâAs at 297 K. For comparison, a < 1 x 10 3 cm"1

for other concentrations is also shown. (After Casey, Jr., et al4 1975) (Ref. 31).

Also associated with high carrier concentrations is free carrier absorption, a f c , which is characterised by a gradually increasing absorption and af c is proportional to λη, where 1.5 < n < 2.5. A good example of this is the measurements by Lacey (Ref. 35) on the absorption coefficient of n-type GaP which are shown in Fig. 2*17.


|| , , , , 1 1 5 0 0 6 0 0 7 0 0 ΘΟΟ 9 0 0 ΙΟΟΟ ΙΙΟΟ

X(nm) — * ·

Fig. 2.17 Absorption coefficient of GaP doped with S and Te. (After Lacey, 1970) (Ref. 35).

2.4 OPTICAL REFLECTIVITY

The most useful optical experiment from the point of view of obtaining a reasonable full picture of the band structure as a function of k-space is reflectivity. The reflectivity spectrum of GaAs, for example, shows features at the critical points and the energy at which these features occur corresponds to a vertical electron transition or absorption of radiation. The first differential of the reflectivity can be measured by electroreflectance and this shows up even more electronic transitions because of the much higher resolution that is obtained. Figure 2.18 shows the electroreflectance spectrum of GaP (Ref. 20) and for simplicity only the three major electronic transitions which occur at the critical points marked in Fig. 2.11 are indicated on the figure. These transitions are:

(a) EQ the Γ- 5 - Γ· direct-band gap,

(b) E the Λ3 - Λ transition,

(c) E the X- - X transition.

The other features of the spectrum and the electroreflectance experiment have been fully described elsewhere (Refs. 18, 20, 36.)


Fig. 2.18 Electroreflectance spectrum of GaP. (After Thompson et al. 1966) (Ref. 20).

2.5 PHONON SPECTRA OF ZINC BLENDE STRUCTURES

The diamond and zinc blende structures have two atoms per unit cell and therefore there are six branches in the vibrational spectrum (Ref. 28.) The branches of the phonon spectrum may be divided firstly into transverse or longitudinal depending upon whether the long wavelength modes have polarisations transverse or along the wave vector k. Secondly, depending upon whether the two atoms of the cell vibrate in or out of phase with each other we obtain either a slow or high-frequency oscillation which is termed an acoustical or an optical vibration.

Therefore the branches may be characterised as two TO (transverse optical) and one LO (longitudinal optical), two TA (transverse acoustical) and one LA (longitudinal acoustical). Curves calculated by Johnson and Cochran (Ref. 37) for GaAs are shown in Fig. 2.19. These show that the two TO and two TA branches are each degenerate in the<100> and<111> directions. In addition the LO and LA branches in the diamond structure are degenerate at the zone edge in the< 100> directions.


10.0 r 10.0

υ CVJ

O

>N

"5.0 c 0) D

S U_

[HO]

—

„TO

^ \ L O

/ L A

TA|^^-

TA2

1 _. J 1.0 0.5 1.0

2.6

Fig. 2.19 Calculated phonon dispersion curves for GaAs. (After Johnson and Cochran, 1962 - Ref. 37.)

ELECTRICAL PROPERTIES

2.6.1 Introduction

One of the fundamental characteristics of semiconductor crystals is that their electrical conductivity can be changed by many orders of magnitude by the controlled addition of small quantities of impurities. It is this character-istic that is used to form the p-n junction in an LED. One side of the junction is doped with impurities which make the material n-type and the other side is doped p-type. Three other reasons why doping is needed for an LED are:

(a) to form a low impedance diode from crystals with a band gap greater than 1.3 eV and consequently a very low intrinsic carrier density;

(b) for good light-injection efficiency at the junction;

(c) the diode light-emission properties can be strongly influenced by the concentration of impurities.

In the case of GaAs the group II elements Cd and Zn are used to obtain p-type doping and the group VI elements S, Se and Te are added when n-type conductivity is required. The group IV impurities silicon and germanium are unique in GaAs in that they can be used to produce both n- and p-type materials under certain growth conditions. They are referred to as AMPHOTERIC impurities. Silicon-doped GaAs in which silicon is the only majority carrier present has produced the highest power efficiency LEDs ever produced (Ref. 38). Their power efficiency is 32% .


The simplest calculation of impurity level energies is based on the hydrogen atom model for which the ionization energy E H is given by

4 m0 q

E H = y p = 13.6 eV (2.9) 32 i\l ε0 h

where ε0 is the permittivity of free space, mQ is the rest mass of the electron.

If we replace mo by the electron (or hole) effective mass m* and ε0 by the permittivity of the semiconductor then the ionization energy E^ (or Ea) of the donor (or acceptor) will become

Ed = (EJ = | — ] | — | E D (2.10)

The ionization for donors in GaAs and GaP calculated from equation (2.8) is 7 meV and 35 meV respectively. Such a model assumes that the ionization for donors or acceptors is independent of the impurity and cannot account for the ionization energy of deep level impurities. However, the calculated values do predict the correct order of magnitude of ionization energy for the shallow impurities which are used in most LEDs. In practice each impurity produces a characteristic level and even though shallow impurities have ionization energies close to the hydrogenic value defined by equation (2.10) by careful luminescence measurements at liquid helium temperatures, as we will see in Chapter 7, specific impurities can be identified and their density estimated.

2.6.2 Carrier Concentration

The intrinsic carrier concentration of a semiconductor, n^, decreases rapidly with increasing band gap according to the equation

n.2 = N N exp (-Eg/kT) (2.11)

valence band as given by Smith (Ref. 28). When the material is doped with either donors or acceptors at a concentration of N^ or Na, the carrier concentration is increased. If we suppose that the impurity ionization energy E^ or E a is small compared with the band gap and that the temperature is sufficiently high to completely ionise these impurities then, provided Nd' Na > n i ' a condition of extrinsic conduction exists.

For the case of only donors present, charge neutrality gives n-p = Nd, and the density of carriers is given by:

n = N^ and p = n.2/N_ (2.12) d i d


This corresponds to an n-type semiconductor in which the majority carrier electrons do not vary appreciably with temperature, but the minority carrier density is temperature sensitive.

The p-type semiconductor occurs when the material is doped with acceptors Na. In this case:

p = Na and n = n±2/N (2.13)

A more complex situation occurs when both donors and acceptors are present. However, over a limited temperature range it is possible to assume that when Nd >> Na t n e n Nd *n e<ïuati°n (2.12) can be replaced by N^ - Na. Similarly where Na >> N^, Na in equation (2.13) can be replaced by N a - N^.

When the temperature is sufficiently lowered so that not all of the donor and acceptor states are completely ionized, and the carrier concentration depends upon the ionization energy of the impurity states. For a partially compensated material in which N^ >> Na an approximate expression for the electron density is then:

IN, - N d a N exp (-E,/kT) (2.14) c a

when N is negligibly smaller than Nd this simplifies to:

n % ^(NdV*5 exp(-Ed/2kT) (2.15)

The variation of the carrier concentration for an n-type semiconductor over a wide range of temperatures is shown in Fig. 2.20 (Ref. 28). In this figure, which shows the electron density plotted against the reciprocal of temperature, three distinct regions can be observed. In the high temperature range, where the material is intrinsic the graph has a slope of E /2k. Over the normal operating temperature the electrons density is constant, and then at lower temperature corresponding to carrier freeze-out the curve has a slope of Eg/2k or Eg/k.


Jtn n

INTRINSIC RANGE SLOPE = Eg/2k

SATURATION n« Nd-Na

EXTRINSIC RANGE

SLOPE - ED/2k OR ED/k

DEIONIZATION RANGE

l/T Fig. 2.20. Variation of carrier concentration for an n-type semiconductor over a wide range of temperatures. (After Smith, 1959) (Ref. 28).

To measure the carrier concentration directly the most common method uses the Hall effect. The basic set up is shown in Fig. 2.21 where an electric field,

* Jx

Fig. 2.21. Basic Hall effect measurement.

Ex, is applied along the x-axis and a magnetic field, Bz, along the z-axis. The Hall voltage developed across the y-faces of the sample is given by the equation: (Ref. 28)

R„ J B W H x z (2.16)

where Jx is the current density and the Hall coefficient RH is given by:


(Ref. 28)

R L(P^JUL) b = ^ r=<^-> (2.17) H q\(p+*n)l) VP <x>2

where yn and μρ are the drift mobilities of electrons and holes respectively. The parameter τ is the mean free time between carrier collisions. It can be shown that for phonon scattering r = 1.18 and that for ionized impurity scattering r = 1.93.

If one carrier type predominates, as is the case for the extrinsic semi-conductor , then :

if n » p , RH = q n r

(2.18)

if p » n , R H = + £_

Thus from the measurement of the Hall voltage the density of carriers can be obtained. By making measurements over a suitable range of temperatures where carrier freeze out occurs then impurity ionization energies can be calculated.

2.6.3 Carrier Mobility

The conductivity σ of semiconductor crystal is given by the expression

σ = q(y n + μ p) (2.19)

where μη and yp are the drift mobilities of the electrons and holes. In extrinsic semiconductors, of course, we need only consider the majority carriers, as the contribution by the minority carriers can be neglected.

The mobility of carriers is related to the mean free time between collisions, τ, and the effective mass of the carrier, m*, by the expression

μ = (qx/m*) . (2.20)

The band structure of III-V compounds at the Γ^ point leads to the direct-gap materials having a low effective electron mass and so we can expect them to have a high electron mobility. The limit to the mobility for low impurity crystals is likely to be set by polar optical scattering. This is the interactions of carriers with the relative movement of the two different atoms which gives rise to a polarisation of the crystal. The upper limit of mobility due to polar optical scattering is given by: (Ref. 37)

ttjW'^H^H^H"2»-1· = 1.7xl03°[3-1 l^-l ' ^ Τ ' Μ ν ^ Fl-^l I exp |f-| -1 I cm'v xs x (2.21)


where qc is the effective charge, (Ref. 39) M is the reduced atomic mass and v the volume of the unit cell and θ- is the equivalent temperature for the characteristic longitudinal optical vibration frequency. The function F(6;L/T) is a slowly varying function.

For electrons and holes in compounds with an indirect gap the effective mass is much larger than in direct-gap semiconductors and for these materials, in a high purity form, the scattering caused by acoustic lattice vibration will be more important. Shockley and Bardeen (Ref. 40) have shown that these vibrations limit the mobility to

.5/2

(?)' μΔ = 3.2xicf5 pu^ [ -% } cm2/^1 s'1 (2.22)

where Εχ is the deformation potential, p is the density of the material and u is the »peed of sound in it.

A third scattering mechanism which can be dominant for intentionally doped materials is impurity scattering, i.e. scattering of carriers by defect or impurity ions. The mobility limit due to this process is given by the Brooks-Herring (Ref, 41) formula

(2.23)

/mM T £ where b = 1.29 x 10 1 4

where ε, the dielectric constant, m* the effective mass and Nj and n are the densities of ionised impurities and carriers respectively.

For neutral impurities Erginsoy (Ref. 42) calculated the mobility limited due to neutral impurities as

, , 1Λ22 / m* \ 1 2 - 1 - 1 ,0 nA. yN = 1.4 x 10 — — cm v s (2.24) \ mo / ε %

where NN is the concentration of neutral impurities.

For a specific slice of material the effective carrier mobility, y, may be expressed by combinations of the mobility of the various scattering mechanisms, V^: by the formula


Σ i- (2.25)

The most usual way of determining the relative contributions of the different scattering mechanisms is by measuring the mobility as a function of temperature and fitting the theoretical expressions to experimental results. In particular a combination of Hall effect and resistivity measurements have been used by a number of investigators to determine the quality of undoped materials in which the residual impurities generally produce n-type conduction.

Vapour and liquid phase epitaxial techniques have been developed to improve the purity of GaAs and layers in which the total ionised impurity concentration of 1013 - 1014 cm"3 can be obtained. However, undoped GaP is still in the range lO1^ - 1016 cm'

the electron concentration of 3

Measurements of the mobility in undoped GaAs against carrier concentration obtained by various workers shows considerable scatter as is shown in Fig. 2.22. This scatter has been attributed to the compensating effect which occurs when both donor and acceptor atoms are present. This is indicated in Fig. 2.22 by the curves showing the theoretical mobility calculated (Ref. 43) as a function of carrier concentration (N<j - Na) for several fixed compensation ratios (Na/Nd). Calculations by Wolfe et al. (Ref. 44) give lattice limited scattering electron mobilities of 2.4 x 10b cm2 V"1 s"1 at 77 K which is in good agreement with the highest measured mobility of 2.1 x 105 cm2 V"1 s"1. Wolfe and Stillman (Ref. 43) were also able to obtain good agreement between theory and experiment of the temperature dependence (4-100 K) of electron mobility in GaAs as shown in Fig. 2.23. They achieved this by making allowance for the piezoelectric effects which are a result of the lattice movement in long wavelength acoustical modes (Ref. 44).

10°

Ώ O _ £ I05

N A /N D *0

NA/ND=0.9

i i I i i i i i I J 1 I I I I 0 1 3 I0 1 4 I 0 1 3

Carrier concentration (cm-3) Fig. 2.22. Variation of 77 K mobility of GaAs with electron concentration (ND - NA) showing the effects of variable NA/ND. (After Wolfe and Stillman, 1970) (Ref. 43).

In t roducing LED Crys ta l s 33

10°

> CVJ

E υ >»

I '0= o 5

P N0(cm'3) NA(cm"3)

t O4 .8x l0 1 3 2.1 xlO13

h Δ 4 . 6 χ Ι 0 1 3 3.Ox I013

Γ · 7 . 2 xlO13 3.7x I013

h A | . 9 x l 0 1 4 2 . 6 x l 0 1 3 .<d2g

L .!_.. 1 1 1 .1 1 I 1 . 1 1

Calculated

1 1 1 1 1 1 1 L, J 10' I02

Temperature (°K)

Fig. 2.23. Temperature dependence of the mobility of GaAs for several high purity samples. (After Wolfe and Stillman, 1970) (Ref. 43)

For intentionally doped n-type GaAs normally used for the fabrication of LEDs, the temperature dependence of the theoretical mobility is not in good agreement with experimental results. In order to reduce the discrepancy a "mobility-killer" scattering mechanism (Ref. 47) with a T"^ o r τ~1·5 dependence on temperature has been proposed although the nature of this "killer" has not been determined. More recently, however, it has been shown (Ref. 48) that mobility reductions caused by the non-parabolic shape of the Γχ conduction-band valley and electron-electron scattering play an important role for this material.

The mobility of high purity n-type GaP has been reported by Craford et al. (Ref. 46) to be approximately 3000 cm2 v"1s~1 at 77 K and to vary as T~2 which is somewhat lower than the lattice scattering mobility of 4500 cm2 v'^-s'1

calculated by Rhode. (Ref. 49) The effect of carrier concentration and nitrogen doping on the electron mobility has been measured by Stringfellow et al. (Ref. 45) Figure 2.24 shows the electron mobility at 77 K against Nd " Na· ^ e theoretical curves combine the effects of lattice and ionised impurity scattering. The effect of nitrogen doping on electron mobility is shown in Fig. 2.25. The solid curves are calculated for combined lattice and neutral impurity scattering.


Compensat ion 3000 b ratio

2000

1000

10

J ' ' ' ' ' I l

I 0 1 7

ND-NA(cnr3)

Fig. 2.24. Electron mobility of GaP at 77 K against Nd ~ Na· T h e curves are theoretical calculations for combined lattice and ionized impurity scattering for values of (Nd + Na)/(Nd - Na) of 1, 2, 5, 10 and (After Stringfellow et al., 1975) (Ref. 45).

20.


I U

I03

I02

;\o

ΙχΙΟι β \

^ v 6 0 4

2xl018 \ J

-5χΙθ ' 8

1 x I0'9

2x l0 1 9

1 1 1 1 1

■ \

^605 \ ^

\ 6 0 6 ^ \ λ

J L_

\

^1 ^

1 1 100

Temperature (°K) 500

Fig. 2.25. Electron mobility of GaP against temperature for the following nitrogen concentrations: 8.8x1ο1' cm (*) , 4.9xl018 cm"3 (A), and 8xl018 cm"3 (·). The solid curves are calculated for combined lattice and neutral impurity (nitrogen) scattering and the nitrogen concen-trations indicated. (After Stringfellow et al., 1975 -Réf. 45).

Finally, Table 2.4 (Ref. 50) compares the electron and hole mobilities at room temperature for a range of group IV elements, III-V compounds, two ternaries which show both n- and p-type conductivity and $-SiC. In the alloys very little good experimental data exists because of the difficulty of preparing good single crystal material with a uniform composition; however, some initial calculations of theoretically expected mobility for some mixed III-V compounds have been made. (Ref. 51)


Table 2.4. Room Temperature Mobility in cm2 V^s"1 of a Range of Semiconducting Elements and Compounds

Mobility

c Si Ge a-Sn BP AlAs AlSb GaP GaAs GaSb InP InAs InSb 3-SiC CdGeAs2 CdSnAs2

Electron

1800 1900 3800 2500 500 1200

200-400 300

8800 4000 4600 33000 78000 4000

70 22000

Hole

1400 500

1820 2400 70

420 550 lOO 400 1400 150 460 750

25 250

REFERENCES

1. B. R. Pamplin, J. Phys. Chem. Solids, 25, 675 (1964).

2. E. Mooser and W. B. Pearson, J. Electronics, 1_, 629 (1956) .

3. P. Migliorato, B. Tell, J. L. Shay and H. M. Kasper, Appl. Phys. Letters, 24, 227 (1974).

4. P. W. Yu, S. P. Faile and Y. S. Park, Appl. Phys. Letters, 2^, 384 (1975).

5. P. W. Yu, Y. S. Park, S. P. Faile and J. E. Ehret, Appl. Phys. Letters, 2(5, 717 (1975) .

6. J. C. Woolley and E. W. Williams, J. Electrochem. Soc. 113, 899 (1966).

7. C. Hilsum and A. C. Rose-Innes, Semiconducting III-V Compounds, p.6, Pergamon Press (1961).

8. L. Pauling, Nature of the Chemical Bond, Oxford University Press (1960).

9. J. A. Van Vechten, Phys. Rev. 187, 1007 (1969. See also: J. C. Phillips, Bonds and Bonds in Semiconductors, Academic Press (1973).

10. Tables 2.1 and 2.3 were adapted from Tables 1 and 3 in H. C. Casey, Jr. and F. A. Trumbore, Materials Science and Engineering, 6_, 69 (1970) .

11. J. L. Shay and J. H. Wernick, Ternary Chalcopyrite Semiconductors : Growth, Electronic Properties and Applications, Pergamon Press (1975).

Introducing LED Crystals J7

12. M. L. Cohen and T. K. Bergstresser, Phys. Rev. 141, 789 (1966).

13. T. K. Bergstresser, M. L. Cohen and E. W. Williams, Phys. Rev. Letters, 15, 662 (1965).

14. Table 3 was adapted from Table 3 in Reference 10, p.73.

15. P. T. Landsberg, Solid-State Electronics, 10, 513 (1967).

16. L. Nordheim, Ann. Phys. Lpz. £, 607, 641 (1931).

17. J. C. Woolley and K. W. Blazey, J. Phys. Chem. Solids, 25, 713 (1964).

18. E. W. Williams and V. Rehn, Phys. Rev. 172, 798 (1968).

19. A. G. Thompson and J. C. Woolley, Can. J. Phys. 45, 255 (1967).

20. A. G. Thompson, M. Cardona, K. L. Shaklee and J. C. Woolley, Phys. Rev. 146, 601 (1966).

21. J. C. Woolley, C. M. Gillett and J. A. Evans, Proc. Phys. Soc. (London) ΊΊ_, 700 (1961) .

22. J. C. Woolley and J. A. Evans, Proc. Phys. Soc. (London) 78, 354 (1961).

23. J. C. Woolley and J. Warner, Can. J. Phys. 42, 2030 (1964).

24. M. B. Panish and H. C. Casey, Jr., J. Appl. Phys. 40, 163 (1969).

25. D. G. Thomas, J. J. Hopfield and C. J. Frosch, Phys. Rev. Letters, 15, 857 (1965).

26. J. C. Campbell, N. Holonyak, Jr., M. G. Craford and D. L. Keune, J. Appl. Phys. _45, 4543 (1974) .

27. B. 0. Séraphin and H. E. Bennett, Semiconductors and Semimetals, Vol.3, Academic Press, Chap.12, (1967).

28. R. A. Smith, Semiconductors, Cambridge University Press (1959).

29. J. I. Pankove, Phys. Rev. 140, 6A, 2059 (1965).

30. P. J. Dean and D. G. Thomas, Phys. Rev. 150, 690 (1966).

31. H. C. Casey, Jr., P. D. Sell and K. W. Wecht, J. Appl. Phys. 46, 250 (1975).

32. E. Burstein, Phys. Rev. 93, 632 (1954).

33. E. Haga and H. Kimiva, J. Phys. Soc. Japan, 19, 1596 (1964).

34. R. Braunstein, J. Phys. Chem. Solids, 8_, 280 (1959).

35. S. D. Lacey, Solid State Communications, Q_, 1115, (1970) .


36. M. Cardona, K. L. Shaklee and F. H. Pollak, Phys. Rev. 154, 696 (1967).

37. F. A. Johnson and W. Cochran, Proc. Int. Conf. on Physics of Semi-conductors , Exeter, 498 (1962).

38. I. Ladany, J. Appl. Phys. 42, 654 (1971).

39. H. B. Callen, Phys. Rev. 76, 1394 (1949).

40. W. Shockley and J. Bardeen, Phys. Rev. 77, 407 (1950). (See also Phys. Rev. 80, 72 (1950).)

41. H. Brooks, Advances in Electronics, 8_, 85 (1955) .

42. C. Erginsoy, Phys. Rev. 79, 1013 (1950).

43. C. M. Wolfe and G. E. Stillman, Proc. Third Int. GaAs Conference, Inst. of Physics, (1970).

44. C. M. Wolfe, G. E. Stillman and W. T. Londley, J. Appl. Phys. 41, 3088 (1970).

45. G. B. Stringfellow, H. T. Hall, Jr. and R. A. Burmeister, J. Appl. Phys. 46», No.7, (1975) .

46. M. G. Craford, W. 0. Graves, A. H. Herzog and D. E. Hill, J. Appl. Phys. 42_, 2751 (1971) .

47. L. Weisberg, J. Appl. Phys. 33, 1817 (1962).

48. E. H. Stevens and S. S. Yee, J. Appl. Phys. 44, 715 (1973).

49. D. L. Rhode, Physica Stat. Sol. B, _53, 243 (1972).

50. Handbook of Chemistry and Physics, 57th edition, CRC Press (1977).

51. J. W. Harrison and J. R. Hauser, J. Appl. Phys. 47, 292 (1976).

3 CRYSTAL GROWTH

3.1; INTRODUCTION

A good single-crystal growth method is essential for high-quality LED material. Ιτ\ the earlier work crystals were grown from the melt and diodes were fabri-cated by diffusing in impurities from the vapour state into slices cut from these crystals. Although the melt-growth technique has improved considerably in recent years it was not until low-temperature epitaxial growth was developed that the LED became a commercial success.

For both melt and epitaxial growth,a detailed picture of the phase diagram is required since this defines the growth conditions precisely in terms of composition and the equilibrium conditions for the vapour phase over the liquid and solid phases. Hence the phase diagrams of compounds and alloys in the three LED materials classes that were listed in Chapter 1 will be reviewed;

This description of the phase diagrams will be followed by a short account of crystal-growth processes. The reader who requires a more detailed study of this subject should consult an earlier volume in this series. (Ref. 1). In this earlier book on "Crystal Growth" both the melt and epitaxial growth techniques were extensively reviewed.

The growth technique in which an epitaxial layer is grown on top of a single crystal melt-grown substrate has produced the most efficient LEDs. Green, yellow, amber and red GaAsi-xPx and GaP lamps are made more economically by vapour epitaxial growth. Solution epitaxial growth, however, produces the most efficient GaAs infra-red diodes and Gai-xAlxAsx red diodes. This latter technique has also been used with considerable success to produce the complex multilayer structures required for the double heterojunction GaAs-Gai_xAlxAs lasers that will be described in Chapter 6.

3.2. PHASE DIAGRAMS

3.2.1 The Gibbs' phase rule

The Gibbs phase rule (Ref. 2) relates the number of degrees of freedom, F, in a system to the number of components, C, and the number of phases, P. When pressure is treated as a variable the rule is stated thus:

F = C - P + 2, (3.1)

F is the number of independent variables that have to be known in order to completely define the thermodynamic state of a system. For condensed systems in which the pressure is not considered as a variable equation (1.1) is simplified to:

39


- P (3.2)

For a binary compound phase diagram for, say, GaAs, C = 2 and we can write:

F = 3 - p. (3.3)

This means that for compounds like GaAs in a single phase field such as the liquid or the solid phase, P = 1 and this means that there are two degrees of freedom. These degrees of freedom of temperature and composition can there-fore be varied independently.

However, in a two-phase field, where both solid and liquid are found, P = 2 and hence F = 1. In this case if the temperature is chosen the composition of the two phases is defined by the points of intersection of the horizontal line drawn on the temperature against composition phase diagram with the phase boundary lines.

Finally, if there are three phases present then there is no degree of freedom and we have a three-phase point on the diagram. A three-phase point, F = 0, and a two-phase line, F = 1, has been indicated on the GaAs phase diagram shown in Fig. 3.1. We will discuss this diagram in more detail as we go on to consider the phase diagram of the III-V compounds.

I200

11 C O —

_ lOOO| U o

£ 90θ| 3

< . £j ΘΟΟ Û.

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600H

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VAPOUR GROWTH

LUTION

OWTH 1

I

o Ga

IO 20 30 40 50 60 70 80 90 IOO ATOM °/o As As

Fig. 3.1. Ga-As phase diagram. (After Casey, Jr., and Trumbore, 1970) (3).

Crystal Growth 41

I .400

1 3 0 0

1 2 0 0

3

ω MOO cc 3 1-< ce I.OOO LU

o. Σ £ 9 0 0

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VAPOUR GROWTH

SOLUTION GROWTH

__i—

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Ga

4 0 50

ATOM °/o

6 0

P

7 0 8 0 9 0 IOO

P

Fig. 3.2 Ga-P phase diagram. (After Casey, Jr., and Trumbore, 1970 (3).)

3.2.2 The III-V Compounds

The Ga-As and Ga-P phase diagrams are shown in Figs. 3.1 and 3.2 respectively. (Ref. 3). They are typical of the majority of the III-V compounds in that GaAs or GaP is formed at a composition where the liquidus reaches a maximum and is therefore a congruently melting-point compound. The line corresponding to III-V compound formation has a finite width which although it is less than 1% of the composition can still lead to considerable changes in the electrical and optical properties as we will see later. In the case of GaAs it has been shown that the line broadening increases above 800°C and explains why non-stoichiometry is such a problem in melt-grown material.

Another reason why it is often difficult to melt grow stoichiometric III-V crystals with exactly 50 atom % of each constituent element is when the vapour pressure of one of the constitutents is very high at the melting point of the compound. (Ref. 4). This is the case for both GaAs and GaP and before steps could be taken to prevent the loss of arsenic or phosphorus the pressure-temperature curve (Ref. 3) for arsenic or phosphorus vapour in equilibrium with the liquidus shown in Figs. 3.1 and 3.2 had to be determined.

The melting points of the majority of the III-V compounds are shown in Table 3.1 (Ref. 2). InSb has the lowest melting point and this fact, together with the low vapour pressure of both constituents over the melt, means that very high purity stoichiometric material can be prepared relatively easily in this case.


Table 3.1 Melting Points of LED Compounds in K

InSb

InAs

InP

GaSb

GaAs

GaP

GaN

798

1215

1330

980

1510

1750

1500

AlSb

AlAs

AIP

CuGaSe2

CuInSe2

AgInTe2

1330

1870

1770

1310

1250

965

(See References 2 and 7 for the melting point of other binary and ternary semiconducting compounds.)

3.2.3 III-V Ternary Alloys

For III-V alloys like GaAsi_xPx and Ga;L_xInxAs the full phase diagrams has three dimensions because three elements are present. However, in almost every case the two dimensional pseudo-binary phase diagram for an alloy between the stoichiometric compounds can be used with a reasonable degree of accuracy. Once again, as with the compounds, when one of the elements is very volatile the pressure-temperature curve for the vapour of this element in equilibrium with the liquidus should be known before preparation of the alloy is under-taken .

The approximate pseudo-binary phase diagrams for GaAS]^xPx (Ref. 5) and Inl-xGaxs b (Ref· 6) are shown in Figs. 3.3 and 3.4 respectively. In the GaAs;L_xPx case, the two^phase field of solid (S) plus liquid (L) is relatively small IhcT alloy growth over the whole composition range is readily achieved by using the low-temperature epitaxial techniques described below. For Ini_xGaxSb the two-phase field is broader so that production of the alloys is more difficult to achieve as fluctuations in the growth conditions can lead to large changes in the alloy composition.

I500

υ o

< a.

i UJ

•~ I 3 0 0

I 2 0 0 O 2 0 4 0 6 0 8 0 IOO

GaAs Mol °/o G a P Go P

Fig. 3.3 Pseudo-binary phase diagram for GaAs-GaP alloys. (After Antypas, 1970 (5).)

42

Crystal Growth 43

Fig. 3.4. Pseudo-binary phase diagram for InSb alloys. (After Woolley and Smith, 1958).

GaSb

3.2.4 Ternary Semiconducting Compounds

The ternary phase diagrams in which ternary compounds are found are just as complicated as the ternary alloy diagrams in that a three-dimensional phase diagram is again required. In many cases an approximate pseudo-binary phase diagram can be drawn. For example, for the II-IV-V2 chalcopyrite compounds the solubility of the group IV element with the II-V2 compound can be considered. The phase diagram for the pseudo-binary system CdP2-Sn shown in Fig. 3.5. (Ref. 7). CdSnP2 is formed peritectally from the two-phase field of 3 CdP2 + liquid at 565°C. CdSnP2/lnP heterojunctions have been made into infra-red LEDs confirming that in spite of the complex nature of the phase diagram a reasonable quality ternary semiconductor can now be prepared.

900 r

Fig. 3.5. CdP2~Sn pseudo-binary phase diagram. (After Shay and Wernick, 1975 (7).)


A very comprehensive review of ternary phase diagrams has been given recently by Shay and Wernick. (Ref. 7).

3.3 MELT GROWTH

3.3.1 Melt Growth of III-V Compounds

Growth at the melting point of III-V compounds, where both solid and liquid phases are in equilibrium with the saturated vapour, proved quite difficult because of the high pressure involved for some of the most useful LED materials. Use of liquid encapsulation with transparent B2O has made the growth of the phosphides and arsenides much safer and more reproducible but a high pressure of inert gas is still required. In the case of GaP, for example, the inert gas pressure above the B2O3 has to be 39 atmospheres to prevent loss of phos-phorus from the stoichiometric melt. A simplified diagram of single crystal pulling from a liquid encapsulated melt is shown in Fig. 3.6.

o

INERT GAS

Fig. 3.6 Single crystal pulling from a liquid encapsulated melt.

In spite of the breakthrough made with B2O3 encapsulation, melt grown crystals are not used for the manufacture of complete LEDs but they have a very important part to play as a substrate material for the lower temperature epi-taxial growth processes. The main reason for this is that the high temperature in melt growth produces non-uniform doping and a large number of vacancies in the crystal lattice. These vacancies act as "killer centres" because of the non-radiative processes that they produce as we will hear in more detail in Chapter 8. In GaAs, for example, there are approixmately lO1^ vacancies per cm^ in melt grown crystals and LEDs made from this material have very low efficiencies. With the lower temperatures of growth that are possible with the epitaxial techniques described below, this concentration can be reduced by many orders of magnitude. For example, for a 700°C growth temperature the vacancy concentration can be as low as 1014 per cm^ in undoped GaAs.

Crystal Growth 45

3.3.2 Ternary Alloys and Compounds

The complex nature of the phase diagrams of alloys and compounds leads to problems in melt growth. For the alloys the width of the solid and liquid two-phase field at most compositions in the alloy system not only makes the preparation of homogeneous alloys (of constant composition) difficult but the preparation of melt grown single crystals is almost impossible to achieve. However, in the case of ΙηΑε^.χΡχ some very impressive homogeneous liquid encapsulation melt grown ingots have been produced. (Ref. 8). Although the ingots were polycrystalline, grain sizes as large as a few mm2 by 5-10 mm long with the long axis parallel to the growth direction have been achieved.

Similarly with the ternary compounds the production of single crystal melt grown ingots is extremely difficult and only by very careful preparation techniques have large single crystal grains been possible. The growth of II-IV-V2 compounds has recently been reviewed by Pamplin (Ref. 9) and these methods are also applicable to I-III-VI2 compounds. To give one example, in the case of the CuInSe2, (Ref. 10) which was the first ternary chalcopyrite compound from which light-emitting diodes were prepared, the elements had a purity of 99.999% and they were all heated up together very slowly in an evacuated quartz ampoule until the temperature reached 1050°C. After leaving them at this temperature for about 1 hour the melt was slowly cooled at 2°C/ hour. Nucleation occurred at the ends of the tube and single crystals grew by directional freezing caused by the temperature gradients in the furnace.

3.4 VAPOUR EPITAXIAL GROWTH

The temperature range used for vapour epitaxial growth of GaAs and GaP is shown at the right-hand side of Figs. 3.1 and 3.2 respectively. This range also applies for the GaAs^_xPx alloys to which this growth method is best suited.

The technique is an epitaxial one because a thin layer of crystal is grown on top of a single crystal melt grown substrate. The epitaxial layer has exactly the same atomic arrangement as the melt grown substrate.

Because of the difficulties of handling elemental phosphorus, the vapour epitaxial growth of GaP and GaAsi_xPx is achieved by the use of phosphine and arsine in the manner shown in Fig. 3.7. (Ref. 11). There are three constant temperature zones: the gallium source zone at 775°C, the reaction zone at 850°C and the deposition zone at 750°C. Arsine, phosphine and hydrogen selen-ide (for n-type selenium doping) or ammonia for nitrogen doping are combined in a mixing chamber before they are swept into the furnace reaction zone by the hydrogen carrier gas. Just below the hydrogen gas inlet another tube permits hydrogen selenide to by-pass the mixing chamber when an abrupt n-type impurity concentration gradient is needed. The gallium is transported as the chloride with hydrogen chloride and hydrogen, p-type doping is produced in a separate furnace by flowing hydrogen over molten zinc and introducing the zinc into the reaction zone. Any composition of GaAs^_xPx may be produced on the GaAs or GaP melt grown substrates to an accuracy of ±1 atom %.

Enlarged versions of the reactor shown in Fig. 3.7 are now used for the commercial production of GaP and GaAs^_xPx for light-emitting diodes. (Ref. 11). This system has proved to be so competitive in cost and power efficiency for GaP that it is preferred over liquid epitaxy for green, yellow and amber diodes. In the case of nitrogen doping in green GaP and GaAs^«xPx LEDs a


MIXING CHAMBER

A s H 3 + H2 P H 3 + H 2

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SUBSTRATE

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Fig. 3.7. Vapour epitaxial growth of GaP. (After Tietjen and Amick, 1966 (11).)

higher nitrogen-doping level can be achieved. An added bonus is that the quality of vapour epitaxial growth is such that, provided the substrate is flat, the epitaxial growth will be uniform and no polishing is required in contrast to liquid epitaxy where the growth of GaP is normally uneven.

FURNACE WINDINGS (APPROX 2KW)

POWEPÔUT GAS OUT

WINDING FORMER GOLDED SILICA

FURNACE TUBE (OUTER) m

ITHERMO COUPLE

TO CONTROLLER

POWER IN FROM EUROTHERM CONTROLLER

BUNG

SUBSTRATE

PECTROSILX BOAT '

INDIUM'MELT FURNACE WINDING

Fig. 3.8. Solution epitaxial growth of InP by the horizontal Nelson "tipping" technique. (After Astles et al., 1973 (14).)

3.5 SOLUTION EPITAXIAL GROWTH

Although growth from the vapour is the best technique for growing epitaxial p-n junctions for visible GaAsP and GaP LEDs the method of solution growth has proved far better than vapour growth for the production of GaAs, (Ref. 12)

Crystal Growth 47

InP, (Ref. 14) Ali-xGaxAs (Ref. 3) and Ga]_-xInxAs (Ref. 15) LEDs.

There are four very important reasons why solution growth has proved so important:

(1) The low temperature of growth means that vacancy concentrations and the concentration of contaminating impurities are relatively small (see Figs. 3.1 and 3.2 for the temperature ranges used for solution growth of GaAs and GaP).

(2) The use of gallium or indium melts for compounds or alloys based on gallium or indium further reduces the gallium or indium vacancy concen-tration. As will be seen later gallium (or indium) vacancies reduce the efficiency of the diodes.

(3) Metallic impurities like copper also act as killer centres for electro-luminescence just as the gallium vacancies and these are leached out by the gallium or indium.

(4) GaAs p-n junctions with silicon as both the p- and the n-type dopant can be grown during a single run as will be described later on.

A multitude of solution growth techniques have been developed for the prepara-tion of LEDs. (Ref. 1). Many of these will be referenced later on when the individual compound and alloy diodes are discussed. For the present just two of the methods will be described.

Firstly, the horizontal Nelson (Ref. 16) "tipping" technique which was used for the growth of indium phosphide is shown in Fig. 3.8. (Ref. 14). 99.9999% pure indium is placed at one end of the spectrosil boat and the single crystal melt-grown substrate is placed at the other end. The boat is positioned so that it is centrally in the furnace in the constant-temperature zone. The furnace tube is flushed out with oxygen-free nitrogen gas for 15 minutes and then palladium-diffused hydrogen gas for 15 minutes. After this the tempera-ture is raised to the growth temperature of, say, 700°C under the flow of hydrogen gas. At 600°C any residual indium oxide on the solution surface is removed by reaction with the hydrogen. Phosphine, which dissociates at temperatures above about 150°C into phosphorus and hydrogen, is then passed at a rate of 50 ml/min over the indium melt until it is saturated and small crystals of indium phosphide are seen floating on it. The temperature is then slowly raised at 1°C at a time under the flow of pure hydrogen again until the crystals in the melt are dissolved. The temperature is then taken up a further 1°C to allow for etch-back of 5 microns off the substrate to remove surface irregularities. The solution is then gently TIPPED over the substrate and the controlled cooling down of the furnace is started. After cooling at l°è/min for 20 to 30 minutes the solution is tipped off without leaving any of it on the substrate.

Secondly, the most popular technique is probably the "multipot" one which has been so successfully used for the growth of Ga]_-xAlxAs for LEDs and single and double heteroepitaxy lasers. (Ref. 17). Figure 3.9 (Refs. 17,18) shows two variations of the "multi-pot" method. In 3.9(a) the graphite slider holds both the GaAs substrate and GaAs source slice. Each solution in the pots through which the slider is drawn has had some gallium arsenide added to it to give partial saturation at the growth temperature. During the growth cycle the source slice goes before the substrate into each pot in turn and produces complete saturation. Each pot is doped with the appropriate dopant or


GaAs SUBSTRATE

GaAs SOURCE

WÉËm

MELT

QUARTZ PULL ROD

(a)

GRAPHITE SLIDE GaAs SUBSTRATE

WEIGHT (QUARTZ OR GRAPHITE) SOURCE WAFER MELT

QUARTZ PULL ROD

(b)

Fig. 3.9. Solution epitaxial growth of GaAs and Ga1_xAlxAs using the sliding "multipot" method. (After Kressel and Nelson, 1973 (17).)

aluminium is added to the melt if a Gai_xAlxAs alloy layer is required. Figure 3.9(b) shows a modification in which a very small amount of solution is squeezed against the slider by a weight on top of a GaAs wafer in each pot. With this modification a 0.1 micron/min growth rate has been achieved at 900°C and as many as six solution-grown layers for double heterojunction GaAs: Gai_xAlxAs lasers have been produced in one growth cycle with the thickness being controlled to at least 0.1 micron.

3.6 MOLECULAR BEAM EPITAXY

Molecular beam epitaxy (MBE) is a very sophisticated way of preparing thin films of compound semiconductors. Not only can layers of much less than a micron in thickness be grown but also the doping concentration can be changed abruptly within a 10 nm thick layer. (Ref. 19). Although GaAs LEDs (Ref. 20) produced by this method have only had very poor efficiencies in comparison to solution-grown material it is still a relatively new technique and undoubtedly more efficient LEDs will soon be forthcoming. The recent observation of pulsed lasing action in A l ^ ^ a ^ s - GaAs - ΑΙ^χσαχΑε double hetero junction MBE prepared lasers is evidence of the rapid improvements that are occurring. (Ref. 21).

A cross-section of a typical growth system is shown in Fig. 3.10 (Ref. 22). It is a high-vacuum ion-pumped system with the melt-grown GaAs substrate mounted on a heating block pivoted so that the sample can be rotated about the centre of the system. On the periphery four individually heated pyrolytic BN evaporation cells are surrounded by a liquid nitrogen-cooled shroud. These shrouds collimate the beams from the ovens and trap out the condensing gases. For analytical study layers were grown on <100> orientated Cr-doped GaAs sub-strates which were chemically polished with a methanol-diluted solution of bromine and cleaned in the system just prior to deposition by ion sputtering. The substrate temperature was in the range of 560-600°C during the epitaxial growth. Pure elemental ot-As and Ga were used as source materials with a 900°C temperature for the Ga and 300°C for the As oven. The pressure before growth was about 10"^ tprr and during growth this rose because of the arsenic pressure to 2 x 10"7 torr. Growth rates of about 1 micron per hour were achieved.

48

Crys ta l Growth 49

VIEW PORT

FLUORESCENT SCREEN

VIEW PORT

OUADRUPOLE MASS SPECTROMETER

Λ

EVAPORATION OVENS

CYLINDRICAL ANALYSER

LIQUID NITROGEN SHROUD

ELECTRON GUN

MON SPUTTERING GUN

Fig. 3.10 Molecular beam epitaxial growth. Illegems and Dingle, 1975 (22).)

(After

The epitaxial surface structure was determined by glancing-angle electron diffraction as it was grown and the surface composition was determined with an Auger spectrometer. Finally, a quadrupole mass spectrometer was used to meas-ure the molecular beam constituents and the contaminating gases that were still present in spite of all the precautions taken.

3.7 ELECTRON BEAM PLASMA TECHNIQUE

This method has been used to prepare Gai-xInxN (Ref. 23) and AIN (Réf. 24). These materials are of interest because their high-energy gaps (see Table 2.3, Chapter 2) means that blue-violet through to red LEDs might be possible. For the preparation of Gai_xInxN a mixture of Ga and In was placed inside a water-cooled anode. Nitrogen at a pressure of 0.3-0.5 torr was passed through a hollow cathode reaction chamber and was chemically activated by the d.c. dis-charge. The metal mixture in the anode was heated by electron bombardment and reacted with nitrogen. During the reaction the semiconductor alloy was deposited on sapphire (OOOl) orientated substrates.

3.8 CONCLUSION

In conclusion a few words should be said about the relative costs of these various crystal-growth methods. The epitaxial vapour- and solution-growth techniques are by far the most economical and often require very simple


equipment. The high expense of an ultra-high vacuum system and the slow production rate for molecular beam epitaxy will mean that this technique will probably be restricted to specialised applications other than LEDs and to research on nucleation. The semiconductor layers grown by electron beam plasma, ionized-cluster beam technology (Ref. 25), ion plating (Refs. 26,27), sputtering (Ref. 28) and other associated evaporation or ion-assisted techniques (Ref. 29) are still not up to the vapour and liquid epitaxial standard and more research is needed before it can be established whether they will compete or not.

Finally, melt growth, although expensive because of the specialised high-pressure equipment that is required for visible LED materials, is essential because it provides the substrates for epitaxial growth.

Crystal Growth 51

REFERENCES

1. B. R. Pamplin, Crystal Growth, Pergamon Press (1975).

2. Handbook of Chemistry and Physics, CRC Press (1977).

3. H. C. Casey, Jr., and F. A. Trumbore, Mater. Sei. Eng., β_, 69 (1970), and references therein.

4. C. Hilsum and A. C. Rose-Innes, Semiconducting III-V Compounds, Pergamon Press (1961).

5. G. A. Antypas, J. Electrochem. Soc. 117, 700 (1970).

6. J. C. Woolley and B. A. Smith, Proc. Phys. Soc. 72, 214 (1958).

7. J. L. Shay and J. H. Wernick, Ternary Chalcopyrite Semiconductors, Pergamon Press (1975).

8. A. G. Thompson and J. W. Wagner, J. Phys. Chem. Solids, 32, 2613 (1971).

9. B. R. Pamplin, J. Crystal Growth, 26, 239 (1974).

10. P. Migliorato, B. Tell, J. L. Shay and H. M. Kasper, Appl. Phys. Letters, 24_, 227 (1974).

11. J. J. Tietjen and J. A. Amick, J. Electrochem. Soc. 113, 724 (1966). The vapour-growth reactor of the type described in this reference is now produced commercially by Applied Materials Technology, Inc.

12. E. W. Williams and D. M. Blacknall, Trans. Mater. Soc. AIME, 239, 387 (1967).

13. H. Rupprecht, GaAs Conference, Institute of Physics, p. (1966).

14. M. G. Astles, F. G. H. Smith and E. W. Williams, J. Electrochem. Soc. 120, 1750 (1973).

15. R. E. Nahory, M. A. Pollack and J. C. DeWinter, J. Appl. Phys. 46, 775 (1975).

16. H. Nelson, RCA Rev. 24, 603 (1963).

17. H. Kressel and H. Nelson, Physics of Thin Films, ]_, p. 115, Academic Press (1973).

18. H. F. Lockwood and M. Ettenburg, J. Crystal Growth, 15, 81 (1972).

19. A. Y. Cho, J. Appl. Phys. 46, 1733 (1975).

20. A. Y. Cho and I. Hayashi, J. Appl. Phys. 42, 4422 (1971).

21. A. Y. Cho and H. C. Casey, Appl. Phys. Lett. 25, 288 (1974).


22. M. Illegems and R. Dingle, Conf. on GaAs and Related Compounds, Inst. of Physics, p. 1 (1975). See also A. Y. Cho, J. Vac. Sei, and Technol. £, 531 (1971).

23. K. Osamura, S. Naka and Y. Murakami, J. Appl. Phys. 46 , 3432 (1975).

24. Y. Uemura and M. Iwata, J. Crystal Growth, 29, 127 (1975).

25. T. Takagi, I. Yamanda and A. Sasaki, IPAT 77, Proceedings of the Conference on Ion Plating and Allied Techniques, p.50, CEP Consultants (Edinburgh) (1977).

26. D. G. Teer, IPAT 77 Proceedings, p.13, CEP Consultants (1977).

27. E. Henderson, IPAT 77 Proceedings, p.122, CEP Consultants (1977).

28. C. W. Pitt, IPAT 77 Proceedings, p.149, CEP Consultants (1977).

52

4 FABRICATION

Over the last 10-15 years, LEDs have developed from a laboratory phenomena to a commercially viable product. Initial development concentrated on the material aspects in improving the efficiency and the range of colours available, but more recently effort has been directed to improving the assembly and packaging technology.

Figure 4.1 gives a flow diagram of the processes that are used for the well-established devices made from GaAs, GaAs^_xPx and GaP. This diagram does not include the finer details such as the photolithographic techniques which are now commonly used in many of the LEDs that are commercially available. Because of the variations in processing for the different materials we shall concentrate on LEDs fabricated from GaAsi-xPx since these are the most commonly used and it will give the reader enough information to appreciate the processing steps involved.

GaAs GaAsP GaP

Pulled crystals of GaAs for substrate

X LPE

growth of n-and p-layers

by amphoteric silicon doping

GaAs or GaP substrates

Pulled crystals of GaP for substrate

VPE of n-layer of GaAsP

X VPE of

n - layer of GaP

X

Junctions formed by diffusion of p-type impurity - zinc

LPE of n-layer of

GaP t

Junction formed by LPE of p-layer

Contacts evaporated on to both n and p-type faces of wafer

Wafer scribed and then broken into individual die

Die mounted on to lead frame and then wire bonded

Tinted epoxy applied to complete the LED

Fig . 4 .1 Fabr i ca t ion schedule of LEDs made from GaAs, GaAs1_xPx and GaP

53


4.1. MATERIAL

The starting point for the fabrication of GaAsi_xPx LEDs is the growth of vapour epitaxial material as described in the last chapter. Layers of GaAsi_xPx are grown on to substrates of GaAs or GaP wafers depending upon which provides the best lattice matching. Monsanto produce this material to four different specifications as given in Table 4.1.

Table 4.1 Specification for GaAsP Materials (Monsanto)

Emission

Substrate

Growth type

Conductivity type

Dopant

Carrier concentration

Resistivity

Orientation

Epitaxial layer

Conductivity type

Dopant

Carrier concentration

Mobility

Film thickness

Graded

Constant

Phosphorus content

Nitrogen doping

Red

GaAs

Czochralski

n-type

Silicon or tellurium

2x1ο17 atoms cm (min)

0.008 ohm cm (max)

(100; Off (2°±1°) [lio] direction

in

Vapour grown

n-type

Tellurium

0.4-1.2xl017 atoms

2000 cm2/V s (min)

20 microns (min)

40 microns (min)

37-41%

Nil -

-3 cm

Red/Orange, Yellow or Green

GaP

Czochralski

n-type

Sulphur

0.35-2xl018 atoms cm"3

(100) Off (5°±1°) in Q-lo] direction

Vapour grown

n-type

Sulphur

0.3-lxlO17 atoms cm-3

lOOym

^65%

Red/ Orange

lOym

60ym

85-90%

Yellow

70ym

100%

Green

At least the last lOym of epitaxial layer is doped with 1-2 x lO1^ nitrogen atoms per cm ,3

Fabrication 55

4.1.1 Gallium Arsenide Substrates

GaAs substrates are used for the well-established GaAsQ.6po.4 l a v e r s which give a deep red emission. The n-type GaAs substrates are sliced along the {lOO} planes which cleave readily at right angles to each other thus facilitating the production of square chips. Due to the high mobility of n-type material it is possible to obtain substrates with a resistivity of less than 10"2 ohm cm. The epitaxtial layer is divided into a region of graded composition on top of which is grown a layer of constant composition, typical thicknesses of which are given in Table 4.1. This grading of composition in the first part of the epitaxial layer reduces the strain produced by lattice mismatch of the substrate and the constant composition layer since it is graded in composition from GaAs to the final alloy layer constant composition.

4.1.2 Gallium Phosphide. Substrates

Gallium phosphide, as well as providing a better lattice match to the higher phosphorus content GaAsi_xPx layer, also provides better optical coupling than GaAs. This results in increased edge emission and extraction efficiency.

Although the substrates are doped n-type to approximately 101 8 atoms/cm^, the lower electron mobility of GaP results in a resistivity typically an order of magnitude higher than GaAs. Because the phosphorus-rich epitaxial layers have an indirect band gap, nitrogen doping of approximately 10-·-" atoms/cm-^ is introduced into the growth process in the last 10-15 microns. This allows relatively efficient emission of red/orange, yellow or green to be achieved depending upon the mole ratio of arsenic/phosphorus atoms in the epitaxial layer.

4.2 PHOTORESIST TECHNIQUES

The photoresist procedures which are universally used in silicon transistor and microcircuit technology can usually be adapted to LED crystals provided that the wafers have a sufficiently smooth surface and are not too small in area. For both of these reasons wafers of pulled crystal and layers deposited by vapour-phase epitaxy normally present less problems than layers produced by liquid growth.

Photoresist techniques are used in a number of processing steps during the fabrication of GaAsP LEDs. Depending upon the exact details of the processing procedure they can be used

(i) to obtain, in conjunction with these deposited films, isolated diffusions, i.e. the well-known "plan r" diffusion technique;

(ii) to produce complex contact patterns aimed at minimising spreading resistance without appreciable loss of light-emission;

(iii) to selectively etch away part of the top p-layer of a p-n junction to obtain a "mesa" construction.

Photoresist systems involve a light-sensitive material which after being coated on to a substrate, exposed to light through a mask and receiving the required development treatment, leaves a pattern on the substrate similar in geometry to that of the mask used. There are two general photoresist types: positive and negative. A negative system is one from which a negative of the


mask used remains behind on the coated substrate, i.e. the resist remains where it is exposed to the ultra-violet light. A positive resist system leaves a positive image in the irradiated resist film after the expose and develop process, i.e. the photoresist remains behind where it is not exposed to light.

The choice of photoresist depends upon factors such as its adhesion to the substrate, its ability to withstand the etchant and the ease of alignment.

Typical processing steps in the use of photoresist are as follows and are illustrated in Fig. 4.2.

j) COATING OF SUBSTRATE

dispense head

wafer

.vacuum table

v—s controlled spinner

i i i ) EXPOSURE OF RESIST

i i i i i uniform u.v. light (tungsten halogen)

/ photo -1 1 I 1 I 1 1 I I I I I I J J 1 lithographic

mask IM H I I I I I I I I I M I I I ΙΊ

alignment jig *—wafer coated with photoresist

iv-v) DEVELOPMENT AND BAKE OF RESIST hardened resist which will .withstand selected etchants

x£2 ^wafer

enlarged view of processed wafer

F i g . 4 . 2 T y p i c a l p h o t o r e s i s t p r o c e d u r e

(i) Coating of Substrate - The most widely used method for the coating of substrates is by dispensing a controlled amount of photoresist on to a wafer which is held by vacuum onto a spinner. The spinner sequence of acceleration, spin duration and deceleration are accurately controlled to distribute the resist as evenly as possible.

(ii) Drying the Photoresist - Normally this is achieved by baking in an oven at 80-100°C for approximately 30 minutes.

(iii) Exposure of Resist - Using commercially available alignment jig the mask pattern is aligned with the substrate and the wafer is exposed for typically 1 minute under a U-V lamp.

(iv) Development of Photoresist - Most photoresist manufacturers supply and recommend a proprietory developer for their respective photoresists.

Fabrication 57

For negative resists the developers are usually blends of common hydrocarbon solvents whereas for positive resist dilute solutions of alkali are used.

(v) Post-development Bake - Conditions can vary from no bake at all for some positive resists to typically 30 minutes at 100-150°C for negative resists.

(vi) Etch Procedure - After developing the required pattern the pattern is transferred to the substrate by etching. The demands of the etch dictate the type, thickness and bake time of photoresist.

(vii) Photoresist Removal - The ease of removal of the processed photoresist is sensitive to the post-development bake conditions. The resist is normally removed by soaking the wafer in a solvent stripper.

4.3. DIFFUSION

The p-n junction at which the light is generated is formed by the thermal diffusion of zinc into the n-type epitaxial layer. Due to the volatility of the group V elements, i.e. As and P, it is normal to use the sealed-ampoule technique.

In this technique the wafer is loaded into a clean ampoule together with the required amounts of zinc and group V elements. The ampoule is pumped down to a good vacuum (typically less than 10~° torr) and then sealed off. The ampoule and its contents are then placed in a tube furnace in which the temperature profile is constant at least over the length of the ampoule. After diffusion for the required time and temperature the amouple is with-drawn from the furnace and the ampoule quenched at the end opposite to that where the wafer is located. This minimises the condensation of zinc onto the surface of the wafer.

4.3.1 Planar Techniques

For the more sophisticated LED dice required for monolithic 7 segment displays and to a lesser extent for standard single chip lamps, the planar diffusion technology is used. The basic procedure shown in Fig. 4.3 is to coat the wafer with a thin film which does not react with it and is impervious to zinc at the diffusion temperature. Then by the use of photoresist selected regions of the film can be removed prior to the diffusion step described above, thus resulting in localised diffusions.

Although phosphorus-doped silicon dioxide can be used as a diffusion mask, silicon nitride is now probably more widely adopted since it can be deposited in an rf plasma at low temperatures (Ref. 1) (MOO°C) . One of the difficulties with S13N4 is its resistance to etchants which do not attack the photoresist. This can be overcome by using a layer of S1O2 on top of the S13N4 and etching with a hot solution of H202 and phosphoric acid. However, etching by a mixture of CF4/O2 in an rf plasma is also possible. (Ref. 2).


I) Initial wafer

epitaxial / layer

«*— substrate

2) Oxide / n i t r i d e film deposited

3) Photoresist technique used to expose selected areas

4) Oxide / nitr ide film etched in exposed areas

5) Zinc diffuses into wafer at exposed areas

-•—oxide/ nitride film

photoresist developed in /selective areas

PAW/J

selectively etched oxide/ nitride

/ Ι777Π1 tllli\

zinc diffusion

i w m HVI\AI

6) Top view of completed diffusion

Fig. 4.3 Planar diffusion procedure

4.3.2 Review of the Prior Art

Our present knowledge of the physics of diffusion into GaAs]__xPx alloys is at a very elementary stage and most commercial techniques have been established by trial and error from a knowledge of diffusion in GaAs and GaP. The diffusion of zinc into these materials depends upon the temperature, and the partial pressures of zinc, arsenic and phosphorus. The partial pressures can only be determined from a knowledge of the condensed phases present in the ampoules. Thus, as a general rule, the starting source composition is not the actual source composition, which decides the Zn, As and P partial pressures.

Having established the required amounts of dopants and hence the partial pressures of the vapour phase the diffusion of zinc is determined by its equilibrium surface concentration and the vacancy concentration in the crystal. From a mathematical viewpoint the diffusion can be described by the one-dimensional form of Fick's second law.

3C(xt) at

3_ 8x (i) (4.1)

where C is the concentration gradient of the diffusing species at position x

Fabrication 59

after time t and D is the diffusion coefficient. When D is independent of concentration and the surface concentration, Co, is constant, the solution is given by

C = Co erfc j ^ · where erfc y = 1 -^ç. f e'z2dz (4.2)

Although profiles with this shape are found to describe many of the dopants used for silicon and germanium, initial measurements of zinc diffusion into GaAs showed profiles with an abrupt front (Ref. 3) which can be accounted for (Ref. 4) by a diffusion coefficient D a C^ as shown in Fig. 4.4. More recently Tuck and Kadhim (Ref. 5) have reported a "double-profile" in which the curves appears to be made up of two shapes - a shallow profile close to the surface and another more penetrating one. This they have attributed to the possibility of variation in the gallium vacancy density.

10'

feX-**-:

10"

I017

si [Experimental

— Theoretical

26 39

l 1 1 9.0 _1° I

25 50 75 100 Depth (μ)

125 L

150

Fig. 4.4 Comparison of experimental and theoretical variation of zinc concentration with distance at 1000°C in GaAs. (Experimental data after Cunnell and Gooch, (3) i960 and theoretical variation assuming C α D^ from Weisberg and Blanc, (4) 1963.)

Figure 4.5 shows a "double-profile" obtained by the diffusion of zinc in GaP with and without the inclusion of elemental phosphorus. The diffusion conditions used in the above experiment (Ref. 6) are similar to those used in the fabrication of green emitting diodes reported by Stringfellow et al. (Ref. 7) who found that the junction depth is proportional to the square root of the phosphorus vapour pressure and that the diode efficiency increased with decreasing partial pressure.


Zinc

Concentration

( atoms cm"3 )

IO 20 3 0 4 0 50 6 0 Penetration (microns)

Fig. 4.5 Profile of zinc diffusion in GaP. (After Tuck, unpublished data (6).)

Comparison of the diffusion of zinc into red, yellow and green emitting GaAsi_xPx at 750°C has been made by Odell. (Ref. 8). He found that his results could be explained by reference to the ternary phase diagrams of Casey and Panish (Ref. 9) and Panish (Ref. 10) which show two monovariant regions 1 and 2 which correspond to different diffusion sources. A summary of his results on the dependence of junction depth on diffusion source and material is shown in Table 4.2. When the phosphorus or arsenic pressure is increased further so that the diffusion conditions no longer lie within monovariant regions then the diffusion depth becomes a function of the ratio of arsenic to phosphorus in the ampoule.

Table 4.2 Junction Depth of Diffusion at 75Q°C (Data from Odell) (Ref. 8)

Junction depth (microns/hr5) Material

GaAS0.6P0.4

GaAS0.15P0.85

GaP

Zinc only (Region 1)

Zinc + P/As (Region 2)

8-12 ^4

V7.5 2.7 - 2.9

^6 2.3 - 2.7

To summarise, the physics of the diffusion of zinc into GaAsi_xPx alloys is still at an elementary stage, and the limited amount of work on III-V compounds has been limited to either GaAs or GaP. From a practical viewpoint the diffusion procedure have been largely empirical and has been aimed at producing efficient emission within certain constraints. For example, the

Fabrication 61

temperature and duration of the diffusion has

(i) an upper limit determined by the ability of the oxide/nitride to resist the process;

(ii) a lower limit determined by the practical consideration of producing a junction of typically 2-3 ym within a reasonable time period. Within these constraints a schedule has to be determined such that a clean surface is maintained (this can be calculated from the liquid phase regions of the phase diagram) and that the surface conductivity is sufficiently high for good ohmic contacts to be made.

4.4. OHMIC CONTACTS

For efficient and reliable operation of LEDs it is essential to use low-resistance ohmic contacts. The contacts should also be sufficiently thick to provide a suitable bonding pad for the wire bonding and to minimise the voltage drop in the contact material. For dice with planar diffused junctions, the top contact pattern is normally designed so that the bonding pad is located on the oxide. Commercially available gallium phosphide dice, shown in Fig. 4.6, show how the planar diffusion technology can be used in conjunction with the photoresist defined metal top contacts to produce a sophisticated geometry which minimises the loss of emission from the top face.

METALLIZATION

0 , 2 5

( •O IO)

O, 4 6

( Ο Ι Θ )

o CM C)

00

O O

Maximum diameter of slice 45 mm Dimensions in millimetres

(inch conversions in brackets )

Fig. 4.6 Gallium phosphide light-emitting dice. (Available from Ferranti, type XP 72.)


Contact to the n-type side of the LED is normally achieved by evaporating over the whole of the back face. For the absorbing GaAs substrates the metallisation has, of course, no effect on the extraction efficiency of the light, but for epitaxial layers grown on GaP the alloyed back contact is found to be absorbing. Consequently in some designs the back face of the wafer is first covered by S1O2 of which a small fraction is removed using the photo-resist technique. Metallisation is then evaporated over the entire face. Since the S1O2 acts as an efficient reflector this reduces the effective absorption at the back face but still provides an adequate low resistance contact.

A typical ohmic contact procedure for GaAsi_xPx is illustrated in Fig. 4.7. The starting point (i) is a wafer which has been through the planar diffusion step and still has a silicon nitride layer. Step (ii) consists of lapping the back face of the wafer to obtain a suitable thickness and then mounting it in an evaporator where metallisation is deposited on both the front and back faces. Step (iii) uses photoresist procedures to define a contact pattern on the top face.

silicon nitride isolated zinc diffusion

i) initial wafer with planer diffused junctions

ii) metallization evaporated on top and back faces

_γΜΡΜ\_ \wiw\ = ^ j - · — epitaxial layer

—substrate

^ \ p - diffusion into substrate

top metallization

-·—epitaxial layer

-4—substrate

back metallization on lapped substrate

isolated zinc diffusions

i i i ) top contact ψϋ^ \ ψ ! ^ defined and wafer sintered

ΛψΒΒ^—

top contacts

^p«\ -epitaxial layer

-substrate

^sintered back contact

Fig. 4.7 Procedure for contact processing

The metallisation is then alloyed into the wafer in a hydrogen atmosphere at 450-550°C depending upon the material composition and the metallisation.

Two important aspects affecting the production of suitable contacts are surface conditions and carrier concentration at surface layer. Although it is fairly evident that cleanliness and the removal of any deposits from the diffusion or photolithographic processes are necessary it may not be so well appreciated that surface roughness is a critical factor not only in obtaining low resistance but in assisting good adhesion. The importance of carrier concentration in producing ohmic contacts is well known. Klohn (Ref. 11) has

Fabrication 63

measured the variation of contact resistance of evaporated metals onto both n- and p-type GaAs and has shown that the resistance drops rapidly as the surface impurity concentration is increased. The surface concentration of GaAs]_-xPx layers diffused with zinc may vary from mid lO

1^ to above 10 2 0 atoms/ cm·3, depending upon the diffusion conditions. As given in Table 4.1 minimum substrate concentrations are 2 x 10-1-7 atoms/cm3 for GaAs and 3.5 x 101 7 atoms/cm3 for GaP.

the

An investigation of metallic contacts for n-type GaAs by Paola (Ref. 12) has shown In-Au alloys to be less sensitive to ambient influences than the Sn-Au alloys. In and also Ni are used for their "wetting" propreties in a variety of alloy contacts. Contact to n-type GaP using Au-Te-Ni alloys have been reported by Natasuka et al. (Ref. 13) who claim a specific contact resistance of 4 x 104 ohm cm. Bergh (Ref. 14) has reported the use of a Au-Si alloy for the n-type GaP which is reported to have the added bonus that it acts as a highly efficient reflector.

A well-used contact alloy for GaAs is Au-Ni-Ge (Refs. 15, 16) which is a Au-Ge entectic (12% Ge) with a small amount of Ni for wetting. Using this alloy ohmic contacts to Gao.6Alo.4As> GaAso.6p0.4 anc^ G a P were studied by Blum and Shih (Ref. 17) and their results are summarised in Table 4.3. Odell (Ref. 18) has also found that the alloy is a good ohmic contact to p-type GaAs0.4Po.6

b u t less suitable for GaAs0.15P0.85 a n d GaP.

Table 4.3 Contact Resistance of Au-Ge-Ni on Ga fiAln a

A s*

GaAs. Λ Ρ Λ . and GaP with various Electron-free 0.6 0.4

Carrier Concentrations at 300 K (After Blum and Shih (Ref. 17).)

Electron carrier concentrations

Semiconductors

Gart ,-Al^ „As 0.6 0.4

GaAs^ ^ΡΛ „ 0.6 0.4

GaP

0.8-lxlO17

(cm )

4.5xlO"4

(600°C)

4xlO"3

(600°C)

2-3xl017

(cm"3)

lxlO"3(a)

(520°C)

2.1χ1θ"4

(550-600°C)

5.5xlO"4

(600°C)

17 4-5xlO±/

(cm )

5xlO-4

(520°C)

1.7xlO~4

(550°C)

3xlO"4

(600°C)

7xl017

(cm~3)

2.5xlO~4

(500°C)

l-3xl018

(cm )

2.1xlO*~4

(500°C)

8x10"5

(600°C)

(a) The figures in parenthesis indicate the optimum sintering temperature for 2 minutes to produce the minimum contact resistance (in ohm cm2) indicated in the same box.

The contact resistances of Au-Zn and Al to p-type Alo.4Ga0.6As, GaAs0.6Po.4 and GaP with carrier concentration of ^2 x lO1^ cm"3 are listed in Table 4.4. (Ref. 17). It is seen that both Au-Zn and Al provide good contact to p-type GaAso.6po.4' b u t t h a t only Au-Zn alloy was suitable for GaP after sintering to 550°C."

Table 4.4 Contact Resistance Measured at 300 K of Au-Zn and Al to p-type Ga„ rAl ,As

* **

0.6 0.4

GaAs

P

and GaP at different Heating Temperatures for 2 minutes

(After Blum and Shih (Ref. 17).)

Wafer material

Α1Λ ,Ga r

t As

0.4 0.6

GaAs^

rV

^

A

0.6 0.4

GaP

Contact

material

Au-Zn

Al

Au-Zn

Al

Au-Zn

Al

Contact resistance (ohm-cm^) after different heating temperatures

As

deposited

2.5xlO-2

N-0

l.lxlO-6

lo"4

N-0

N-0

400 C

1.9xlO~5

N-0

1.3xl0"6

5.5χ1θ"5

N-0

N-0

450 C

lO"5

N-0

2.9xl0"6

2χ1θ"5

N-0

N-0

500 C

8xlO"6

lO"4

6xl0"6

1.5xlO~6

N-0

N-0

550 C

5xl0"5

lo"3

N-0

600 C

2xlO"5

6.5χ1θ"4

N-0

650 C

1.5xl0"3

N-0

All the materials are p-type with carrier concentration approximately 2x10 cm

(b) N-O (non-ohmic)


Fabrication 65

4.5 SCRIBING AND BREAKING

The operation of scribing and breaking of LED dice is similar to that in widespread use for silicon. However, since the wafers of LED material are not as uniform in shape or size as silicon slices it is not so easy to standardise the procedures to the same extent. Fortunately the vapour phase grown epitaxial layers are normally grown on to a (100) oriented substrates so that provided that the scribing is aligned to be parallel to the cleavage planes, reasonable yields can be achieved.

4.5.1 Scribing

The two most widely used means of scribing LED materials are diamond-tip scribing and diamond-blade-saw scribing.

Scribing machines using truncated pyramid diamonds as the scribing tool is the most widely used technique today. The scribe mark produced by this technique produces a triangular cross-section kerf structure with a compressive stress pattern developed below the apex of the cut. The depth of the scribe, d, typically <0.003 cm is, of course, only a small fraction of the total thickness of the wafer, t, as shown in Fig. 4.8(a). If the pressure on the diamond is increased to give a deeper cut, the GaAsi-xPx material crumbles or cracks.

V ^ |«

V a) diamond tool scribe

^ h-

b) diamond saw scribe

Fig. 4.8 Kerf patterns for diamond scribing.

The kerf pattern for a diamond-saw scribe can be considerably deeper and exhibits parallel sides with rounded bottom as shown in Fig. 4.8(b). In fact, with recently available saws (Ref. 19) and blades it is possible to cut completely through the wafer. For a diamond-impregnated blade with a thick-


ness of 0.0043 cm the resulting kerf width, w, is less than 0.005 Cm.

4.5.2 Breaking

Breaking of semiconductor wafers is accomplished in most cases by the conventional roller-break technique in which the scribed wafer is placed scribed face down on a soft flexible support. A thin piece of plastic is stretched over the top of the wafer and a roller is passed across the top of the wafer under pressure. For the diamond-scribed wafer it is found that successful breaking can only be achieved if the ratio of the die length, 1, to wafer thickness, t, are not less than a given value. Typically, however,

this value varies depending upon the exact composition of the epitaxial layer and its thickness.

These restraints, of course, do not apply for diamond-sawn scribing. If the technique adopted is to mount the wafer on a substrate and to cut right through the wafer then the only limitation is that the wafer thickness is limited by the geometry of the wheel to typically 0.030 - 0.038 cm. No breaking is then required. Alternatively if the wafer is only cut partially through the wafer, typically through two-thirds of the thickness, then conventional breaking is adopted.

4.6. DICE MOUNTING AND WIRE BONDING

There are a wide variety of techniques available for these operations, and the choice of technique is linked to the application for which the LED dice is being used (i.e. whether it is for an indicator lamp, as part of a numeric display or possibly some hybrid application). In this book we are primarily concerned with single indicator lamps and we will limit the processing to the most common type which is mounted on a lead frame with axial leads as shown in Fig. 4.9. Lead frames made from both beryIlia-copper and nickel-iron (Kovar) are in common use. The frames are generally nickel and then gold plated although sometimes selective gold plating is limited to the upper part of the frame.

Fabr ica t ion 67

die mounting platform

wire bond

Frame thickness » 0 - 0 2 0 "

0 2 0

Fig. 4.9 Typical lead frame for LED with axial leads.

4.6.1 Dice Mounting

Mounting of GaAsP die can be achieved by using a suitable preform. The preform needs to provide adequate wettability and strength and to melt at a temperature above that required for the wire-bonding operation but not too high so that degradation of the chip occurs. A Au-Ge eutectic (12% Ge) which melts at 356°C has been used with some success although with the availability of conducting epoxies, new die-attach techniques have been established.

Commercially available conducting resins contain a metal or a mixture of metals, e.g. Ag-Au-Cu-Al. Of these, probably the Au- or Ag-based mixes are the most suitable for die-attach applications. For economic reasons Ag-based resins are preferable although in some applications Ag ion drift under applied field (Ref. 20) has been observed. Where this is unacceptable the more expensive Au-based resins are probably more suitable. The resins are made both as single- and two-component mixes and occur with a variety of viscosities. A procedure which has proved suitable for a one component Ag-filled resin (Epotek H31) (Ref. 21) is described.

(i) After cleaning the lead frame a small blob of epoxy is dispensed from a 0.025 cm syringe on to the platform of the lead frame. The height of the syringe above the lead frame and the duration of the dispense operation must be determined experimentally.


(ii) Using a vacuum pick-up lower the chip on to the centre of the epoxy blob applying a light pressure.

(iii) The lead frame is then baked in an oven at 150°C for 2 hours to cure the epoxy.

It has been found that these dies will withstand subsequent processes up to 450°C and that they provide a satisfactory low impendance die attachment provided the current density through the die is not excessive.

4.6.2 Wire Bonding

There are two types of wire bonding in widespread use in the semiconductor industry - ultrasonic bonding using aluminium wire which can be performed at room temperature and thermocompression bonding using gold wire which has to be performed at an elevated temperature. Of these two, it is found that the Au-Au thermocompression bonding is more suitable for LED fabrication.

In this process the Au wire (typically 0.0025 cm diam.) is threaded through a heated capillary (150-200°C) slightly larger than the wire diameter. A ball is formed at the end of the wire by oxy-hydrogen flame cut-off melting. A weight of 50 g or more is used to force the ball down on to its bonding pad which is heated to 300°C or more. The heated lead frame is normally enveloped in a forming gas (10% H2/ 90% N2) atmosphere to prevent any oxidation. The dwell time to form a reliable bond has to be determined experimentally. The effect of substrate heat, physical conditions of the heated wire and pressure create a Au-Au weld.

4.7 ENCAPSULATION

The penultimate process in the fabrication of semiconductor lamps is the encapsulation of the lamp in partially transparent or translucent epoxy resin. The resin dome serves three main functions: (i) it protects the wire bond and provides the only support for the leads, (ii) it increases the light output from the chip since it reduces the refractive index mismatch between the chip and the air, (iii) it determines the radial distribution of the emission.

There are a wide variety of transparent liquid epoxy resins commercially available. They are normally a two-component formula which must be mixed together immediately prior to use. Good mixing and degassing is necessary. At this stage the required dye and white pigment should be added. The epoxy is allowed to stand to allow all bubbles to escape. It may be helpful to heat the mix as this will reduce the viscosity and allow bubbles to rise more easily.

The shape of the epoxy dome is determined by the polypropylene moulds which are mounted in a base plate as shown in Fig. 4.10. The moulds are then filled with the liquid epoxy and the lead frame, clamped in a support, is dipped into the filled moulds to a predetermined depth. The complete encapsulation jig is heated to gel the epoxy after which the lead frame can be removed from the moulds and the epoxy cured for a longer period.

Fabrication 69

lead frame clamp

lead frame

Fig. 4.10 Encapsulation jig

The final procedure in the fabrication is to separate the individual lamps. This is carried out on a pneumatic cropping machine. For identification purposes it is often arranged to make one lead longer than the other. Unfortunately it is not universally established which should be the longer lead. A completed lamp is shown in Fig. 4.11.

c r o p p e d ^

lead frame

?T<S (

Fig. 4.11 Completed LED indicator lamp


In this chapter we have attempted to describe the basic procedure for the fabrication of LEDs. We have not mentioned the quality-control checks which must exist to ensure a reliable product nor have the final inspection and test procedures been described. The lamp characteristics have been omitted here since they are shown in detail in Chapter 9.

Undoubtedly there are many variations of LED processing which are used and will continue to be developed both to improve the lamp performance and to simplify its construction and so further reduce its cost. Although the processing procedure described has been for one specific lamp construction, similar techniques are used for other lamp types and indeed for the fabri-cation of displays.

Fabrication

REFERENCES

1. R. C. G. Swann, R. R. Melta and T. P. Cauge, J. Electrochem. Soc. 114, 713 (1967).

2. H. A. Clark, Solid State Tech. 19, 51, (1976).

3. F. A. Cunnell and C. H. Gooch, J. Phys. Chem. Solids, 15, 127 (1960).

4. L. R. Weisberg and J. Blanc, Phys. Rev. 131, No. 4 (1963).

5. B. Tuck and M. A. H. Kadhim, J. Mater. Sei. Ί_, 585, (1972).

6. B. Tuck, private communication.

7. G. B. Stringfellow and D. Kerps, Solid State Electron. 18, 1019 (1975).

8. E. C. Odell, Thorn Lighting Ltd., Tech. Memo RM 516 (1975).

9. H. C. Casey, Jr., and M. B. Panish, Trans. Mater. Sei. AIME, 242, 406 (1968).

10. M. B. Panish, J. Electrochem. Soc. 113, 224 (1966).

11. K. L. Klohn and L. Wardinger, J. Electrochem. Soc. 116, 507 (1969).

12. C. R. Paola, Solid State Electron. 13, 1189 (1970).

13. H. Nataksuka, A. J. Domenico and G. L. Pearson, Solid State Electron. L4, 849 (1971).

14. A. A. Bergh and J. R. Strain, Ohmic Contacts to Semiconductors, p.116, Ed. B. Schwartz, J. Electrochem., N.Y. (1969).

15. N. Braslau, J. B. Gunn and J. L. Straples, Solid Atom Electron. 10, 386 (1967).

16. J. S. Harris, Y. Nannichi, G. L. Pearson and G. F. Day, J. Appl. Phys. 40, 4575 (1969).

17. J. M. Blum and K. K. Shih, J. Luminescence, 1_, 162 (1973).

18. E. C. Odell, Thorn Lighting Ltd., Lamp Res. Tech. Memo, RM 508 (1975).

19. D. M. Penning and H. R. von Arx, Solid State Tech. 15, 27 (1972).

20. J. R. Fitzpatrick, B. F. Jones and K. J. Downton, J. Phys. D, 9_, L129 (1976).

21. Available from Teknis Ltd.

71

5 LUMINESCENCE AND EXCITATION PROCESSES

Up to now we have considered the properties of semiconductors in thermal equilibrium. In this state there is a balance between the thermal generation of electrons and their subsequent recombination. In the rest of this book, however, we are concerned with non-equilibrium situations where excess carriers are created and we observe how the system is restored to equilibrium. Under these unstable conditions recombination of carriers may occur either radiately or non-radiately. Radiative recombination results in light emission, whereas in the case of non-radiative recombination the potential energy of the recombining carrier is dissipated as heat. The processes can be further subdivided as follows:

Radiative

1. Recombination of an electron from conduction banri (or donor site) with a hole in the valence band (or acceptor site). For indirect-band gap semi-conductors phonon participation is required.

2. Radiative recombination of excitons.

3. Recombination via a recombination centre.

Non-radiative

1. Auger recombination - a three carrier process which may be band-to-band or involve recombination centres.

2. Multiphonon process which may also involve recombination via defect levels.

The creation of excess electrons and holes in a crystal may be achieved in a number of ways, i.e. bombardment with energetic electrons, ions or other particles. We are concerned, however, with two processes:

Photoluminescence - this is the radiative recombination of excess carriers which have to be generated by the absorption of light.

Electroluminescence - this is the radiative recombination of carriers which have been electrically injected into the crystal. In particular we shall concentrate on d.c. electroluminescence due to the minority carrier injection at p-n junctions.

Although our prime interest is in radiative recombination, we cannot ignore non-radiative processes since they compete in the recombination processes. Thus the development of efficient luminescent materials is aimed at improving the radiative processes and reducing the effectiveness of the non-radiative ones.

72

Luminescence and Excitation Processes 73

5.1 RADIATIVE RECOMBINATION

The process of recombination is shown in terms of the energy band diagram in Fig. 5.1 for direct- and indirect-band-gap materials. These transitions

Fig. 5.1 Radiative band-to-band recombination in (a) direct gap and (b) indirect gap semiconductors.

conserve both energy and momentum so that for no phonon participation as shown in Fig. 5.1(a), the frequency of the emitted radiation hv is given by

hv = Εχ - E2 (5.1)

Since the momentum of the photon is negligible then the momentum of the recombining carriers must be equal, i.e. a vertical transition.

However, for indirect-band-gap material the lowest energy conduction band electrons have a non-zero momentum. Radiative recombination in this case requires momentum transfer due to phonon participation as depicted in Fig. 5.1(b). In this case the frequency of the emitted radiation hv is given by

hv = ΕΊ - E0 ± hv 1 2 p (5.2)


where Vp is the phonon frequency.

The recombination rate for a direct-band-gap semiconductor under equilibrium conditions is given by the van Roosbroeck-Shockley relationship. (Ref. 1). This makes a "detailed balance" of the generation and recombination process at various photon frequencies v. The radiative recombination rate R0(v) in an interval, dv, is given by

R (v) dv = P(v) p(v) dv , (5.3) o

where P(v) is the probability per unit time of absorbing a photon of energy hv and p(v) dv is the density of photons in an interval, dv. The latter is given by Planck's law in equation 5.4. n is assumed to be independent of v. 2 3

/ N -, 8πν n dv /r „N p v dv = -— .. /Ί m -,

5.4) 3 exp(hv/kT -

1) ,

c The absorption probability is directly related to the absorption coefficient a(v) of the photon in the semiconductor by the expression

P(v) = a(v) - , (5.5) n

where c is the speed of light and n is the refractive index.

Substituting equations (5.5) and (5.4) into equation (5.3) and integrating over all frequencies gives the total number G0 of electron-hole pairs generated per unit volume per second and hence the recombination rate R0 is given by

rrn2 / k r \ 3 Γ g(v) u2

z2 \h j J o exp(u- 1) Go = Ro = ^ T ~ 1^1 I ---'-" - d u ' ( 5 · 6 )

where u = hv/kT

Equations (5.3) and (5.6) can be evaluated from experimentally measured values of the absorption coefficient or from theoretical values as discussed in Section 5.2, to determine the recombination spectra and the total recombination rate.

The above relationships are derived for thermal equilibrium conditions. To extend this to non-equilibrium conditions we first postulate that R is proportional to the product of the density of free electrons n and holes p,

i.e. R = B n p , (5.7)

where B is the probability for radiative recombination.

The recombination rate under thermal equilibrium RQ would then be

Ro " B noPo - B n i 2 < (5.8)

where n and p Q are the thermal equilibrium values.


Thus by eliminating B from equation (5.6) and (5.7)

R np o *

2 n. 1

(5.9)

Values of the theoretically calculated recombination probability, N, for both direct and indirect gap semiconductors are given in Table 5.1. As expected the direct-gap semiconductors will have higher values for the probability of radiative recombination than indirect-gap materials. This implies that if we wish to make efficient LEDs from indirect-gap materials, then we cannot rely on band-to-band transition but, we must change the recombination kinetics by choosing suitable dopants.

Table 5.1 Theoretical Recombination Probability, B, for Direct- and Indirect gap Semiconductors at 300 K (V.P. Varshni - Ref. 6)

Semiconductor Gas type B cm s"1

GaAs Direct 7.21 x 10~10

GaSb Direct 2.39 x lo"10

-9 InP Direct 1.26 x 10

InAs Direct 8.5 x 10~

InSb Direct 4.6 x Ιθ"11

Si Indirect 1.79 x lo"15

-14 Ge Indirect 5.25 x 10

-14 GaP Indirect 5.37 x 10

5.2 MINORITY CARRIER LIFETIME

When the density of carriers in an extrinsic semiconductor is increased above its equilibrium value by an external influence then the recombination rate increases to restore the balance. If the disturbance is a steady-state phenomena, then a new equilibrium value will be established for which a characteristic time can be associated with the lifetime of the excess carriers. We will now show that it is the minority carriers and not the majority carriers which determine this characteristic time.

To demonstrate this, consider p-type material in which excess electron and hole densities Δη and Δρ recombine radiatively according to equation (5.9). The total electron density np = npo + Δη will, of course, be much smaller than the hole density pp = ρρο + Δη. The radiative recombination rate R may be written R = RQ + àR0f so that rewriting equation (5.9) gives


R + AR R (n + Δη) (p + Δρ) o po po _

o 2 n. 1

(5.10)

whence AR % Δη Ap

o po ^po

If we assume that there is charge neutrality in the material then Δη = Δρ. Since η ρ ο << p p o we can to a very good approximation rewrite equation (5.10) as

AR/R = Δη/η , (5.11) o po

i.e. the recombination rate is dominated by the minority carrier density. It is normal to define the lifetime of excess minority carriers τ η by the expression

AR = Αη/τη . (5.12)

So that combining equations (5.11) and (5.12) gives the expression for the lifetime of carriers which recombine radiatively

Tr, = n„JK <5·13> n po o

For n-type material the recombination of excess carriers is determined by the lifetime of excess holes i.e. τ ρ =

Pno/R

0·

It is found that the concept of a minority carrier lifetime may be extended to recombination by other mechanisms, i.e. to each mechanism is associated a characteristic recombination lifetime τ±. The effective lifetime xeff when various mechanisms occur simultaneously is given by

— - V -reff ^_ H

(5.14)

These recombination processes may be conveniently divided into radiative and non-radiative ones, each with effective carrier lifetimes TR and τ ^ . Equation (5.14) can then be rewritten

1 - X ' λ . (5.15) Teff TR TNR

We can now relate the radiative efficiency of a material, η, to the minority carrier lifetimes for radiative and non-radiative recombination by the expression


Thus to achieve high radiative efficiency we must keep τ ^ as long as possible as shown in Fig. 5.2.

IOO CL

percentage

radiative

e f f i c i e n c y

Ό °/o

5.3

Fig. 5.2 Dependence of radiative recombination efficiency on carrier lifetime of excess carriers, τ and τ .

K NR

RECOMBINATION VIA DEFECT CENTRES

In this section we shall consider the kinetics of the recombination of excess carriers via a defect centre which can be associated with a single energy level within the forbidden gap. The theory outlined was first developed by Shockley and Read (Ref. 2) and used to account for the electrical characteristics of p-n junctions by Sah, Noyce and Shockley. (Ref. 3). Although the kinetics were derived for non-radiative recombination centres it can be used as a basis for the kinetics of radiative recombination via defect centres.

The recombination of the excess carriers consists of the four steps shown in Fig. 5.3. In calculating the balance of these competing processes it is convenient to use the expressions n^ and p, which are defined as

n1 = n± exp £(Et - E±)/kTj and ρ± = n± e x p ^ . Ε _) /k^l

where E is the trap energy level and E^ is the intrinsic Fermi level. The recombination rate of excess carriers, U, may then be written

V n VthNt(pn - ni >

σ (n + n1) + σ (ρ + ρχ) (5.17)


initial state final state

Ec

Et □ O □ Ξ Ξ □ a) electron capture Ei —*»

Ev

Ec

■■*m-- i w %

Et Ξ Ξ □ □ B □ b) electron emission Ei —+>

Ev

Ec

ψ·^;τ T^F

Et Θ □ □ □ □ □ c) hole capture Ei —»-

Ev + *

Ec Et □ □ □ □ □ □

d) hole emission Ei —»-

Ev l i l i i l

Fig. 5.3 Basic process of carrier generation and recombination through traps. (After Sah et al. (Ref. 3).)

where σρ and ση are the hole and electron capture cross-sections respectively, vth t h e thermal carrier velocity and Nt is the density of traps. Note that in thermal equilibrium U = O.

If we now consider a p-type semiconductor in which p = p p o + Δρ and n = npo + An and Δη = Δρ, then the lifetime of excess electrons is given by

τ = Δη/U n

- 1 /ηρο + η 1 + Δ η \ , 1 / P p o + P l + A n \

V t h N t \^ Ppo + npo / V t h \ \ Ppo + npo /

(5.18)

If the capture cross-sections σρ and ση are of the same order of magnitude and


Δη is small, then the minority carrier lifetime in p-type material is approximately given by

n th t -1 (5.19)

and similarly for a n-type material τ ρ ( V t h N t ' Thus if the recombination via these traps is non-radiative, then the radiative efficiency of the material will be improved by keeping the trap density N. to a minimum.

5.3.1 Zn-0 Complex in Gallium Phosphide

A specific case for radiative recombination via a defect centre is the Zn-0 complex in p-type gallium phosphide. This complex forms a deep level recombin-ation centre which is electrically neutral and has a large capture cross-section for electrons. When an electron is captured the complex becomes negatively charged and attracts a hole by Coulombic forces into a shallow exciton hole level. As indicated in the band diagram in Fig. 5.4 the trapped electron may return to the valence band non-radiatively with an associated lifetime T N R or recombine radiatively as a bound electron-hole pair with an associated lifetime TR. The rate of emptying the filled traps due to both radiative and non-radiative processes can be described by a decay constant T D

given by

NR

(5.20)

We can now look at the decay of excess electrons in the conduction band, which may have been generated optically or injected via a p-n junction. As shown in Fig. 5.4 there are two major routes available. Route 1 is a non-radiative

Zn , O doped p — type gallium phosphide

Ec

Ev

j^j electron top level Et due to Zn - O complex

__ hole Fermi level

Shallow exciton hole level

Fig. 5.4 Band diagram of recombination via Zn-0 complex in GaP. (After Rosenweig, et al., (4).)


band-to-band decay (possibly Auger mechanism) with a time constant τ η, so that

Δη/τη (5.21) U. = rate of electron decay (via Route 1)

Whereas Route 2 is a more complex decay involving the electron traps, of density Nt, due to the zinc-oxygen complex. From the kinetic model proposed by Rosenweig (Ref. 4) it may be shown that

Δη

(5.22) U = rate of electron decay (via Route 2)

\ t th nj t

where vth is the electron thermal velocity and ση is the electron capture cross-section by the traps. A comparison of the decay rates via the two routes as a function of the excess electron density is shown in Fig. 5.5. It

electron combination

rates U|,U2

Nt To

Z^ slope = (T,

l,y*^ initial slope 1/ = Ntvth^n

)-'

trqp.s filled

-" ί

excess electron density* Δη

Fig. 5.5 Dependence of recombination rate on density of excess electrons in p-type GaP, (Ζη,Ο)

is noted that the recombiantion rate U2 which includes the radiative mechanism increases linearly for small values of Δη but eventually saturates at a value of Nt/iD. This in effect imposes a maximum light output from the material.

The radiative efficiency of the material is given by first determining the proportion of electrons which decay via route 2 and multiplying that by the radiative efficiency for that particular route (as given in equation (5.16)),

i.e. radiative efficiency η = ,.(J1_)(_L_). \U1 + U2 / \ 1 + T R / T N R /

(5.23)

5.4 AUGER RECOMBINATION

One of the main competitors of a radiative electron transition is recombination


via the Auger effect in which a second electron absorbs the energy given up by the first electron. Corresponding to each type of single electron transition (i.e. band-to-band, band-to-donor/acceptor or donor-to-acceptor) there are equivalent Auger mechanisms as shown in Fig. 5.6. It is due to the complexity of the mathematical treatment and also to the difficulties in performing sufficiently precise experiments that our understanding of the processes is still at an early stage.

Single Electron Process. Correspond!no Auger Process.

Bond -band

Band - donor

Band -acceptor

Donor -acceptor! I I 4

* - ^

Fig. 5.6 Correspondence between single electron and Auger processes for several transitions. (After Landsberg and Adams (5).)

It should be noted, however, that an argument for the derivation of the Auger recombination process analogous to the Roosbroeck-Shockley formula for radiative theory has been developed by Landsberg and Adams, (ref. 5). However, the treatment is beyond the scope of this book. In principle this derivation offers a means of calculating the Auger recombination rates from impact ionization data.

5.5 BASIC EQUATIONS FOR NON-EQUILIBRIUM CONDITIONS

Before we can fully explore the mechanisms of carrier generation and injection it is necessary to examine the transport equations related to non-equilibria. To simplify the mathematics but still obtain useful expressions we shall restrict the analysis to a one-dimensional carrier flow in a semiconductor of uniform cross-section which lies in the x-direction.


Continuity equations. Consider an element of the semiconductor, δχ, which is at a distance x from the origin. The continuity equations relate the rate of increase in carriers in that element to the contributions from net generation, net recombination and net current flow into the element. The equations apply to both electron and holes giving

3p

Jt

9n

3tT G„ _1

q

p dx

un

dx

1 (5.24)

Jp and Jn are the current densities of holes and electrons and the sign convention corresponds to the normal current flow. Up and Un are the recom-bination rate of excess minority carriers and can be related to the minority carrier lifetime τ by the expressions

(5.25)

where p and n are the carrier densities and p 0 and nQ are their equilibrium values.

Current flow equations. In the absence of any magnetic field, current flow consists of two terms. The first is due to the effect of the electric field on the charged carriers and the second is associated with the concentration gradient of carriers.

Thus the hole and electron current are given by

= q

= q

yp P ε

μ η ε n

q D

q D

d£ dx

dn

dx

\ (5.26)

Jn + Jp· For non-degenerate semi-and the total current J is given by J conductors the carrier diffusion constants D are related to the mobilities by the Einstein relationship D = (kT/q)y.

5.6 OPTICAL GENERATION

The basis of photoluminescence is the generation and recombination of electron hole pairs by irradiating a wafer of semiconductor with radiation of suitable wavelength. Provided the energy of the irradiating source is slightly greater than that of the forbidden gap, then each photon absorbed produces one electron-hole pair. The penetration of the irradiating source and hence the position of the generated electron-hole pair will depend upon the absorption coefficient of the semiconductor. In general, the carrier generation will not be uniform throughout the wafer, so that concentration gradients will occur and carriers will diffuse in such a direction as to reduce the gradients. The steady-state distribution of excess carrier density will be dependent on the


minority carrier lifetime. This distribution will then determine the spatial variation of the electron-hole recombination and hence the re-emitted radiation. It is thus evident that the detailed analysis of the photolumin-escent spectra is not a straightforward operation. We shall now examine a simple examplet the steady-state distribution of optically generated excess carriers.

Consider radiation with energy hv which is incident on to a p-type semi-conductor wafer of unit area as shown in Fig. 5.7(a). It is assumed that

(a)

extrinsic p — t y p e semiconductor

( b )

photon flux Φ (cm

curves drawn for

IX

f'sec"1)

\ ( i i )

(i) ocp = i

(ii) °Cp = f (iii)~p-!$P

distance x W

( c )

excess earner density (Δη-Δρ)

for curves A: Ln = y

for curves B: Ln - jg-

other notation as above

distance

Fig. 5.7 Optical generation through a thin wafer as a function of absorption coefficient and diffusion length.

photons/sec are transmitted into the material at x = 0. Since each photon absorbed, generates one electron-hole pair, then the generation rate at distance x is given by

d£ dx

(5.27)

The variation of the photon density φ is related directly to the absorption


coefficient corresponding to the wavelength of incident radiation,

i.e. φ = φ exp(-a x) . (5.28) o ^ p

This variation is shown in Fig. 5.7(b) for different values of absorption coefficient ap. As the absorption coefficient is increased, the electron-hole pairs will be generated closer and closer to the front face. The concentration gradient of electron-hole pairs will cause a flow of carriers away from the front face. For most calculations the field component of the current is negligible compared with that due to the diffusion current so that charge neutrality at all points is assumed. With the knowledge that the minority carrier lifetime will determine the carrier flow we will solve the continuity equation for electrons. Putting 3n/9t = 0 into equation (5.24) and substitu-ting for Gn from equation (5.27) gives

_ d£ / np - npo \ + i

- dx -[ τ η | + q — = 0. (5.29) q dx

Substituting for the current density Jn from equation (5.26) and for the flux density from equation (5.28) gives after rearrangement:

!% Ii _ _ Λ 2 I D τ I D dx I n n / n

npo (5.30)

The general solution of this equation is

lx \ I x \ αρΦο e xP ("V0

n - n = A exD [ — ] + B exp [ - -— 1 - — — . — oV (5.31) P P° V W l M °n (a/- (1/Lp)

2] where Lp is the minority carrier diffusion length (=(Dnxn) .)

The constants A and B are determined by suitable boundary conditions which in this case is the surface recombination velocity of carriers s ; given by

D. s =

~nP \ d x /

Figure 5.7(c) shows the result of this calculation when it is assumed that s = o both at x = 0 and x = W. The excess carrier concentration has been plotted for three different values of absorption coefficient and for two different diffusion lengths. Upon inspection it is seen that the major effect of increasing the diffusion length is to increase the density of excess carriers. Although this occurs throughout the wafer the effect is more significant at the back face. The profile of the excess carrier density depends upon both an and on Ln. As the absorption coefficient is increased, however, and carrier generation takes place closer to the front face, the carrier density becomes less dependent upon ap and approximates to the form

n = n + Δηη exp (-x/L ) , po ° r n


p = p + Δη0 exp (-x/L ) . (5.32) po n

For front face photoluminescence, therefore, it is normal to irradiate the sample with a wavelength which is absorbed rapidly. The exact wavelength is not critical since the excess carrier distribution is not sensitive to small changes in absorption coefficient. For LED layers grown on to different substrates, front face photoluminescence offers a non-destructive means of assessing the quality of the layer.

5.7 ELECTRICAL INJECTION

Before looking at the details of the p-n junction it is worthwhile first examining the flow of carriers which are injected into one end of a long bar of semiconductor as shown in Fig. 5.8(a). It is assumed that the injection rate at x = O is sufficient to maintain an excess minority carrier density Δη0 at x =0. Charge neutrality is achieved by the flow of holes from the far end of the bar, and the current which flows out of the ohmic contact at the far end of the bar must equal the electron injection current.

The diffusion of the minority carriers injected into the bar may be determined by the continuity equations. As with the previous example of photogenerated carriers, it is reasonable to assume that the electron drift current is negligible in comparison to the diffusion current. The continuity equation for electrons given in equation (5.24) is

31 ~n ~n q dx ^ = G - U + - — - (5.24)

In this case there is no net generation of carriers so by substituting for Un

and Jn from equations (5.25) and (5.26) yields

d2n _ An 2 2

3x2 Lp2

= 0 . (5.33)

For a uniformly doped bar npo is independent of x,

n - n = A exp (- x/L ) + B exp (x/L ) (5.34) p po n n

where A and B are arbitrary constants.

If we assume that the bar is sufficiently long (i.e. >> Ln) then it is reasonable to assume An = 0 at the far end of the bar, i.e. B = 0, whence

n = n + Δη exp (- x/L ) (5.35) po o n

This is plotted in Fig. 5.8(b).


(a)

(b)

excess minority carrier density Δη

distance x

(c)

current flow Jn , Jp

distance x

Fig. 5.8 Current flow due to electrical injection of minority carriers

The electron diffusion current Jn(x) is given by

J (x) qDn dn n = "J~ dx _ -q Dn

L exp (-x/L ) (5.36)

Thus the electron diffusion current decays in the same exponential manner as does the excess electron density. The total current J(0) must however be independent of x so that the variation of the hole current is given by Jp(x) = J(o) - JD(x) as shown in Fig. 5.8(c) where J(o) is the electron current at x = 0,

J(o) = q Dn L

Anr (5.37)

5.8 p-n JUNCTION

One of the most convenient ways of injecting carriers into a semiconductor is by forming a p-n junction. In its simplest form, depicted in Fig. 5.9(a) it


p-type

depletion 1 region

Φ

Nd - X p

\

charge

I -n -

density

Χη

Na

field

"""Em

. potential

- type

^ " ^ 1

/ Vbi

1

Ec

p-type N.

EF J

Ev t <*vp

O/Vbi

n - t y p

(a) schematic physical diagram

(b)

of charge

(c)

( d )

potential distribution

(e)

i band diagram

| iCyVn

" F e

Fig* 5.9 Abrupt p-n junctions in thermal equilibrium

consists of neutral n- and p-type regions which are separated by a barrier region depleted of carriers. When the doping concentrations Na and N^ are uniform we have an "abrupt" junction where the depletion region extends a distance xp into the p-type material and xn into the n-type material and they are related by the expression

N x a P

= N x d n (5.38)

The total depletion width, W, is then W = xn + xp. This width can be determined from capacity measurements, i.e. c = dQ/dV = ε/W. The field developed in the depletion region and the potential barrier can be related to


the junction width by integration of Poisson's equation across the depletion width as shown in Fig. 5.9,

9

dx2

3j P(X) (5.39)

The built-in potential V^ can also be related directly to the carrier concentrations in the p-type and n-type material by noting that the Fermi level is constant throughout the junction under thermal equilibrium conditions. Thus if the majority carrier densities in the p and n-type material are p and nno, respectively, then

kT

q in

.1 Current Flow

(5.40)

The variation in the carrier concentration under thermal equilibrium and forward bias is shown in Fig. 5.10. Under thermal eqilibrium it is noted that

(b) carrier density at equilibrium

Fig. 5.10 Carrier density in a p-n junction

88


the pn product is everywhere equal to n-j/ even within the depletion region. When a forward bias V is applied to the p-n junction, the pn product changes throughout the barrier region by the Boltzman factor, so that

pn = n.2 exp (qV/kT) . (5.41)

Thus within the depletion region we have an excess number of carriers and hence there must be an increased recombination rate. Before estimating the current due to this net recombination within the depletion region, let us first estimate the excess carriers which are injected into the neutral regions.

Consider the electron and hole densities at the boundaries of the depletion region. At the boundary with the p-type neutral region, let the excess minority carrier density be Δη0, so that the total electron density is npo + Δη0. To maintain charge neutrality the increase in the majority carrier density must also equal Δη0. Substituting into equation (5.41) gives

(p + ΔηΩ)(n + Δη0) = n.2 exp (qV/kT) .

po ° po u i

Since Pnonno = n i 2 an<^ Poo > > n p c then this expression reduces to

Δη % n [exp (qV/kT)-l] . (5.42) o po u

Similarly the excess hole carrier density Δρ0 at the boundary with the neutral n-type region is given by

Δρ0 = P [exp (qV/kT)-l] . (5.43)

As we have seen in the previous section this excess carrier density results in the diffusion of the minority carriers into the neutral regions, with an exponential decay determined by the minority carrier diffusion lengths. The resulting diffusion currents Jn and Jp are given by substituting equations (5.42) and (5.43) into the current expression (5.37),

[exp (qV/kT)-l] ,

(5.44)

[exp (gV/kT)-l] .

So that the total diffusion current due to minority carrier injection Jd is given by

Jd = J d o E e X p <<lV/kT)-l] (5 .45)

where J^ = q n. (D /L Na + D /L Nd) . do l P p n n u

We must now determine the current which flows to maintain the net recombination within the depletion region. For the wide-band-gap semiconductors with which we are concerned, the dominant mechanism is recombination via deep traps as

j =Ξ_ΞΕ p LP

T _ <* D n Jn - "IT-

n po

P n o


given in equation (5.17). The recombination rate is then

σ σ v., N. (ρη - n.·2) p n th t ^ i_ σ (n + n, ) + σ (ρ + ρη ) n J- P 1

(5.17)

Under the simplifying assumption that ση = σρ = σ the maximum value of U, which occurs when n = p, is given by

σ v Nt ni fexp (qV/2kT)-ll (5.46)

The current which flows is then given by

r o

qUdx

rgo fexp (qV/2kT)-ll (5.46)

where rgo

qWn.

2τ and τ

o 0 V t h N t

Then for bias voltages V > kT/q, the total forward current J is given by

J £ J, exp(qV/kT) do

J exp (qV/2kT), (5.47)

Since J^Q a n^ whereas Jrgo a ni then it is anticipated that the proportion of the current due to recombination in the space charge layer will be greater in wide-band-gap materials which are used for LEDs. The typical forward I-V characteristic of an LED exhibits three regions. This is illustrated in Fig. 5.11 which compares the characteristics of three LEDs fabricated from GaAsi_xPx with the compositions x = 0.4, 0.85 and 1. At small currents, region A, the slope of log I against V can be approximated by a straight line with a slope of qV/nkT where n % 2. In this region the current flow is primarily due to recombination in the space-charge layer. As the foward bias is increased the slope of the characteristic increases and n ■> 1. This is region B where the diffusion current is dominant. Finally as the current is further increased to region C, the series resistance of the contacts and the neutral regions becomes apparent and the voltage increases rapidly.


Devices (i) Ga As.6 P.4

fabricated from

(iii) Ga P - N

i o -

lO"

lO"3l·

\0~*\-Current

(amps)

lO~

io-

io -

lO"

~ l" / ^

Region c / <■«')

{ I //

| Region B / / / ^

! r/l \ Region A / / /

1 1 / 1 / l / 1 1 1 1

— Tiiï>

1 IO 12 14 1-6 18 20 22

Forward bias (volts)

Fig. 5.11 Typical forward I-V characteristics of LEDs fabricated from GaAs, P .

1-x x

Since radiative recombination is associated with recombination of the diffusion current, then to obtain efficiency p-n junctions, region B should be as wide as possible. This is achieved by reducing JrgQ by minimising the density of non-radiative traps and also by reducing the series resistance of the device by better processing.

5.8.2 Emission Efficiency

A detailed review of the diode emission will be given in a later chapter. However, it is appropriate to look in this section at the various definitions of emission efficiency. From photoluminescent measurements it is found that for most diodes fabricated from GaAS]__^x (O < x < 1) the majority of the useful emission occurs by recombination of electrons in the p-type layer. The electron injection efficiency is therefore an important parameter in the design of the LED. If, for the range of current of interest, the space charge recombination current can be neglected then the electron injection efficiency ηθ is given by ne = Jn/(JO + J n ) . Substituting from equation (5.44) gives

Dn npo / Dn npo Dp Pno (5.48)

This can be expressed in a more convenient form by using Einstein's relation-


ship and by putting the equation in terms of majority carrier concentrations so that

η e 1 +

L n y p po

P n l " 1

1 + — — I (5.49) _ P

where pn and pp are the resistivities of the n- and p-type layers at the junction.

The internal quantum efficiency nint is dependent upon the product of electron injection efficiency ne and the radiative recombination efficiency of the excess electrons nr as given in equation (5.23). It is defined by the expression

photon generation rate in the LED int electron flow rate through the LED

The external quantum efficiency next is the number of photons leaving the LED per electron which flows through the junction. This is generally much less than the internal quantum efficiency because of the difficulty of extracting the photons from the semiconductor chip, due to high refractive index value.

The power efficiency of the LED, ηρ, is very simply defined as

optical power output . ,,_ _, . η = -f—,_ . Ί τ (5.51)

ρ electrical power input

If the LED has a radiant power distribution S_(X) with a forward bias V_, then power efficiency can be related to the external quantum efficiency by the expression

jff S(X)dX

P = W —5v ' ( 5 · 5 2 )

For visible light-emitting diodes the luminous efficacy of the LED is a very important parameter because it relates the diode emission to the response of the human eye. The normalized response of the human eye is known as the relative luminosity function, ν(λ), and as shown in Fig. 5.12 it reaches a

(5.50)

Luminescence and Exc i ta t ion Processes 93

o z 2 D

I O,

O S

0 - 2

O· I

0 0 5

0 0 2

i O O I

j 0 0 0 5 J

O 0O2

O O O I

6ΘΟ 5 0 0 4 0 0 3 0 0

2 0 0

IOO 6 0

h 4 0

; 3° ω

§ 10 -1 a 8

6 4 3

2

0 - 8

3 0 2-6 2-6 ENERGY («V)

2 4 2 2 2 O 1 8 1 1 1 1 1 1 1

y

\i \ _AJ 4 0 0 4 5 0 5 0 0 550 6 0 0 WAVELENGTH (nm) λ

650 7 0 0

Fig. 5.12 Relative luminosity function

maximum value at 555 nanometers and covers the range from 400 to 720 nm. The luminous efficacy ηρ of any spectrum is simply

η^ = 680

/ V(X) S(A) dX o

Γ S(X) dX o

lumen/watt (5.53)

and the luminous efficacy of emission from an LED η is defined as

_ lumen output I electrical power input

(5.54)


REFERENCES

1. W. van Roosbroeck and W. Shockley, Phys. Rev. 94, 1558 (1954).

2. W. Shockley and W. T. Read, Phys. Rev. 87, 835 (1952).

3. C. T. Sah, R. N. Noyce and W. Shockley, Proc. IRE, 45, 1228 (1957).

4. W. Rosenweig, W. H. Hackett and J. S. Jayson, J. Appl. Phys. 40, 4477,

(1969).

5. P. T. Landsberg and M. J. Adams, J. Luminescence, 7_, 3 (1973).

6. V. P. Varshni, Phys. Stat. Solidi, _19, 459 (1967).

SOLID-STATE LASERS MADE FROM LED MATERIALS

Lasing action at room temperature in an LED material is relatively simple to achieve provided that:

(a) direct-gap semiconductors are used;

(b) the non-radiative centres like the gallium vacancy-complex in GaAs, which will be discussed extensively in Chapter 8, are kept to a minimum.

Direct-gap compounds and alloys are essential because, as was seen in Table 5.1 the recombination probability is so much higher than.in the indirect-gap materials. Non-radiative centres can be reduced to be a considerable extent by using low-temperature solution or vapour growth of the materials in the manner already described in Chapter 3.

6.1 CRITERIA FOR LASING

GaAs was the first LED material to be made into a solid-state junction laser. (Refs. 1-4). The special "laser-structure" that is required to produce lasing action is shown in Fig. 6.1 (Ref. 5). Two sides are cleaved along the (110) planes to give mirror finishes and two sides are cut either with a mechanical saw or an ultrasonic cutter. Laser action occurs when the current density is increased beyond a critical value. Associated with the coherent emission which is a characteristic of laser emission is a significant increase in the quantum efficiency.

FORWARD BIAS

CLEAVED < I I O > GaAs MIRROR SURFACES

ACTIVE R E G I O N - H

L A S E R L I G H T

Fig. 6.1 Schematic diagram of a GaAs semiconductor laser.

(After Gooch, (5).)

95


Correlation between these phenomena is related to the onset of stimulated emission as illustrated in Fig. 6.2. For low applied bias as shown in Fig. 6.2(a) the excess electron density is low and spontaneous radiation occurs

Δ«η small (

T N R

J

<

•—1

T R

1

f

hV

a) Spontaneous emission

Δ-η large

b) Stimulated emission

Fig. 6.2 Band diagram associated with spontaneous and stimulated emission

with a decay constant, T R S P / as given by the van Roosbroeck-Shockley relation (Ref. 6). The competing non-radiative recombination, in general, has a shorter time decay, T^R, SO that the internal efficiency, ni which is given by

1 + ττ

(6.1)

*sp< / NR

is small. When the current is increased and the density of excess electrons become sufficient the emission generated stimulates other excess electrons to decay with a much shorter lifetime,TR .,as illustrated in Fig. 6.2(b). The stimulated emission has the same wavelength as the exciting radiation and is coherent with it. Since the lifetime of the stimulated emission, TRst, is now much shorter than T^R,the differential internal efficiency is much greater. A plot of the radiation output as a function of current is shown in Fig. 6.3, where the change in the slope of the lines indicates a significant increase in efficiency as the current passes through the threshold value Jt.

Solid-state Lasers 97

CURRENT DENSITY DENSITY, J

Fig. 6.3 Radiation power output as the current density in the laser increases. Jt is the threshold current density for laser action.

The threshold for lasing occurs when there is enough stimulated emission to produce a gain which is equal to the optical losses due to absorption and transmission at the Fabry-Perot mirror faces. Consider a cavity as shown in Fig. 6.4 which has a length I and two partially reflecting surfaces with

mirror faces with reflectance R

Fig. 6.4 Fabry-Perot cavity

reflectance R. The system is characterized by a gain per unit length, g, and a loss per unit length a. Consider a point in the cavity emitting a flux SQ

in forward direction. After travelling a distance 21 this flux S is given by

S = S R exp (2g£ - 2ai) , (6.2)

For stimulated emission to occur the radiation at any point remains unchanged after the 21 path, i.e. action gt is given by

Thus the gain required for threshold of laser

a + τ £n ( * ) ·

(6.3)


The gain is proportional to the density of excess electrons and hence to the forward current. Thus the threshold current J"t is proportional to gt, i.e. gt = BJt.

Substitution into equation (6.3) gives

6.2

J t -

NEAR- AND

a B

FAR-

. *n

-FIELD

a/R;.

n PATTERNS

(6.4)

When the current is increased above the threshold value,first one and then more bright field spots appear at the line of intersection of the p-n junction with the Fabry-Perot mirror face, as shown in Fig. 6.5(a). By micrographing the front and back faces of a number of lasers it has been established that the spots appear in matched pairs at the ends of lines or filaments running perpendicular to the Fabry-Perot mirror faces. A model of filamentary formation by Jonscher and Boyle (Ref. 7) suggests that the stability of these filaments is associated with the increase in refractive index which occurs when the local electron concentration is reduced by the shorter decay-time constant associated with stimulated emission. This is illustrated in Fig. 6.5 (b). The filament then acts as a light pipe and constricts the light along its length.

a) filament formation , ( + )

(-)

Δη

fas.! I I

b) excess electron density through a filament

Fig. 6.5 Near-field pattern of diffused large area laser junctions.


Filamentary lasing has been avoided by the use of the stripe geometry illustrated in Fig. 6.6. For a normal diffused junction the stripe width is

Typical values I = 200um d = \0 μπ\

w = I μτη

Fig. 6.6 Mesa stripe structure of GaAs laser diode

usually chosen comparable to the filament width ( ΙΟ ym). For such a structure standing waves occur when the cavity contains an integral number of half wavelengths. For a cavity of length I this condition is expressed by

™λ - o 2n " *

(6.5)

where m is an integer and λ/η is the wavelength of the radiation in the semi-conductor. The spacing between the modes is given by

Δλ 21 FI)

(6.6)

Figure 6.7 shows how the longitudinal modes of a cavity modulate the emission spectrum (a) just below threshold and (b) just above threshold. (Ref. 5).


[>\ ■ ■ - γ m**»*!

1 / /Ί

k \J 1 l i l 1

8550Â" 8450Ä

(a)

m**^"m'

1 ill M M Jl ki

»■■ — PU" .J 8550A 8450A

(b)

Fig. 6.7 Laser spectra near threshold: threshold, (b) above threshold.

(a) below

In addition to longitudinal modes it is possible to propagate modes having a transverse component. If the sides of the junction are plane and parallel, distance, d apart, then the radiation emerging from the ends will show intensity peaks at an angle ΦΝ where

rN [j--1 h] (6.7)

as shown in Fig. 6.8. In the plane of the active layer the width of the beam as half maxim intensity Δφ is given by Δφ = λ/d where d is the width of the active layer. In the plane transverse to the active layer, the radiation is also subject to single-slit diffraction. Thus the beams diverge with a much larger angle of half intensity δθ given by δθ = λ/w, where the thickness of the active recombination layer is typically %1 ym.


6.3

Fig. 6.8 Far field pattern of stripe laser junction

CONTINUOUS OPERATION

One of the major problems which have limited the application of the junction laser is that CW operation has been limited to low-temperature operation and that as the temperature is increased the threshold current for stimulated emission is found to increase exponentially as shown in Fig. 6.9. (Ref. 8).

40 **\J

2 0

IO

Θ

6

4

2

1

-

-

o

i 1

K* exp (T/I05)-^

/ o

vC °

o Y

o /

1

\

A

V I

A 1

~-\ -

-

I _

20

H lot

"1 8

H 6

H 4 o X ιΛ

Ο ΙΟΟ 2 0 0 3 0 0

TEMPERATURE (°K)

Fig. 6.9 Dependence of threshold current on temperature for a GaAs diode. (After Pankove (8).)


This means that the laser will only operate continously if the temperature rise due to the thermal resistance does not cause the threshold current to increase above the operating current. Initial attempts to reduce the thres-hold current at room temperature were made by growing laser structures using liquid epitaxy techniques (Refs. 9-11) rather than the diffused structures originally adopted. Pilkuhn and Rupprecht (Ref. 12) pointed out that this improvement was partly due to more effective carrier confinement resulting from change in the effective band gap. This led to the use of heterojunctions using the ternary Ga-^-ÂlxAs system where the band-gap variation could be optimised by using the appropriate Ga/Al mole ratio. Using this material both single and double heterostructure lasers have been developed with the structures shown in Fig. 6.10. As a result reduced current threshold values have been achieved.

Double-heteros+ructure Single-heterostructure Homostructure

0.1-1%

-Ιμ.ιτι ~0.l i.m Ίμ,πτι ~\μ,ΓΓ\

Fig. 6.10 Schematic representation of the band edges under forward bias conditions, refractive index changes, and optical field distribution in homostructure,. single heterostructure, and double heterostructure lasers. (After Hayashi et al. (13).)

Figure 6.10 shows the band diagrams, variation in refractive index and confinement of radiation and carriers associated with the homostructures and single and double heterostructures. (Ref. 13). In the single heterostructure the penetration of the radiation into the p-type Gai_xAlxAs layer is minimal due to the refractive index change and with the double heterostructure pene-tration of the radiation is further confined to the p-type GaAs active layer, thus increasing laser efficiency. The interface between the active p-type GaAs region with the n- and p-type ternary compounds acts as a potential barrier to electron and hole penetration. This also improves the laser gain


by increasing the density of excess carriers in the active region.

6.4 DEGRADATION

Experimental measurements on stripe geometry D.H. lasers show marked improved CW lasing as compared with broad area contacts. Using this structure continuous operation at HO°C measured on the heat sink has been achieved. (Ref. 14). However, the operating life of CW operation is still very short, typically up to 100 hours. The degradation has been divided into two degradation mechanisms - a bulk process which causes a gradual decrease in efficiency and a catastrophic mechanism which is related to the operating conditions. In particular this latter "self-damage" mechanism has been associated with the effect of the strong optical flux density at the mirror surfaces. Figure 6.11 shows the relative effect these two mechanisms have on the differential efficiency. (Ref. 15).

1.0

Aging • ·

o

x 0.5

Self -damage o

J I I I I I I I L 0 . 5

Ai + h / i t h

Fig. 6.11 Degradation test for heterostructure diodes showing the change of differential efficiency η/η0 against change of threshold current Alth/ïth- · = non-catastrophic degradation. O ,D = catastrophic degradation at 77 and 300 K, respectively. (After Eliseev (15).)

Solid-state lasers made from other alloys, e.g. GaAsi_xPx (Ref. 16) and Ini_xGaxP, (Ref. 17) are generally less efficient than the double hetero-junction Gai_xAlxAs lasers although recently a n-InP/p-Gao.16

Ino.84AsO.39p0.6l/ p-InP double heterojunction laser emitting at 1.15ym has shown continuous CW operation for 1500 hours at room temperature.(Ref. 18). Additionally, a visible double heterojunction laser made from GaAsQ#7P0.3/lno.34Ca0 ggP has now been fabricated with a room-temperature threshold current of 3,400 A/cm2

at a lasing frequency of approximately 700 nm and this current is three times less than the best reported Ga^_xAlxAs red lasers. (Ref. 19).


REFERENCES

D. N. Nasledov, A. A. Rogachev, S. M. Ryokin and B. V. Tsarenkov, Sov. Phys. - Solid State, 4_, 782 (1962).

T. M. Quist, R. J. Keyes, W. E. Krag, B. Lax, A. L. McWhorter, R. H. Rediker and H. J. Zeiger, Appl. Phys. Letters, ^, 91 (1962).

M. I. Nathan, W. P. Dumke, G. Burns, F. H. Dill and G. Lasher, Appl. Phys. Letters, 1_, 62 (1962).

R. N. Hall, G. E. Fenner, J. D. Kingsley, J. J. Soltys and R. 0. Carlson, Phys. Rev. Letters, 9_, 366 (1962).

Gallium Arsenide Lasers (edited by C. H. Gooch), Wiley (1969).

W. van Roosbroeck and W. Shockley, Phys. Rev. 94, 1558 (1954).

A. K. Jonscher and M. H. Boyle, Gallium Arsenide 1966 Symposium Proceedings, Inst. Phys. and Phys. Soc. (1967).

J. I. Pankove, IEEE J. Quantum Electronics, £, 119 (1968).

Zh. I. Alferov, V. M. Andreev, D. Z. Garbuzev, Yu. V. Zhilgaev, E. P. Morozov, E. L. Portnoi and V. G. Trofim, Fiz. Tekh. Poluprow, 4_, 1862 (1970).

I. Hayashi, M. B. Panish, P. W. Foy and S. Sumski, Appl. Phys. Letters, 17, 109 (1970).

P. R. Selway, A. R. Goodwin and C. M. Phillips; 4th Annual Conference on Solid State Devices (unpublished).

M. H. Pilkuhn and H. Rupprecht, J. Appl. Phys. 38, 5 (1967).

I. Hayashi, M. B. Panish and F. K. Reinhart, J. Appl. Phys. 42, 1929 (1971).

J. E. Ripper, J. C. Dyment, L. A. D'Asaro and T. L. Paoli, Appl. Phys. Letters, 18,155 (1971).

P. G. Eliseev, J. Luminescence, 7_, 338 (1973).

M. H. Pilkuhn and H. Rupprecht, J. Appl. Phys. 36, 684 (1965). (See also the review article by M. H. Pilkuhn on "The Injection Laser11, Phys. Stat. Solid, 25, 9 (1968).

J. C. Campbell, W. R. Hitchens, N. Holonyak, Jr., M. H. Lee, M. J. Ludowise and J. J. Coleman, Appl. Phys. Letters, 24, 327 (1974).

C. C. Shen, J. J. Hsieh and T. A. Lind, Appl. Phys. Letters, 30, 353 (1977).

H. Kressel, G. H. Olsen and C. J. Nuese, Appl. Phys. Letters, 30, 249 (1977) .

1.

2 .

3 .

4 .

5.

6 .

7 .

8.

9 .

10 .

11 .

12 .

13 .

14 .

15 .

16 .

17.

18 .

19 .

7 MEASUREMENT TECHNIQUES

7.1 PHOTOLUMINESCENCE

7.1.1 The Basic Photoluminescence Equipment

Figure 7.1 shows the simplest type of photoluminescence experiment that can be set up. Lenses are used to focus the exciting light source on to the sample surface and to focus the photoluminescence emitted by the sample on to the entrance slit of a spectrometer. Hence this is known as front-surface luminescence. The two filters F3 and F2 ensure that the light falling on the sample has an energy greater than the band gap of the semiconducting sample. The third filter Fi stops the exciting light that is scattered from the front surface of the sample from going into the spectrometer. Careful positioning of the sample is essential to make sure that it does not act like a mirror and reflect the exciting light directly into the input slit. Directly reflected light must be avoided because Fi will not be a perfect filter and strong visible or ultra-violet light will pass through it to some extent.

* LAMP

& 3F ,

SAMPLE

|PRE-AMP

LOCK- IN AMPLIFIER!

1REFERENCE TSIGNAL

1—1 P. M. I

'■ /

SLIT

SLIT

X - Y

PLOTTER

SPECTROMETER

CHOPPER

Fig. 7.1 lenses.

Front-surface photoluminescence equipment with

When first setting up the experiment the spectrometer entrance slit is closed and Fi is removed. The scattered light image from the sample is then care-fully focused until it just fills the width of the entrance slit. F]_ must then be replaced before the slit is opened or the photomultiplier (PM) will be overloaded. The chopper is of the miniature tuning-fork type which produces

105


so little vibration that it can be mounted on the spectrometer entrance slit. The spectrometer disperses the luminescence and the output slit selects a particular wavelength band of light to fall on the PM detector. The output from the PM then goes first to a phase-sensitive detector and finally to an X-Y recorder. The reference signal from the chopper is connected to the phase-sensitive detector so that only chopped light signals are amplified.

Many different light sources are available as the list in Table 7.1 shows. Tungsten iodide sources are very good for routine measurements where high sensitivity is not required. Lasers are excellent light sources and give more power than conventional lamps provided that the laser line used for excitation has at least 30 to 100 milliwatts of power in it. Filtering is a problem with gas lasers because weak spurious gas emission lines often occur in the wavelength region where the photoluminescence is being studied.

Table 7.1 Photoluminescence Light Sources

Source Wavelength range (nm)

Power required

Tungsten-iodide

Mercury

Xenon

Deuterium

GaAs LED

GaAs laser

Helium-neon laser

Krypton laser

Argon laser

Visible + IR

UV + visible

Visible

UV

930 to 940

900

632.8

647.1 and various other lines up to 476.2

514.5 and other lines out to 457.9

Helium-cadmium laser 441.6

Dye lasers Tunable 430 to 660

150 watts

100 to 200 watts

200 watts

20 watts(a)

200 milliwatts

7 watts (pulsed)

100 milliwatts

30 to 150 milliwatts depending on which line

50 to 700 milliwatts depending on which line

50 milliwatts

(15 to 20% of argon laser line pump power)

Deuterium sources that are currently available are of such a low power that they are only suitable for materials like the II-VI compounds which have a very high photoluminescence efficiency.

The lens arrangement shown in Fig. 7.1, although it is very easy to set up, results in a considerable loss of the photoluminescence emission from the sample because the F number of the final lens just in front of the entrance slit of the spectrometer does not match with the larger F number of the spec-trometer. (F number is the ratio of the focal length to the diameter of the lens or mirror.)

For samples which have very weak luminescence off-axis paraboloid mirrors which exactly match the F number of the spectrometer can be used in the

Measurement Techniques

optical set up shown in Fig. 7.2. (Ref. 1).

107

5PEX U METER

SPECTROMETER

CL

CRYOSTAT

Ga As SAMPLE

(F l ) Cu S 0 4 : H 2 0

MERCURY LAMP

SCREEN

Fig. 7.2 Photoluminescence equipment including off-axis paraboloid mirrors to reduce luminescence losses. (After Bebb and Williams, 1972 (1).)

When LED materials with an energy gap in the infra-red are studied then saturated copper sulphate solution is used for the filter immediately in front of the exciting mercury or tungsten lamp as shown in Fig. 7.2 for a gallium arsenide sample luminescence study. Copper sulphate cuts off all the infra-red very efficiently.

Although filter selection is very important the sample surface condition is also vital. Single crystal samples give the largest luminescence signals in general, because their surfaces can be etched more effectively than poly-crystalline samples. For "transmission" luminescence, both sides of the sample must be mechanically polished to a mirror finish to maximise the transmission and then just before the luminescence is measured they must be etched with a polish etch to remove work damage. For front-surface lumines-cence obviously only one side needs to be etched just before the run commences. For melt grown slices mechanical polishing followed by a chemical polish etch to remove work damage is required - say 20 microns etched off when the final


mechanical polish was with a 1-micron alumina or diamond abrasive compound. Epitaxial samples will often give reliable results without etching. Vapour-grown epitaxial samples are better than solution grown in this respect. When epitaxial layers are etched they must be etched very slowly to make sure that only a small amount of the epitaxial layer is removed. Etching with a few drops of bromine in 20 cm^ of methanol give an excellent polish etch for GaAs, GaP and InP. (Ref. 1). A list of etches for seven III-V compounds and SiC has been given by Gatos and Levine (Ref. 2) and Table 7.2 lists a selection with an emphasis on polish etches since these are the most suitable for photoluminescence studies because of the mirror-smooth surface that they produce. Note that the etch times given in the table apply for melt-grown samples. Shorter times will be needed for epitaxial samples. If methanol is used larger quantities of methanol will slow down the etch rate considerably.

Table 7.2 Etchants for LED Materials

Compound

AlSb

GaP

GaAs

GaSb

InP

InAs

InSb

SiC (a and

Etchant

1 HF 1 H202 followed by 1 HCl

1 H20 followed by 1 HN03

Cl2 bubbled slowly through CH3OH

5-20% Br2 in CH3OH

1 Br2

10 CH3OH

1 Br2

10 CH3OH

15 HF 75 HNO3 15 CH3COOH 0.06 Br2

(a) Modified

CP4

2 HF

1 HNO3

1 CH3COOH

(b) I 2 in CH3OH

ß) Fused N a 2 0 2

Conditions

1 min and

25 sec.

CH3OH is

saturated

with C l 2

then sample

is immersed

while the

gas is

bubbling

20 sec

20 sec

5 sec

5-30 sec

350-9CO°C

Remarks

The two times differentiate

A and B surfaces,

respectively

(A is all Al atoms,

B is all Sb atoms)

Polishing

Polishing

Polishing

Shallow pits on A surface

Polishing

Etch pits on the A surface

Chemical polishing - pits on

A surface

Polishing

Develops etch figures

Measurement Techniques 109

After etching the samples must be thoroughly washed in a suitable solvent. Distilled iso-propyl alcohol has been found to be suitable after methanol-based etches.

The freshly etched sample is then mounted into a cryostat since low-tempera-ture photoluminescence measurements give much more detailed information about the impurities present in the sample. This is because the line width narrows and the intensity increases as the temperature is lowered. Often, for quick surveys of LED material quality, the simple liquid nitrogen dewar shown in Fig. 7.3 'will be quite sufficient. Rubber solution is used to mount the sample on the copper block since this causes no strain on the sample when it is cooled.

LIQUID NITROGEN

SAMPLE

Fig. 7.3 Simple liquid nitrogen photoluminescence dewar. ID is the incident high-energy exciting light intensity and IpL is the lower-energy photoluminescence intensity.

For detailed studies of the luminescence the samples are mounted in a helium cryostat. One of the most versatile is the continuous-flow helium cryostat (Ref. 3) that is shown in Fig. 7.4. It can be used from 6 K up to room temperature. Rubber solution is used to mount the sample on to a metal frame which is attached to a thin-walled cupro-nickel tube. With a 1-inch metal frame three samples can be run at one time provided that they are all mounted on the same side. If two samples are mounted opposite each other on both sides of the frame internal reflections from the glass cause both samples to be excited so a combination of the two photoluminescence spectra will result.


^-—CONNECTIONS TO THERMOCOUPLES AND GAS HEATER

SAMPLE THERMOCOUPLE

HELIUM REFERENCE THERMOCOUPLE

SAMPLE THERMOCOUPLE

CONNECTION TO IMMERSION HEATER

Fig. 7.4 "Continuous flow" helium cryostat. Maeda, (3).)

(After

Cooling of the samples in the immersion cryostat is accomplished by boiling off helium gas with an immersion heater. This heater is connected to a variac which is adjusted to control the boiling rate. Course control with the variac works quite well for temperature in the range 6 K to 40 K but above this a finer temperature control is required and this can be achieved in either of two ways. Firstly, some of the helium is allowed to escape from the top of the helium dewar through a valve which is connected to the helium recovery line so that the rate of helium flow over the samples is appreciably slowed down and they will be able to rise in temperature. The only snag is that it takes about 30 minutes to set up each temperature that is required for each measurement and the temperature control is only accurate to about ±1°C. The second method takes much longer to set up because a temperature controller is used to control the current through a heater that is wound just below the sample (see Fig. 7.4). Variations in temperature can now be reduced to fractions of a degree and the time between measurements is now only a few minutes.

The temperature of the samples is obtained by using a gold-chromel thermo-couple mounted just behind the samples on the metal frame and a reference junction immersed in liquid helium at the bottom of the helium dewar. The reference thermocouple is threaded through a thin walled cupro-nickel tube of thin diameter which is welded onto the bottom of the sample frame. For very accurate temperature measurements one sample only should be put on the sample frame and the thermocouple junction mounted on the side of the sample. In this way the exact sample temperature will be measured provided that the thermocouple is calibrated with a precalibrated gallium arsenide diode thermometer just before the luminescence measurements are begun.


To prevent condensation on the dewar it is silvered over the whole of the inside with the exception of a window region which coincides with the position of the sample frame. Changing of the samples is relatively easy and as many as twelve samples can be run at helium temperatures in 1 day with a helium consumption of only 2 to 3 litres.

Grating spectrometers are usually used for helium temperature measurements because high resolution can be obtained with them. For example, with a 1 metre grating spectrometer a resolution of 0.8 nm/mm of slit width is achievable.

Regular calibration of the spectrometer whether it is of the prism or grating type is essential. This calibration can be carried out either with calibrated interference filters or with a low pressure Hg or Cs gas-discharge lamp.

Detectors have always been a problem in the infra-red region of the spectrum but the recent development of infra-red sensitive photomultipliers, such as those that use lnAs^_xPx cathodes, has considerably improved this situation. Cooling with gas from a liquid-nitrogen source significantly improves the signal-to-noise ratio of photomultipliers. The use of slit-shaped cathode-type photomultiplier further improves the signal-to-noise ratio. Although photomultipliers are the most sensitive type of detector their wavelength range is still limited to 200 to 1000 nm. Hence other detectors must be used for the infra-red studies beyond this range. Some of the possible detectors that can be used are listed with the photomultipliers in Table 7.3.

Table 7.3 Detectors for Luminescence

Type

SI S5 Sll S20

Range

300 to 200 to 330 to 300 to

(a]

(nm)

1100 650 700 800

ι Photomultipliers

Peak

350

response (nm)

and 800 330 450 420

Temperature (K)

300 to 300 300 300

80

(b) Semiconducting Solid-state Detectors

Material

Si PbS InAs InSb Ge:Hg Ge:Cu HgCdTe

Type

PV PC PV PC PC PC PC

Material

Range

340 500

1000 500

2000 2000 3000

to to to to to to to

(nm)

1100 3000 4000 5600 13000 25000 15000

(c) Pyroelectr:

Peak response (nm)

900 2000 3600 5300

lOOOO to 15000

8000 to

LC Detector

Range (nm)

11000

15000

Temperature (K)

300 300 300 77

<35 4.2 77

Temperature (K)

Triglycine sulphate 2000 to 25000 300


7.1.2 Microphotoluminescence

Microphotoluminescence is the technique of studying photoluminescence with a microscope. Figure 7.5 shows the method that is used. (Ref. 4) A helium-neon laser light source is focused on to the sample surface so that the reflected beam misses the microscope and the luminescence from the illuminated spot on the sample enters the microscope. The laser light spot on the sample surface is about 20 microns and the energy of the laser light must be greater than or equal to the band-gap energy of the semiconductor sample or lumines-cence will not be observed. For studies of GaAs^_xPx (Ref. 4) this technique proved useful in the study of luminescence in the region of dislocations. The resolution is limited, even though magnifications of the microscope of up to 400 have been used, because the poor photoluminescence efficiency of most LED materials at room temperature means that large light spot sizes have to be used.

LUMINESCENCE ENTERS THE MICROSCOPE

Fig. 7.5 Microphotoluminescence equipment. (After Stringfellow and Greene (4).)

The technique of cathodoluminescence which will shortly be described has a much better resolution and is better suited to the study of dislocations and the junction region of an LED.

7.1.3 Lifetime and Time-resolved Photoluminescence

For lifetime and time-resolved photoluminescence studies of photoluminescence a pulsed lamp or laser is used and the spectrum or the decay at a wavelength corresponding to the maximum emission of one of the emission lines is studied with an oscilloscope. (Ref. 5) The pulsed nitrogen laser is suitable for most materials because it emits a 7 nsec pulse with a peak pulse output of about 150 kW at 337.1 nm.


7.2 CATHODOLUMINESCENCE

7.2.1 Introduction

Cathodoluminescence or the excitation of luminescence with electron beams is a powerful tool for the study of LEDs. A resolution of 1 micron or less can be achieved as opposed to 20 microns with microphotoluminescence. Cathodo-luminescence is ideally suited to the study of the LED not only because of this high resolution but also because in most cases the beam can readily be scanned over the junction. Furthermore, equipment like the scanning electron microscope can also be used for looking at the current profile in the junction and with a simple modification both the diffusion length and the lifetime of minority carriers can be measured.

The shape or energy peak of the cathodoluminescence spectrum can with care be used to measure the carrier concentration non-destructively in the same way as photoluminescence. This means that the carrier concentration profile in the region of the junction of an LED can be studied.

Most of what is said in the next chapter about the photoluminescence associated with impurities also applies to cathodoluminescence so this will not be repeated here. Suffice it to say that the two techniques are often used to complement each other and confirm a particular luminescence mechanism (Refs. 6,7).

The inherent electron beam energy is greater than the energy of photo-luminescence-excitation sources so that cathodoluminescence can often be used to study room-temperature luminescence of materials that show no photo-luminescence unless they are cooled. This is particularly useful for LED studies since the cathodoluminescence and diode properties can be directly compared without having to extrapolate the luminescence measurements from lower temperature.

This same beam power, however, has three disadvantages associated with it:

(a) the sample temperature can be raised by many tens of degrees at high beam currents - this causes large shifts in the emission lines;

(b) the penetration depth is so large that epitaxial materials which have micron-thick layers cannot be studied at high beam voltages and it is generally true to say that front surface photoluminescence should only be used in this case;

(c) for large penetration depths in heavily doped n-doped semiconductors like gallium arsenide and those with smaller energy gaps the absorption of the light emitted within the material is appreciable and causes distor-tion of the cathodoluminescence spectrum.

7.2.2 The Basic Equipment

In a recent review, T!uir and Grant (Ref. 8) have described a range of cathodo-luminescence techniques that have been used with varying degrees of success. (Refs. 9-12). Here only the type of equipment that is used in a scanning electron microscope (SEM) or electron microprobe will be described since this is normally used for LED junction evaluation. Figure 7.6 shows the set up


used by Casey and Kaiser. (Ref. 13). The total cathodoluminescence can be displayed on a cathode-ray tube as the electron beam is scanned in the X or the Y direction or the cathodoluminescence can be recorded as a function of wavelength by taking the light pipe to the entrance slit of a monochromator and using the same equipment as was shown in Fig. 7.1 for detecting photo-luminescence.

ELECTRON BEAM

CATHODE RAY TUBE

BAMPLE HOLDER

FIBER OPTICS LIGHT PIPE

MONOCHROMATOR] P H O T O -

MULTIPLIER PRE - AMP

P A R .

h

Fig. 7.6 Cathodoluminescence observed in an electron microscope. (After Casey and Kaiser (13).)

A typical cathodoluminescence spectrum of n-type GaAs obtained with the equipment shown in Fig. 7.6 is shown in Fig. 7.7 (Ref. 13). The beam current was 0.3 yA and the acceleration voltage was 40 V. The half width of near-band gap emission peak is shown together with the estimated dashed curve for a deep level line.


too

10

1 ' I 1 I

h

L

l·-L

1 i i i i

1 1 1 1 1 1 1 1

1 1 1 ! 1 1 l\ 1

M i l l

\ ^ _ D E E P

\ \ \

[ 1 1 Λ 1 1 1 1 1

/ + \ / PEAK \ f ENERGY \ HALF-WIDTHX

H I -RESOLUTION

LEVEL

1 1 1 1 1 1 l l 1

τη

-|

A

j

J

j - J I.20 1.30 I.40

ENERGY hM(«V) I.50

Fig. 7.7 Typical n-type GaAs cathodoluminescence spectrum at 300 K observed with the equipment shown in Fig. 7.6. (After Casey and Kaiser (13).)

Provided that the beam energy is small and the concentration of one carrier is much larger than the other, absorption effects due to the penetration of the sample by the beam are relatively small. This is clearly shown in Fig. 7.8 (Ref. 13) where two cathodoluminescence spectra from an n-type sample with a carrier concentration of 3.2 x 101 8 electrons/cm3 are compared. The 40-kV curve shows how the self-absorption causes a very slight narrowing of the line observed with the 20-kV line.


IO 9 8

7

i/) Z 4 UJ

Z

LU > I-

2 2 LÜ

cr

i I I I I I I I I M I I I | I I 1 I

I I ' I ' ' ' ' i ' ' ' ' » ■ ' ■ i I.40 I.50

ENERGY h»(eV)

Fig. 7.8 Absorption effects on the cathodoluminescence caused by increase in electron beam penetration as the beam energy increases from 20 kV to 40 kV. (After Casey and Kaiser (13).)

The penetration depth, d, is related to the energy Ep of the primary electron beam by the approximate equation: (Ref. 14)

aE 3Λ

(7.1)

where a is a constant; d varies from 0.4 to 7 ym as the beam energy increases from 5 to 25 keV. When a diode is studied in an SEM the scanning current mode is used to locate the junction. Figure 7.9 shows the circuit used in this mode to collect the current induced in the diode by the electron beam. (Ref. 15). As expected the current reaches a maximum at the exact position of the p-n junction. (Ref. 16).


ELECTRON! BEAM

/ /

P n

//

V

SCANNING DIRECTION

Lfcs. SPECIMEN T >CURRENT y f AMPLIFIER _J

Fig. 7.9 Scanning current mode in an SEM. (After Calverley and Wight, (15).)

7.2.3 Measurement of the Carrier Concentration

With careful calibration using electrical Hall measurements on a series of samples the cathodoluminescence spectra can be used to calculate the carrier concentration at room temperature or below. Calculation of the carrier concentration in uncompensated samples where one type of impurity dominates is far simpler since the Hall measurements used in the initial calibration need only be done at the temperature of the cathodoluminescence measurements. For compensated sample in which the donor and acceptor concentrations are comparable the Hall measurements have to be made at both 77 K and 300 K so that the degree of compensation can be estimated.

Both the peak energy and the half-width of the near-band edge emission can be used for calculating the carrier concentration in n- or p-type material. Figure 7.10 shows a plot of the peak energy against electron concentration (Ref. 13) for n-type GaAs with a cathodoluminescence spectrum similar to that shown in Fig. 7.8 being obtained with a beam accelerating voltage of 40 keV. The shift to higher energies of the peak begins as the concentration is increased to 5 x 101 7 or above. This energy shift is caused by the donor impurity band formation. This is confirmed by the increase in halt-width at the same concentration as shown in Fig. 7.11 for the same series of samples and taken from the same spectra as the results shown in Fig. 7.10. It is apparent from Fig. 7.11 that the scatter in points is very small and the change in half-width over the concentration range 5 x 101 7 to 6 x lO1^ electrons per cm-* is almost a factor of 3. The peak energy change over the same concentration range is only a few percent and this is why there is more scatter in the points in Fig. 7.10. It is therefore concluded that the half-width of the cathodoluminescence line should always be used in preference to the peak energy for measuring the concentration. This applies to both p- and n-type LED semiconductors. (Ref. 17).

117


ISO

l.48h

l.46l·

l.44h

Ï 1.42

I.40 lO1 17

i 1 1 1 — i — i — » i i

- *—*-

_L· -i-J_

- r 1 1—i—i—ι—Γτ

4 6 8 10 18 Jl I l _ J ' ' ■ '

4 6 8 lO1' ELECTRON CONCENTRATION n 0 ( cm3)

Fig. 7.10 Peak energy of the cathodoluminescence spectrum of n-type GaAs at 300 K as a function of the electron concentration. (After Casey and Kaiser (13).)


O . I 4 | ' 1 — 1 1 1—i i i i | r 1 1 1—I I I I

O.I2

O.IO

119

> 0 8

ï °6I I

Li. - J

x .0.41

.02

J L J_ lO17 2 4 6 8 ID1 8 2 4

ELECTRON CONCENTRATION n 0 ( c m ' 3 ) 8 lO1'

Fig. 7.11 Half-width of the cathodoluminescence spectrum of n-type GaAs at 300 K as a function of the electron concentration. (After Casey and Kaiser (13).)

Since in an LED the carrier concentrations of the n and the p side nearly always fall within the range where the half-width is changing cathodolumines-cence can be used to study the carrier concentration of both sides of the junction. Since it is a non-destructive high resolution technique it is more useful than some of the other methods such as the electrochemical, (Ref. 18) Reststrahl-band minima, (Ref. 19) microwave (Ref. 20) and resistivity techniques.

7.2.4 Relating Cathodoluminescence Efficiency to LED Efficiency

Wight et al. (Ref. 21) have correlated cathodoluminescence efficiency with LED efficiency for green-emitting nitrogen-doped GaP devices. The cathodo-luminescence efficiency was estimated by comparison with a reference sample which was positioned next to the diode under study on the specimen stage. A 40-kV beam of electrons with a beam current of 2.0 microamps and a 50 ym diameter of the focused scanning spot was used for all the efficiency measurements. The spot size was measured by moving the spot across the edge of a Faraday cup so that the incident beam current density could be estimated. The ratio of the peak intensity of the green emission from the reference sample to diode under study was taken as the relative cathodoluminescence efficiency. When the green-emission spectra differed in width the total


integrated intensity of the green light was measured and the ratio of this intensity to the integrated intensity of the reference sample was then used to correct the measured peak intensity.

These relative cathodoluminescence efficiency measurements were made on bevelled diodes with a Ζη,Ν-doped layer on a S-doped (2 x 1 0 " cm ■*) epitaxial substrate. Room-temperature cathodoluminescence efficiency measurements were made at the centre of the p-layer and they were found to show a marked dependence on the diode efficiency as Fig. 7.12 bears out. (Ref. 21) Figure 7.12 shows a plot of the diode and cathodoluminescence (CL) efficiency against the concentration of the ammonia used during the vapour growth of the epitaxial layer. The error bars on the diode efficiency measurements are the standard deviation for six diodes that were assessed for a specific ammonia concentration. Both types of efficiency measurements show the same trends. As the ammonia concentration increases the efficiency increases by almost ten times and then "levels-off" at this value for higher concentrations.

\o~'

■D O Q

10-

H io

c to

1.0

0.04 0.08 0.12 0.16

NH3 gas concentration (%)

Fig. 7.12 Diode efficiency and cathodoluminescence (CL) efficiency in the Ζη,Ν-doped p-layer of a GaP diode as a function of the NH3 gas concentration or the nitrogen concentration in the p-region. (After Wight et al. (21).)

The two efficiencies can be related if an abrupt junction model is assumed. The diode quantum efficiency, η, is given by

= K (Γ R e p

Γ, R ) h n (7.2)

where Rp and Έ^ are the relative bulk cathodoluminescence efficiencies in p-and n-type material, and K is a constant, and Γθ and Γη are the electron and hole injection efficiencies.


This equation is simplified for red GaP diodes and for green GaP diodes where the p-type side of the junction is doped with nitrogen. In these cases Rp » Rn now holds, (Ref. 22) and,

η = ΚΓ R . e p (7.3)

Young and Wight (Ref. 22) made a series of efficiency measurements on green diodes in which the nitrogen- and zinc-doping level on the p-side of the junction were kept constant as the sulphur-doping level was changed from diode to diode. Using equation (7.2) the electron injection efficiency was estimated and compared to the injection efficiency calculated from diffusion length data. This comparison is shown in Fig. 7.13. (Ref. 22) The good correlation between the two methods of estimating Te confirms the assumptions made in the cathodoluminescence efficiency measurements and also confirms that bulk diffusion length values can be used to measure diode performance. The full and the dashed lines in the figure represent the diffusion length calculations for two different constant acceptor concentrations. The square points are those calculated from equation (7.2) from the efficiency measure-ments and the solid circles are electron efficiency measurements on red lamps. The reasons for the fall off in injection efficiency with sulphur doping will be discussed more fully in Chapter 8 when carrier concentration quenching is considered.

10 r

re

0.01

N D - N A (cm-3)

Fig. 7.13 Variation of electron injection efficiency with net donor concentration in GaP lamps.

red lamps, N A - N D = 4 x l O1 8 cm ,

green lamps, N A - N D = 5 x 1 01 7 cm"3.

The full and dashed curves represent the diffusion-length calculations for an acceptor concentration of 5 x 1 0 1 7 and 4 x 1 0 1 cm - 3 , respectively. (After Young and Wight (22) .)


7.2.5 Lifetime and Diffusion Length of the Minority Carriers

The full theory of relating lifetime and diffusion length to cathodolumines-cence intensity has been dealt with elsehwere and will not be repeated here. (Refs. 23-25) The experimental results are fitted to the theoretical curves in order to calculate the diffusion length or lifetime.

With a much more direct technique, Hunter et al. (Ref. 26) have shown that the minority carrier lifetime, τ, and diffusion length, L, on both the p- and the n-side of a p-n junction like an LED can be measured by monitoring the current only. The diffusion length is measured directly by the SEM without any modification to the instrument in the manner shown in Fig. 7.14. Outside the depletion region of the junction either the p or the n side the current, I, generated by the beam is given by:

I exp (-x/L) (7.4)

where I0 is a constant.

ELECTRON BEAM

Fig. 7.14 Measurement of diffusion length in an LED with an SEM. (After Hunter et al. (26).)

The electron beam can be swept with the "Y" sweep of the SEM and the current can be displayed directly on the video screen (see Fig. 7.6). A plot of log I against x gives a straight line with a slope corresponding to L when x is in the range 1 < x/L < 4. For small L, the junction field and the relatively large approximately spherical volume over which carriers are generated by the beam limit the resolution. However, by bevelling the diode (Ref. 27) L values as low as 0.1 ym can be measured and this resolution is sufficient for most LED applications.

In measuring the lifetime the SEM has to be modified by installing either an electrostatic or electromagnetic beam-deflection system. This deflects the


beam for a very short period of time over an aperature so that the sample effectively sees a pulsed beam as shown in Fig. 7.15. (Ref. 26) The beam is not scanned on this occasion but remains at a distance x from the junction where x now lies in the range 2 < x/L < 3. The junction is simply used to detect the current pulse, I, and I is related to the lifetime by the familiar time-decay expression:

I = IQ exp (-t/τ) (7.5)

Hence a graph of log I against the time t gives a straight line whose slope is τ. With the electrostatic switching technique lifetimes down to 50 nsec were measured and with the electromagnetic method 35 nsec was achieved. (Ref. 26) In the electrostatic case the beam is deflected by two parallel plates between the gun and the first SEM condenser lens by applying a voltage pulse of about 300 volts with a pulse generator. In the electromagnetic beam pulsing unit a 5-μΗ induction coil is used instead of the plates.

BEAM PULSING

_π_η_

PULSE GENERATOR

h WIDE BAND

AMPLIFIER

RECORDER

MOSCILLOSCOPE

Fig. 7.15 Measurement of lifetime in an LED with an SEM. (After Hunter et al. (26 ) . )

7.2.6 Dislocation Studies

It is well known that dislocations affect LED diode performance. They shorten the minority carrier diffusion length and increase the concentration of non-radiative defects and impurities.

Although the qualitative effects are well known, quantitative measurements are difficult because of the high resolution required. Although microphoto-luminescence does show up dislocations the resolution is so poor that very little information is gained. (Ref. 4) Cathodoluminescence on the other hand has proved to be very useful when it is combined with photomicrograph and SEM studies of preferentially etched surfaces. (Refs. 28,29) Dislocations are observed as dark spots or dark bands on the cathodoluminescence picture. This confirms that non-radiative traps are produced by dislocations.


The identification of the exact nature of these traps is still unestablished. There is little doubt that with photon correlation techniques (Ref. 30) applied to cathodoluminescence as a function of wavelength for small beam penetration and hence sub-micron resolution could begin to solve this mystery. This identification will be invaluable because steps can then be taken to improve the materials and diode fabrication still further so that higher efficiency LEDs can be produced.

7.3 CAPACITY MEASUREMENTS

Techniques for measuring the minority carrier diffusion length and lifetime and the majority carrier concentrations have already been mentioned above, but a new powerful tool for measurements of deep-level non-radiative traps has been developed from capacity measurements.

Photoconductivity measurements have been used extensively in the study of impurities in semiconductors. They have been particularly useful for the study of deep impurities. For example, oxygen (Ref. 31) and copper (Ref. 32) in GaP and manganese in ZnSe. (Ref. 33) In these materials that were dominantly doped with deep-level impurities the resistance was high and the materials were good photoconductors.

For low-resistance doping the photoconductivity is often smaller than the current noise and since this is the range of doping that is used for both the p and n side of the LED standard photoconductivity measurements cannot be used. An alternative is to study a rectifying junction by forming a Schottky barrier or by measuring the photocapacitance or photovoltage of the LED junction itself. Since free carriers are depleted from the junction region in the Schottky barrier or the LED photo-effects dominate and information can be gained on the impurities present at the junction.

One important advantage of this type of measurement is that information can be gained on both radiative and non-radiative centres. In the case of photo-luminescence and cathodoluminescence only radiative recombination can be analysed. On the other hand, Schottky barrier measurements are often difficult to reproduce because of the importance of surface treatment and surface contamination. Surface or interface states can play a significant role in the variation of the capacitance with wavelength over certain ranges of bias voltage and the understanding of these interface states is still in its infancy in LED compounds although it is much more understood in the case of silicon.

However, for the study of deep-level impurities, photocapacitance and photo-voltaic measurements have been particularly useful in determining the trap type, carrier density in the trap and the activation energy. Only a brief account of photocapacitance will be given as such a full description of these measurements is outside the scope of this book and the interested reader is referred to several other relevant reviews on this topic. (Refs. 34-36)

A basic understanding of how measurement of capacity can lead to the deter-mination of trap density is obtained by inspection of Poissons equation at the p-n junction. In Chapter 5 we have seen that the relationship between voltage, field and charge density is given by

(5.39)


Let us consider an abrupt p-n junction between two uniformly doped p and n regions in which Na » Nd. Then, from equation (5.38) the penetration of the depletion region into the n region xn is given by xn = (Na/Na)xp, so that xn = W. If we assume that in the n region the density of ionized impurities is N+ (this would normally be equal to Nd) then the total voltage V0 across the junction is obtained by substituting p(x) = qN+ into equation (5.39), and integrating twice,

i.e. V = (qx 2 Ν+)/2ε o n

= (qW2N+)/2e . (7.6)

The differential capacity C = dQ/dV where

C2 = (qeN+)/2V (7.7)

where VQ is the sum of the built in voltage V^, and the reverse bias V. Thus, at constant bias the capacity depends upon N+. If we can produce an incremen-tal change in N+ due to charge trapping then this results in an incremental change in C. By differentiation

AC/C α ΔΝ+/2Ν+ (7.8)

There are two standard techniques by which changes in N+ are induced, i.e. by carrier injection (both minority and majority carrier) and by photogeneration of carriers. It should be noted that if electrons are trapped in the n-type depletion region then N+ will decrease producing a negative value of AC. Conversely minority carrier trapping, i.e. holes, will produce a positive AC. Temperature is another important parameter which changes the availability of intrinsic carriers.

7.3.1 Optical Injection

A recent method using optical injection is the so-called dual-source photo-capacitance. Figure 7.16 shows a typical experimental arrangement for GaAs measurements. (Ref. 37) The prime light source which has an energy just below the band gap of the material is used to form a quasi-equilibrium increase in the number of holes in trap levels below the Fermi level and electrons in higher energy levels. On varying the energy of the probe light source either with a monochromator or a set of interference filters the quasi-equilibrium trap densities are changed by pumping carriers to the nearest band edge. Once these carriers go into the depletion region they are accelerated by the built-in field and move out of the semiconductor so that the space charge distri-bution is altered. This results in a change of capacitance and the sign of the change depends on whether it was a hole or an electron trap that was disturbed.


© PROBE

v A A - MONOCHROMATOR

LIQUID N2

Fig. 7.16 Two-source photocapacitance apparatus. The GaAs sample is irradiated through the substrate with light from the prime source with an energy of 1.3 eV and simultaneously with light from the probe source with energy varying from 0.3 to 1.6 eV. (After White et al. (37).)

If there are only one or two impurity or native defect levels present in any quantity in the material then a plot of AC, the change in capacitance with energy hw (or hv), shows a distinct peak for positive or valley for negative changes. The position of the peak or valley can be used to identify the trap depth.

In most cases, however, a large number of traps are present and the capaci-tance signal must be differentiated with time as the probe light energy is slowly varied with the monochromator drive set on a relatively slow speed of the order of minutes. Figure 7.17 shows a differential capacitance spectrum for GaAs cooled to 77 K. (Ref. 37) Four different types of spectrum are shown and the identification of the chromium level F is clearly shown by comparison of the chromium-doped sample spectrum with the others. The interpretation of the other peaks and valleys in this spectrum is discussed elsewhere. (Ref. 37).


Fig. 7.17 Differential photocapacitance spectra. (After White et al. (37).)

7.3.2 Electrical Injection

A technique which uses carrier injection in a reverse biased diode to obtain a measure of the trap concentrations uses the experimental apparatus shown in Fig. 7.18. (Ref. 34). In this method a p+n junction is biased to a reverse voltage V. If a positive voltage pulse is then superimposed to reduce the reverse bias, a new quasi-equilibrium condition is rapidly established. This state corresponds to a narrow depletion width and higher capacity. We then observe how the capacity returns to its original value after the pulse voltage has returned to zero. Lang (Ref. 34) has considered two cases: (i) where the pulse voltage is insufficient to forward bias the junction, the transient capacity is negative as shown in Fig. 7.19. The magnitude of the incremental capacity will depend upon the number of trapped majority carriers. (ii) When the pulse voltage is sufficient to forward bias the diode, the incremental capacity is positive and this corresponds to minority carrier trapping. By analysis of these curves at different temperatures Lang (Ref. 34) has


developed a technique for observing a wide variety of traps in semiconductors.

2 0 MHz r f

OSCILLATOR

VARIABLE

ATTN

PHASE SHIFTER

SDI IOB PULSE

GENERATOR

fs~AMPLË]

<h 1 _ -

PHASE SHIFTER

AND

ATTENUATOR

SDIIOB PULSE

GENERATOR

HEWLETT PACKARD

r f MIXER

±. 200 pf

PAR 115

DC 70 MHz

\ AMPLIFIER

PAR 113

PRE AMP

X - Y

RECORDER

PAR I6O BOXCAR

AVERAGER SCOPE

Fig. 7.18 Block diagram of fast capacitance transient apparatus. (After Lang (34).)


Diode bias ÏT

I I

Diode JLA capacitance

Diode current

Diode capacitance

exp( -e 2 t ) Majority

} carrier trap

7.4

Fig. 7.19 Typical time dependence involved in pulsed bias capacitance transients for majority- and minority-carrier traps. The upper half is a majority-carrier pulse sequence while the lower half is an injection-pulse sequence. (After Lang (34).)

EVALUATION OF DIODE EMISSION

7.4.1 Introduction

For GaAs and other infra-red emitting LEDs evaluation is based upon the energy of emission, its intensity and direction, but to a much lesser extent upon the exact wavelength. This is generally sufficient since the silicon photodiodes with which they are normally used exhibit a response which is relatively insensitive to wavelengths above that corresponding to the bandgap. An exception to this is the anti-Stokes phosphor lamp which will be discussed in Chapter 9. The measurement of radiation energy which covers the range from ultra-violet to infra-red is known as radiometry. The ideal detector for such measurements is one which has a response independent of wavelength.

With the visible emitting LED, lamp assessment becomes much more difficult since we are no longer involved solely with physical measurements but must con-sider pyscho-physical response which varies to some extent with each individual observer. The basic unit of power which describes the luminous flux radiated from LEDs is the lumen. The response of the eye to visible radiation has been


"standardised" by the publication by the CIE of the spectral luminous efficiency function which peaks at 555 nm as shown in Fig. 5.12. At this wavelength 1 watt = 640 lumens. The ideal photometric detector has a spectral response which closely approximates to this curve.

Measurement of the lumen output from an LED is not sufficient to describe the sensation of colour. Colour is determined by the spectral composition of visual energy. It is important to appreciate, however, that there is no unique spectral composition which corresponds to a given colour, i.e. any specified colour can be produced by a range of spectral distributions. The CIE has defined the colour discrimination properties of the standard observer by three primaries X, Y, Z, such that Y is the luminance of the stimulus computed in accordance with the luminance efficiency function V(X).

7.4.2 Characterisation of Emission

An important measure of LED emission is the total flux output; radiant flux φθ measured in watts or luminous flux, φν, measured in lumens. Measurement of φ serves to determine the efficiency of conversion from electrical power input into radiation. Although the lumen output is the prime criterion (other than colour) which is used in assessing lamps used for normal lighting purposes, it is not sufficient for the applications for which LEDs are used, i.e. indicator lamps and displays.

The manner by which the human eye discerns emission is dependent not only on the total output but also on the size of the source. For the standard observer, the threshold distance at which the source begins to appear as a point corresponds to the source subtending an angle of 2 minutes at the eye of the observer. Thus for an LED chip, typically 0.038 cm. diameter, this corresponds to a distance of 61 cm. Therefore a meaningful measure of the emission from a LED indicator lamp is the flux per unit solid angle directed to the observer, i.e. lumens per steradian. This parameter is known as the luminous intensity, Iy, of the source. The encapsulation of an LED indicator lamp is usually designed to give the maximum intensity along the axial direction. The beam pattern of the lamp is defined by the variation of intensity in all directions. The photometric unit of intensity is the candela which equals 1 lumen per steradian in a specified direction.

For larger area sources, e.g. a diffuse LED indicator lamp or an LED alpha-numeric display, the emission no longer appears as a point source so that the appropriate measure of emission is the intensity per unit projected area of emitting surface. This is known as luminous sterance or surface luminance Ly and is measured in candelas per square metre (Ξ1 nit).

One further term which is useful in the assessment of light is the illuminance (or luminous incidence) E v which describes the luminous flux per unit area incident upon a surface. If the distance from the source to the surface is sufficient, then E v will vary as the inverse square law;

i.e. E = I /d2 (7.9) v v

where Iv is the luminous intensity of the source and d is the distance from source to surface. If Iv is measured in candelas and d in metres, then the illuminance is given in lumens per square metre. The terms and definitions used in this section are summarized in Table 7.4(a) and 7.4(b).


Table 7.4(a) Radiometrie Terms

131

Term and Definition Symbol Equation SI units

Radiant flux, rate of flow of energy, Qe

Radiant incidence (irradiance), radiant flux per unit area on to a surface

Radiant exitance (emittance), radiant flux per unit area from an emitting surface

Radiant intensity, radiant flux per unit solid angle from a remote source

Radiant sterance (radiance), radiant flux per unit solid angle per unit area of emitting surface an angle with respect to surface normal

d t dQe dA

dQe

àQe c3ûF~

W a t t s , W

Watts per sq. metre, Wm^

Watts per sq. metre, Wm

Watts per steradian, W sr"1

2£f i (cos0) 1 Watts per steradian per

d2Qe

dû)

(dA cos ΘΓ

sq. metre -1 -9 W sr m

Table 7.4(b) Photometric Terms

Term and Definition Symbol Equation SI units

Luminous flux, rate of flow of luminous energy Qv

Luminous efficacy of radiation, luminous flux φν from a source per unit radiant flux <|>e

Luminous incidence (illumination or illuminance), luminous flux per unit area onto a surface

Luminous excitancy, luminous flux per unit area from an emitting surface

Luminous intensity (candlepower), luminous flux per unit solid angle from a remote source

Luminous sterance (luminance or brightness), luminous flux per unit solid angle per unit area of emitting surface at angle Θ with respect to surface normal

d Q v

Φν/Φ£

M,7 d(j>v

l u m e n , i,m

lumens per watt, im W"1

άφν dA

d<t>v

dA

d<f>v do)

—^-(cos dA

ά 2 φ ν (άΑ

ΘΓ

c o s

-1

ΘΓ

lumens p e r s q . m e t r e , 2,m m~2

or l u x , Zx

lumens per s q . m e t r e , £,m m""2

(Note l u x does n o t apply)

Candela , cd or lumens p e r s t e r a d i a n , Jim s r " 1

c a n d e l a s per s q . m e t r e , cd πΓ 2

or n i t , n t

do)


7.4.3 Flux Measurement

The basic instrument used for the determination of total flux is the integrat-ing photometer. Typically this consists of a spherical shell, usually 15-30 cm. diameter for LED lamps, the inner surface of which is painted with a non-selective uniformly diffusing paint. The lamp under test should be mounted close to the centre of the sphere and the screen should be as small as possible without allowing any rays to fall directly upon the detector window as shown in Fig. 7.20.

digital microammeter

photocurrent detector

diffusing window

Fig. 7.20 Integrating photometer.

The principle of operation is that if a source is placed inside such a sphere then the flux per unit area, E, incident on any part of the surface due to light reflected from the remainder of the sphere is the same and proportional to the total flux φ emitted by the source,

i.e. E (p<J>)/(l-p)4TTr (7.10)

where p is reflectance of the sphere wall and r is its radius. The reflec-tance p of the sphere wall is neither strictly constant nor perfectly diffusing and it is also wavelength dependent even for the best coating materials available (smoked magnesium oxide and barium sulphate). However, by applying suitable methods and corrections it is possible to obtain satisfac-tory results. Detectors most commonly used are silicon and selenium photo-voltaic cells and photomultiplers in conjunction with suitable filters to achieve either a flat response over the required wavelengths for radiometry or a response which closely resembles the spectral luminous efficiency curve for photometry purposes. As well as calibrating the response of the detector over


the required wavelength spectrum it should also be checked for linearity and stability of response.

For these reasons silicon photodiodes are gaining popularity as detectors. The relatively narrow spectral emission of the LED means that it is easy to measure the radiant flux relatively accurately, provided the quantum efficiency of the silicon photodiode is known over the wavelength range of interest, but the luminous flux is more difficult. This is particularly acute with the red-emitting diodes since the spectral sensitivity drops rapidly as the wavelength increases, e.g. the sensitivity is reduced by a factor of 10 for a wavelength change from 645 nm to 683 nm. For this reason it is common to construct and calibrate detector heads which are used solely for radiation longer than 620 nm. A fairly good approximation to spectral response in the 620-680 nm region can be obtained by a silicon PIN photodiode with a filter consisting of a Schott glass type BG-38 at a thickness of 5 mm.

Having obtained a photodiode with a suitable response the integrating sphere must be calibrated by using a lamp of known emission. This would normally be a calibrated low-wattage tungsten filament lamp operated at a known filament temperature. With all possible experimental precautions being taken the error of lumen measurement should be within the range 2-5%.

7.4.4 Intensity Measurement

The majority of LEDs are designed to give an intensity distribution which has cylindrical symmetry, i.e. it is dependent solely on the angle Θ between the axis and the line of sight. Consequently the measurements of intensity patterns can be divided into (i) measurement of axial intensity and (ii) measurement of relative intensity as a function of Θ.

(i) Axial intensity. A diagram of the apparatus required for the measure-ment of axial intensity is shown in Fig. 7.21(a). It consists of a light tight box typically 10-15 cm. long into which a hole is drilled at one end to locate the LED under test and a detector which is mounted on the facing wall on the axis of the LED. The inside of the box is painted matt black and has internal baffles to prevent off-axis light from reflecting from the walls into the photodetector. The distance between the detecting plane and the plane of the source, d, must be measured accurately. The LED intensity can be obtained by a detector calibrated to give illumination using the expression given in equation (7.9).


1 r

J L a) axial photometer

^photocell

digital microammeter

= ± i —

photocell

j ' - t o w geared P fcn motor

dc supply

b) distribution photometer

chart recorder

digital microammeterl

n

Fig. 7.21 Intensity measurement photometers.

(ii) Beam pattern. The beam pattern of intensity is obtained by rotating the LED indicator and monitoring the illumination falling upon a fixing detector, as shown schematically in Fig. 7.21(b). It is convenient to rotate the drive on which the diode is mounted at a constant speed and to use a pen recorder driven from a digital microammeter to plot the variation in photo-current. The beam pattern is normally translated into polar coordinates, as shown in Fig. 7.22 which compares a standard pattern for most LEDs (ii) (angle at half-intensity as 30-40° arc) with a highly collimated beam (i) (<10° half-angle) and a wide viewing angle course (iii) (90° half-angle).


i) highly collimated beam ii) normal beam pattern iii) wide angle emission 1 axial direction

Fig. 7.22 LED intensity polar distribution.

7.4.5 Colour Measurement

Following the recommendations of_the CIE, colour is evaluated by using the colour-matching functions χ(λ) y(X) and ζ(λ) which are shown in Fig. 7.23. The tristimulus coordinates Χ,Υ,Ζ of the spectral emission S(X) are then given by the equations:

X = / S(X) χ(λ) dX, o

Y = Γ S(X) y(X) dX, o

Z = /* s(X) 5(X) dX. o

(7.10)


4 00 500 600

Wavelength λ

700 nm

Fig. 7.23 The 1931 CIE colour-matching functions χ(λ) , y(X) and ζ(λ) .

The colour is specified by the chromaticity coordinates Χ,Υ,Ζ which are independent of the luminous output of the lamp, and given by

X X+Y+Z X+Y+Z and X+Y+Z

(7.11)

Thé chromaticity of a test lamp is uniquely defined by any two of the chroma-tricity coordinates, usually x and y. The 1931 CIE (x,y)-chromaticity diagram is shown in Fig. 7.24. (The periphery of the colour space in the locus of the spectral line emission and the purple line.) Included in this diagram are the regions which represent the "signal colours" of red, yellow, green and blue.


520

Fig. 7.24 1931 CIE (x,y) chromaticity diagram.

There are three different methods of making colour measurements, viz. by matching, by spectral distribution and by the tristimulus filter method. Depending upon the accuracy and speed requirement, all three techniques are commonly used. Matching methods are exclusively visual and since even the eyes of people with so-called normal colour vision differ from each other they are not particularly accurate or indeed rapid.

Figure 7.25 shows a schematic diagram of a spectroradiometer which measures the spectral distribution S(X). Measurements of power through the visible range is taken at equal intervals determined by the slit width of the monochromator. The width of the slit determines the spectral resolution, so that reducing the slit width reduces the amount of power and increases the number of measurements required. Calibration of the instrument is carried out by measuring a lamp of known spectral distribution, usually a tungsten filament lamp. The tristimulus coordinates of the light entering the mono-chromator are given by summation over the visible range, i.e.

X = Σ S(X) χ(λ) δλ,

Y = Σ S(X) y U ) δλ, (7.12)

Z = Σ S U ) ζ(λ) <5λ.


Having obtained x,y and z the chromaticity coordinates are readily evaluated. This technique is probably still regarded as the most accurate method of colour measurement.

condenser lens system

diffusing screen

Ψπ

s monochromator photo-

multiplier

light chopper

chopper supply

data logger

digitizer

synchromous amplif ier

correction

circuits

chart recorder

Fig. 7.25 Schematic diagram of a spectroradiometer.

A much quicker technique of colour measurement is the tristimulus method illustrated in Fig. 7.26. In essence this uses three detectors which have relative spectral sensitivities which correspond to the colour matching functions χ(λ), y(X) and ζ(λ). In practice very close approximations to these functions can be obtained by using multiple layers of coloured glass which are arranged in front of either selenium or silicon photocurrent detectors. The photocurrents generated Ix, Iy and I2 are proportional to the trimulus coordinates X, Y and Z. By using current-to voltage converters and a summing amplifier, the chromaticity values x and y can then be displayed on a ratiometric digital voltmeter. The simplicity of this technique is evident, but its accuracy is only as good as the ability of producing the correct filter characteristics.


inner walls painted w i th di f fusing whi te paint

d i f fus ing glass

screen.

f i l t e r + photodetector to give spectral response proportional to x (A), y (A) and 2 (A)

i ratio

me

tric d.v.m

. readin

g

Il M

+ Λ

_ 3

ratio

me

tric d.v.m

. rea

din

g

Fig. 7.26 Tristimulus colour-measurement apparatus.


REFERENCES

1. H. Barry Bebb and E. W. Williams, Semiconductors and Semimetals (R. K. Willardson and A. C. Beer, eds.), Vol. 8, p.182, Academic Press (1972).

2. H. C. Gatos and M. C. Lavine, Progress in Semiconductors, Vol. 9, p.36 (1965).

3. K. Maeda, J. Phys. Chem. Solids, 26, 595 (1965).

4. G. B. Stringfellow and P. E. Greene, J. Appl. Phys. 40, 502 (1969). Stringfellow and Greene achieved a resolution of 20 microns for room-temperature luminescence using a microscope and a laser. More recently a resolution of 5 microns has been achieved with a laser study of dislocations at helium temperatures. (W. Heinke, Institute of Physics Conference Series, 23, 380 (1975), and W. Heinke and H. J. Queisser, Phys. Rev. Letters, 33, 1082 (1974).)

5. R. Dingle, Phys. Rev. 184, 788 (1969).

6. E. W. Williams, A. M. White, A. Ashford, C. Hilsum, P. Porteous and D. R. Wight, J. Phys. C, 3_, L55 (1970).

7. R. Dittmann, H. Goersch and D. Hahn, Proc. Int. Conf. Luminescence, 2_, 1958 (1966).

8. M. D. Muir and P. R. Grant, Quantitative Scanning Electron Microscopy, 33, 287 (1974).

9. G. A. C. Jones, B. R. Nag and A. Gopinath, Proc. VIth Ann. SEM Symposium II TRI, Chicago, p.309 (1973).

10. E. M. Horl and E. Mugachl, Proc. Vth European Congr. Electron Microscopy, p.502 (1972).

11. R. F. Sippel, Proc. Int. Conf. Luminescence, 2_, 2079 (1966).

12. J. B. Steyn and D. B. Holt, Proc. SEM 73 Inst, of Physics, London (1973) (a diagram of their cathodoluminescence apparatus is shown in reference 8). See also J. B. Steyn, P. Giles and D. B. Holt, J. Microscopy, 107 (1976).

13. H. C. Casey, Jr., and R. H. Kaiser, J. Electrochem. Soc. 114, 149 (1967).

14. A. F. Makhov, Sov. Phys.-Solid State, 2_, 1934 (1960).

15. A. Calverley and D. R. Wight, Solid State Electronics, 13, 382 (1970).

16. See also W. N. Lin and D. B. Witting, J. Appl. Phys. 47, 4129 (1976).

17. D. A. Cusano, Solid State Communications, 2_, 353 (1964).

18. T. Ambridge and M. M. Faktor, Institute of Physics Conference Series, No. 24, p.320 (1975), and also Electron, p.16 in 11th March issue (1976).

Measurement Techniques 1 4 1

19. W. G. Spitzer and J. M. Whelan, Phys. Rev. 114, 59 (1959).

20. M. M. Sayed and C. R. Westgate, Rev. Scient. Instrum. 46, 1074 (1975).

21. D. R. Wight, J. C. H. Birbeck, J. W. A. Trussler and M. L. Young, J. Phys. D: Appl. Phys. 6_, 1622 (1973).

22. M. L. Young and D. R. Wight, J. Phys. D: Appl. Phys.. 7_, 1824 (1974).

23. D. B. Wittry and D. F. Kyser, J. Phys. Soc. Japan, 21, Supplement (Proc. Int. Conf. Phys. Semicond. Kyoto) p.312 (1966).

24. D. B. Wittry and D. F. Kyser, J. Appl. Phys. 38, 375 (1967).

25. D

. B

. Holt

, Quantitative Scanning Electron Microscopy

, 33

, 5 (1974)

.

26. D. R. Hunter, D. H. Paxman, M. Burgess and G. R. Booker, paper presented at an SEM meeting in July (1973). See also the papers: J. F. Bresse and D. Lafeuille, Proc. 25th Anniv. Meeting of Electron Microscopy and Analysis Group, University of Cambridge (Institute of Physics) p.220 (1971), and D. B. Wittry and D. F. Kyser, J. Appl. Phys. 36, 1387 (1965).

27. W. H. Hackett, J. Appl. Phys. 42, 3249 (1971).

28. D. B. Wittry and D. F. Kyser, J. Appl. Phys. 35, 2439 (1964).

29. H. C. Casey, J. Electrochem. Soc. 114, 153 (1967).

30. E. R. Pike, Physics Bull. 27, 109 (1976).

31. S. Braun and H. G. Grimmeiss, Solid State Commun. 12, 657 (1973).

32. H. G. Grimmeiss and G. Olofsson, J. Appl. Phys. 40, 2526 (1969).

33. S. Braun, H. G. Grimmeiss and J. W. Allen, Phys. Stat. Solid(a) 14, 527 (1972).

34. D. V. Lang, J. Appl. Phys. 45, 3014 (1974); also D. V. Lang, ibid., p.3023 (1974).

35. A. G. Milnes, Deep Impurities in Semiconductors, Wiley (1973).

36. C. T. Sah, L. Forbes, L. I. Rosier and A. F. Tasch, Jr., Solid State Electronics, 13, 759 (1970).

37. A. M. White, P. J. Dean and P. Porteous, J. Appl. Phys. 47, 3230 (1976) and J. Electron Materials, _5, 91 (1976). See also H. Lefevre and M. Schulz, Appl. Phys. 12, 45 (1977).

8 LUMINESCENCE OF COMMERCIAL LED MATERIALS

8.1 INTRODUCTION

Luminescence studies have played a key role in the development of high-efficiency LEDs. In the early materials research work luminescence was used to identify undesirable impurities and defects in the crystal lattice. It was also used to identify the recombination mechanisms occurring in an LED. In particular, as was mentioned in Chapter 1, the fact that the majority of the diode emission occurred on the p-side of the junction was verified by comparison of the diode spectrum with the photoluminescence from the p-type region.

This chapter will be restricted to the three most important commercial LED materials at the present time - GaAs, GaP and GaAsi-xPx. Other LED materials and some of their luminescence properties will be briefly outlined in the next chapter.

Only impurity luminescence will be discussed since all LED materials must be doped with either donors or acceptors, or both donors and acceptors, or iso-electronic traps like nitrogen in gallium phosphide. Impurities which are important in the LEDs will be stressed with just a summary table being included of the other impurities.

8.2 IMPURITIES AND VACANCY-IMPURITY COMPLEXES IN GALLIUM ARSENIDE

8.2.1 Simple Substitutional Impurities

Simple substitutional impurities have already been introduced in Section 2.6.1 when doping of III-V semiconductors was described. In direct-gap LED materials like GaAs these impurities are the only impurities that are used when a light-emitting p-n junction is produced.

The simple substitutional impurities that are used are all of the type that contribute only one electron or hole to the binding. Only S electrons are contributed to the binding so these centres are often called hydrogen-like. Two of these so-called hydrogen-like impurity centres are shown in the crystal lattice of GaAs in Fig. 8.1. In (a) Zn acceptors and (b) Se donor impurities are shown. These simple donor and acceptor centres can be either neutral or ionised and this is shown diagrammatically in Fig. 8.2. The full circle represents the nucleus and the charge on the nucleus is positive for the donor and negative for the acceptor. When an impurity centre is ionised the electron or hole moves into the conduction or valence band respectively. This is illustrated in Fig. 8.3. ΔΕ is the activation energy or ionisation energy since this is the energy required to ionise the electron.

142

Luminescence of Commercial LED Materials 143

[m] (As

(a) ZINC

(b) SELENIUM

Fig. 8.1 Simple substitutional hydrogen-like impurity centres: (a) zinc and (b) selenium in the GaAs crystal lattice.

(a)

NEUTRAL DONOR

A

NEUTRAL ACCEPTOR

(c)

(b)

IONISED DONOR

IONISED ACCEPTOR

(d)

Fig . 8.2 Neutral and ion ised donors. The f u l l c i r c l e r ep re sen t s the nucleus and the charge on the nucleus i s shown in s ide t h i s c i r c l e . The e l ec t ron or hole bound to the nucleus i s shown ou t s ide the c i r c l e .


ΔΕ3" CB

(a) (b) NEUTRAL IONIZED

DONOR ACCEPTOR

NEUTRAL IONIZED (c) (d)

ΔΕ ,

VB

Fig. 8.3 Ionization of a donor and an acceptor by the movement of electrons into the conduction band (CB) and holes into the valence band (VB) respectively.

The activation energy can be calculated theoretically from the hydrogen model equation (Ref. 1) as was previously shown in equation (2.10):

^hydrogen = " . 6 (mVnO/ε 2 eV {ΒΛ)

(m is the effective mass of either the electron or the hole, and ΔΕ hydrogen = Ed = E a in equation (2.10).)

The donor or acceptor can be ionised in two main ways:

(1) thermally;

(2) with external radiation.

This means that the ionisation energy as well as being calculated from luminescence studies can also be determined by: (1) studying the change in the band to acceptor recombination intensity as a function of temperature as the donor or acceptor become thermally ionised, or (2) by doing absorption experiments at helium temperatures in the far infra-red to ionise the centres with electromagnetic radiation of the same energy as the binding or activation energy ΔΕ.

Table 8.1 lists the ionisation energy of eight substitutional acceptors in GaAs that have been determined from luminescence data. In the two columns the results from simple band-to-acceptor, BA, luminescence studies at about 20 K (Ref. 2) are compared to those from more sophisticated studies of bound exciton luminescence at 1.5 K. (Ref. 3).


Table 8.1 Ionisation Energies of Some Substitutional Acceptors in GaAs

Impurity Optical activation energy

BA (2)

meV

Exciton data(3)

Beryllium, Be

Magnesium, Mg Ga

Ga Zinc, Zn

Ga Cadmium, Cd, Ga

28

28

30

32.5

28.0

28.4

30.7

34.7

Carbon, C As

Silicon, Si

Germanium, Ge As Tin, Sn

As

18 to 23(a)

28

36 to 40(a)

171

26.0

34.5

40.4

(a) Range of values obtained by different authors.

Substitutional donor activation energies are shown in Table 8.2 (Ref. 2). The photoluminescence values are determined from exciton-neutral donor complexes (Ref. 2) and the absorption and photoconductivity measurements were all made in the far infra-red. The hydrogenic binding energy calculated from equation (8.1) is 5.4 meV (Ref. 4) and quite close to all the values shown in Table 8.2. (The effective mass for electrons at Γ is 0.07.)

Table 8.2 Ionisation Energies of Substitutional Donor Centres in GaAs

Impurity

Optical activation energy meV, ΔΕ

Absorption - „, _,_ . , Z_ _ ^. . Photoluminescence photoconductivity

Sulphur, S

Selenium, Se As

6.10

5.89 6.10

Silicon, Si_ Ga

Germanium, Ge As

5.81

6.08

6.80


8.2.2 Band-to-acceptor Recombination

In GaAs, BA recombination is easy to observe in lightly doped p-type material with an acceptor doping level below 3 x 1017/cm3 over the temperature range 10 K to about 130 K. Below 10 K donor-to-acceptor and exciton complex recombination dominate (Ref. 5) and above band-to-band, BB, recombination is the dominating process. As Fig. 8.4 illustrates, at these low doping levels there is no impurity banding and the ionisation or binding energy ΔΕ^ is therefore unambiguously defined from the peak energy of the photoluminescence line, hVBA· On the right hand side of this diagram the density of states distribution for electrons in the conduction band and holes in the acceptor level are shown.

Fig. 8.4 (a) Band-to-acceptor (BA) recombination and (b) the density (N) of electron and hole states as a function of energy in a lightly p-type-doped direct-gap semiconductors like GaAs.

Figure 8.5 shows how the luminescence spectrum of Cd-doped p-type GaAs with a doping level of 3 x lO1^ holes/cm3 varies as the temperature is lowered from 300 K to 20 K. At room temperature (300 K) only one weak line is observed with a half-width of about 2kT. At this temperature near-band edge or BB recombination dominates as is borne out by the large half-width and the fact that the emission peaks very close to the band-gap energy of 1.435 eV at 300 K. (Ref. 6). The dominance of BB recombination can also be understood in terms of the Fermi level position as is shown in Fig. 8.6. The small signal approximation has been assumed. This approximation means that only the true Fermi level due to the carrier concentration of the impurities in the GaAs needs to be considered when working out which transitions are occurring, and the quasi-Fermi levels in the conduction and valence bands due to the photo-


excited carriers in these bands can be ignored. (For high-level excitation with a laser the small signal approximation does not apply and the quasi-Fermi levels must be considered.)

1.44 1.45 1.46 1.48 1.49 I.50 1.51 1.52

77°K

300°K

I I 1.39 1.41 1.43 1.45 1.47 1.49

ENERGY (eV)

1.51 1.53

Fig. 8.5 Photoluminescence at three temperatures for a lightly cadmium-doped GaAs sample (3.5 x Ι Ο ^ holes/cm^ at 300 K ) . (After Williams and Chapman (7) 1967.)


CB

'fiV BB-<-x^xJ

VB

• · _

'nv BB"

ΔΕΑ

. * v BA

-Θ-

Λ\> BA

f CADMIUM Θ — ACCEPTOR

Ec

300°K 80°K

p - TYPE Go As ( SMALL SIGNAL APPROXIMATION )

Fig. 8.6 Proposed recombination mechanisms for the spectra shown in Fig. 8.5

At 300 K the Fermi level, shown by the dashed line in Fig. 8.6, is above the Cd acceptor level (since it is assumed that no other donors or acceptors are present in any quantity). This means that the acceptors are ionised and there are no holes trapped on the acceptors so BA recombination does not occur. As the temperature is lowered to 80 K the Fermi level passes through the acceptor level and the acceptor impurity traps holes and so becomes neutral. Then both BA and BB are observed. The slight shift in the peak of BB below the band-gap energy for GaAs is almost certainly caused by absorption of the luminescence shifting the peak to lower energies. Just below 80 K the intensity of BB reaches a maximum and then begins to decrease, while the intensity of BA on the other hand continuously increases as the number of neutral acceptors increases as the Fermi level approaches the valence band. Figure 8.7 shows a continuous temperature plot of the intensity variation for the BB line in Cd-doped GaAs with a doping level of 3.5 x 10-^"/cm3 and clearly shows the minimum in l/l (or the maximum in I). (Ref. 7).


Ί 1 1

BB LUMINESCENCE

N C d »3 -5x lO1 16

\y I

4 0 60 80 IOO I20

T ( O K )

Fig. 8.7 Variation of the reciprocal of the band-to-band (BB) photoluminescence intensity for the sample shown in Fig. 8.5 plotted against the temperature T(K).

At 20 K we can see from Fig. 8.5 that the intensity of BB is about 100 times smaller than the intensity of BA. The peak BA + LO at 20 K is the first longitudinal optical (LO) phonon replica of BA. (Phonon replicas in GaAs luminescence have been discussed elsewhere.) (Ref. 2).

It can also be seen from Fig. 8.5 that the total intensity of all the lumin-escence lines that were seen in the spectra increases by two orders of magnitude as the temperature is decreased from room temperature to 20 K. This means three things:

(1) the capture cross-section for BA recombination is very large compared to BB recombination;

(2) the non-radiative BB Auger recombination is relatively strong at 300 K compared to radiative BB recombination;

(3) deep-level emission below 1.1 eV was not monitored and it may be increasing as the temperature increases from 80 to 300 K.

8.2.3 Donor-to-acceptor Pair Recombination

Donor-to-acceptor, DA, recombination has been observed in GaAs and has been reviewed elsewhere. (Ref. 2). Sharp DA pair lines are observed for GaP and will be discussed in Section 8.3.2. below.


At the time of writing sharp pair lines such as occur in GaP at low tempera-tures have not been observed for GaAs so more indirect methods must be used to identify DA recombination lines. These methods of identification have been fully discussed previously (Ref. 2) and are briefly summarised below:

(1) The DA luminescence line shifts to higher energies as the excitation intensity increases;

(2) appreciable narrowing of the line with increasing intensity is observed;

(3) the line shifts towards higher energies as the donor concentration increases;

(4) the DA recombination lifetime is larger than the near band-edge line;

(5) the time resolved spectra show a characteristic energy peak shift.

8.2.4 Auger Recombination

Band-to-band Auger recombination is a non-radiative process in which the energy emitted when an electron and a hole recombine is readsorbed by exciting another electron deep into the conduction or by creating a hole deep in the valence band. Figure 8.8(a) shows the Auger process in which electron 2 is excited up into the conduction band by taking up the energy given out when electron 1 falls to the valence band and Fig. 8.8(b) shows how impact ionisation of fast electrons is the exact inverse of the Auger process. Similarly, for holes, Fig. 8.9(a) shows the Auger recombination of electrons creating a hole deep in the valence band and Fig. 8.9(b) illustrates the inverse process of pair creation by fast impact ionisation. Similar diagrams can obviously be drawn for impurity Auger recombination with the originating or terminating electron or hole state lie on the impurity level. Indeed in LEDs when the material is heavily doped the impurity bands have merged with the conduction or valence band and impurity and band-to-band Auger recombination will be indistinguishable.

/ /

Fig. 8.8 (a) Non-radiative Auger process excites an electron deep into the conduction band, (b) impact ionization of the fast electrons (the inverse of (a)).

2 ·

J


(a) (b)

Fig. 8.9 (a) Non-radiative Auger process excites an electron in the valence band, (b) pair creation by fast impact ionization

Three things should be noted about all the types of Auger recombination:

(i) two electrons and two transitions are involved, as opposed to the single electron transition in photoluminescence or absorption;

(ii) the transitions are not vertical nor necessarilly confined to the Γ point (k = Ο,Ο,Ο), contrasting to the vertical transitions in photo-luminescence at the Γ point in lightly doped direct-gap materials.

(iii) momentum and energy must be conserved in the transition process.

The theory of Auger recombination is complex and has been discussed elsewhere by Blakemore (Ref. 8) and Beattie and Landsberg (Ref. 9). Auger recombination is particularly important in carrier concentration quenching as will be seen shortly and in indirect-gap-materials like GaP. For an indirect and a direct-gap semiconductor with the same minimum energy gap non-radiative Auger recombination will be much stronger in the indirect gap material because the non-vertical transitions required for Auger recombination are the norm.

8.2.5 Carrier Concentration Quenching

In the manufacture of a GaAs LED it is essential to know what majority carrier concentrations are required in the n and the p regions of the device to give maximum efficiency. Figure 8.10 shows the effect of the Sn donor concentra-tion of the substrate on the efficiency of GaAs LEDs that were all prepared by diffusing Zn from a ZnAs2 source at 870°C for 15 minutes to form the p-region in the n-type Sn-doped substrate. (Ref. 10). This clearly shows that there is a critical carrier concentration of about 5 x 101 7 Sn donors per cm3 above which the efficiency at 300 K decreases. A study of efficiency as a function of Si acceptor concentration in Si-Si solution-grown diodes shows that in this case the peak efficiency occurs at the concentration of 1 x 1θ1° holes/cm3. (Ref. 11). Above this density of carriers concentration quenching sets in and


the e f f ic iency f a l l s rapidly.

18

•16 h

•14 h

U z

•12

u.

ÜJ

° 08

•06

• 0 4

A ' O A

' O O ° \

/

/

/ o /

/ ° o / o

'8\ o Λ o \ o

\ \

K /

/

L· I i i i » I -J I I I I I I

\

■ ■. .Vl

IO J7 NI8 I O " IO'

CARRIER CONCENTRATION ( c m - 3 )

IO 19

Fig. 8.10 Effect of carrier concentration on external quantum efficiency of GaAs diodes. All diodes are Sn doped and diffused with SnAs2 at 870°C for 15 minutes. (After Herzog (10).)

A large number of experiments have been carried out using photoluminescence as a function of carrier concentration in order to further understand this process of carrier concentration quenching in the LED. (Refs. 12-19).

The concentration quenching of photoluminescence is illustrated in Fig. 8.11 for Te- and Si-doped GaAs. (Ref. 17). Figure 8.12 shows a comparison of photoluminescence, cathodoluminescence and electroluminescence carrier concentration quenching. (Ref. 20). The Sn-Zn diodes shown in Fig. 8.10 are compared to photoluminescence results on Ge-doped GaAs recorded at 20 K by Williams et al. (Ref. 12) and by Queisser et al. (Ref. 16) on Zn-doped GaAs at 300 K and 77 K; and to cathodoluminescence results by Cusano at 300 K and 77 K. (Ref. 21).


ζ

< a H m ce <

- IO o

O) Z LU

o ÜJ

ce o UJ »-z

I.O

0 .4

" I I I I I

Δ 77°K MEASUREMENT)

h O Si

Δ Te

• Si

BEFORE ANNEALING^ V

\ \ 1

1 AFTER ANNEALING \ \ AT 800°C * Λ Β *

A T e J 3 HOURS FOR

I.» I M i l

1 \ \ d

J I M I N 3xlO" IO" IO"

ELECTRON CONCENTRATION (cm )

Fig. 8.11 Integrated edge emission intensity Qi at 77 K before (solid curve) and after (dashed curve) annealing as a function of room-temperature electron concentration for Si- and Te-doped melt grown GaAs. (After Hwang, (17,18).)


z D

>-QC .

2io-

i h

IO" _2L

IO' 3L

ω

3 ft-4

ELECTROLUMINESCENCE

^.^•^·^-HERZOG (Sn

\s \ / V \ [WILLIAMSetal./ \ \ Γ (Ge.20°K) / \ >

/ CUSANOî / (Te.Se.77°K) ι

1

k OUEISSER etal. / (Zn.300°K) /

GaAs J

-Zn.300eK)

.OUEISSER etal Yîzn,77°K)

/ CUSANO(Zn.300°K) \ \

1 1 1 1

Hio- υ z UJ

IO

HlO"

-3 ^

u UJ _ i Ul

lOK IO' IO" IO19 I O 2 0 IO 2 ' N<j, N a ( c m - 3 )

Fig. 8.12 A comparison of photoluminescence, cathodo-luminescence and electroluminescence carrier concentration quenching for both n- and p-type GaAs. (After Pilkuhn (20).)

Table 8.3 summarises the photoluminescence carrier concentration quenching results obtained for four n-type dopants in GaAs. (Ref. 2). With the exception of the Ge doping the maximum intensity occurs at 2 x 10x o donors/cm-3

in every case. The cathodoluminescence results shown in Fig. 8.12 for Te-and Se-doped n-type melt-grown GaAs also show a maximum intensity at 2 x 10·1-** donors/cm^. However, Cusano observed a lower maximum at about 1 x lO1^ donors/cm3 for cathodoluminescence of Sn doped melt grown GaAs at 300 K and 77 K. (Ref. 21).

Luminescence of Commercial LED Materials ^ 5

Table 8.3 Carrier Concentration Quenching in n- type GaAs

Λ „ Max. Intensity Temp Dopant Growth . 3 2 * Refs.

(n/cm°) K)

Te

Si

Si

Ge

Te

Se

Solution

Melt

Vapour

Vapour

Melt

Solution

2

2

2

3

2

2

X

X

X

X

X

X

10 1 8

I0I8

101 8

10 1 7

loi8

10l8

77

77

300

77

20

77

300

77

25

17

19

12

17

18

25

The lower concentration at which the intensity peak occurs for Ge (Ref. 12) and Sn (Ref. 21) donors can be understood in terms of the higher compensation and in the case of Ge part of the peak shift is caused by the lower tempera-ture of measurement. These statements are confirmed by the work of Tuck on p-type GaAs (Ref. 15) Two of his results are shown in Table 8.4 which summarises the carrier concentration quenching observed in p-type GaAs. (Ref. 2) Zn (1) had a very small degree of compensation and there was no temperature change in the carrier concentration at which the maximum photo-luminescence intensity occurred. Zn, (2) on the other hand, was more heavily compensated and the maximum changed from 3 x 1019/cm3 at 300 K to 1 x 1019/cm^ at 77 K. The melt-grown samples used for Zn (3) were probably also compen-sated since there was a large shift in the peak between 77 K and 20 K. The solution-grown samples Zn (4) also show a temperature shift.

Table 8.4 Carrier Concentration Quenching in p-type GaAs

~ Max. intensity m _ _ Dopant Growth J Temp Refs. (p/cni ) ( K)

Ge Zn(l)

Zn(2)

Zn(3)

Zn(4)

Solution Melt

Melt

Melt

Solution 2 2 to to

1 x 10l9 8 x 101 8

8 x 10l8 3 x 1019 1 x 10l9 1 x 1019 5 x lO18

3 x ICA* 1 x 10l9

12 300 77 300 77 77 20 300 77

13

15

15

14

16

The temperature shift has been explained in terms of movement of the Fermi level as a function of compensation. For the compensated samples, the number of empty states available for recombining photoelectrons will be smaller than for the uncompensated ones; as the temperature is lowered, the number of empty states will be reduced even further as the Fermi level moves through the


impurity band. This explains why the maximum moves to lower concentrations. For uncompensated samples with heavy doping levels greater than 1 x lO^/cm^, the Permi level will be pinned quite close to the valence (or conduction) band and will vary little with temperature, so that there will be a negligible change in the maximum.

Although compensation can change the carrier concentration at which the quenching sets in, it does not seem to effect the quenching at higher concentration. However, there are four other processes which possibly contribute to the quenching and these will now be discussed.

(a) Increasing absorption. In uncompensated samples absorption does not contribute appreciably to the carrier concentration quenching in GaAs. Hill has clearly shown this for both n-type and p-type GaAs. (Ref. 22) His results for n-type material doped with either a group IV dopant (Sn, Si) or a group VI dopant (Te, Se, S) are shown in Fig. 8.13. The photoluminescence highest energy peak is correlated with an absorption coefficient of 300 cm~l as a function of the carrier concentration. All measurements were made at 77 K. At low doping levels the absorption dominates but at higher doping levels it is no longer important and this means that absorption is not a dominant process involved in the carrier concentration quenching of n-type GaAs.

I.60

> c

> £ 'SO Z Ul

Z O t-o I a

I.40 I0 1 7 Ι 0 Ι Θ lO19

ELECTRON CONCENTRATION (cm3)

Fig. 8.13 Correlation of the highest energy photo-luminescence line in n-type GaAs with an absorption coefficient of 300 cm""·'- as a function of carrier concentration. (After Hill, (22).)

Figure 8.14 shows the results that Hill obtained for p-type (Cd and Zn doped) GaAs. As with n-type material this demonstrates once again that quenching at lower carrier concentrations by absorption is important but at higher concen-trations it is no longer affecting the quenching.

-

-

L

1-L

1 1—i i i i i | 1

- ■ ♦ x

• Te doped x Se doped

-•-Sn doped • Si doped

♦ S doped

. . . . . I l l 1

_J£~^

m

1 1 1 1 1 1

l | I 1 1 1

1 1 1 I I I

Γ - 1 - Τ -Γ

• - j

H

J

Ί

1 1 ll


> *1.50 c.

>■

O a. UJ z UJ

Z o 1-

o I I.40 a.

| ' " " 1 ' T - T "

-* X

•Zn doped xCd doped

— 1 M i l l 1 1—i_

r m - η —

JT~*

. ■ . . ,1

1 1—i i i i i 11

oc»300cm^V

• ·

i 1 i 1 i i t 1 \

1 '

•

1 1

—1 1 1 1 1 1

"*"* -\

J - • J J

—j

1 1 1 1 1 1

IO" Ιθ'° IO" HOLE CONCENTRATION (cm-3)

IO"

Fig. 8.14 Correlation of the energy of the photo-luminescence line associated with hydrogenic acceptors in p-type GaAs with an absorption coefficient of 300 cm"1

as a function of hole concentration. (After Hill (22).)

In compensated material absorption can be an important quenching process at heavier doping levels. This is because compensation shifts the absorption edge (and the Fermi level) towards and into the forbidden gap. (Ref. 23)

(b) Band-to-band and impurity Auger non-radiative recombination. We have mentioned that when the donor (or acceptor) band merges with the conduction (or valence) band, band-to-band and band-to-impurity transitions cannot be distinguished. Hence at this merger point you cannot separate out impurity band Auger from band-to-band Auger non-radiative recombination. The concen-tration at which the merger occurs is less than the carrier concentration at which the intensity curve reaches a maximum for both n- and p-type GaAs. This means that Auger recombination must be considered as a very likely cause of concentration quenching.

A look at the model of Beattie and Landsberg (Ref. 9) confirms this. They predict that the probability for Auger transitions from the bottom of the conduction band to the top of the valence band is large when the effective mass ratio, σ = ιηθ/ι%, is very much less than one. Hence for GaAs with σ = 0.15 Auger non-radiative recombination must be considered. In the region where the Auger process dominates, the intensity I, is given by

N (8.2)

(where N is the density of electrons or the density of holes for n- or p-type material respectively). This is because two electrons and two holes are involved in the recombination. The minus sign occurs because the process is non-radiative and therefore leads to a reduction in intensity.

In uncompensated n-type samples with doping levels greater than 2 x 1018/cm3

(and for p-type with doping levels greater than 1 x 10^/CTTT) the intensity quenching rate with carrier concentration increasing is greater than N~2 a s

can clearly be seen from Fig. 8.12. This indicates that other processes must also be involved in the quenching as well as Auger non-radiative recombination.


(c) Precipitate formation. Since precipitates begin to form at high doping levels when a particular impurity begins to reach the so-called solubility limit of that impurity in GaAs, they must contribute to the quenching at very high doping levels. There is no doubt, however, that precipitates do not contribute to the onset of quenching or the point where the increase in intensity of the near-band-edge emission intensity slows down. In this region the carrier concentration is often directly proportional to the impurity concentration in the melt (Ref. 24) or solution (Ref. 11) from which the crystal is grown so no precipitates are formed. At higher concentrations above the maximum in the quenching curve precipitates have been correlated with quenching for Te doping. For example, bright-field transmission electron micrographs and electron diffraction patterns of a melt grown sample containing 5 x 10^° tellurium atoms per cm3 clearly shows that GaoTe3 precipi-tates are present; but when the Te donor concentration was 3 x lCM-°/cm3 no precipitates were observed. (Refs. 25,26)

The degree of non-radiative recombination associated with precipitation has not been measured with photoluminescence and will obviously vary with impurity and as a function of growth conditions.

(d) Vacancy-complex formation. The onset of quenching occurs at about 5 x 10l7/cm3 for n-type gallium arsenide at 77 K. At this concentration the luminescence line that is associated with a Ga vacancy complex is increasing rapidly with intensity. There is evidence to show that strong quenching is associated with Ga vacancy complexes particularly at temperatures above 70 K so this implies that vacancy complexes are partly responsible for the carrier concentration quenching. This subject will be more fully discussed in the next section.

In summary, the onset of quenching in n-type GaAs is believed to be caused by vacancy complexes (Ref. 2) whereas the rapid decrease in intensity at higher doping levels is due to non-radiative recombination associated with Auger recombination and precipitation. In p-type GaAs, on the other hand, Auger non-radiative recombination is probably the dominant process and is respon-sible for both the onset of quenching and the rapid decrease in intensity at heavier doping levels. However, in very heavily doped p-type material precipitates will again contribute to the quenching.

Si DOPED Te DOPED

Fig. 8.15 The two different types of localized Ga vacancy-donor complex for groups IV and VI doping in GaAs. (After Williams (28).)


8.2.6 Vacancy Complexes

(a) Introduction. The breakthrough for the GaAs LED came with the develop-ment of epitaxial growth from a Ga solution. These Si-doped solution grown LEDs had power efficiencies as high as 32% at room temperature. (Ref. 27)

There is now little doubt that one of the main reasons for this high efficiency is due to the absence of Ga vacancy complexes in this solution-grown material. To prove the Ga vacancy complexes exist and that they do have a large effect on doping, diffusion and the efficiency of luminescence and electroluminescence, has taken a considerable amount of time and the exact nature of the vacancy complexes is still not fully established.

The two types of Ga vacancy complexes that have so far been proposed in GaAs (Ref. 28) are shown in Fig. 8.15. In one case the Ga vacancy (Vga) forms a complex with a group VI simple donor centre and in the other with a group IV simple donor. In the first case a nearest-neighbour complex is formed whereas in the latter case a next-nearest-neighbour complex is formed. It is assumed that:

(i) the Ga vacancy is an acceptor; (Ref. 29)

(ii) that it can only be singly charged.

Using these two assumptions it is easy to see why the Ga vacancy-donor complexes shown in Fig. 8.15 occur. They are just near neighbour or next-nearest neighbour donor-acceptor pairs; the electron on the donor is attracted to the hole bound to the vacancy. As expected, the molecular-like configurational coordinate model (Ref. 30) which has been successfully used to interpret luminescence associated with electron transitions at vacancy-complexes applies also to nearest-neighbour simple donor-simple acceptor pairs and the theoretical equations for distant donor-acceptor pairs do not apply.

Another strong reason for believing that these donor-acceptor pairs involving a vacancy are active in III-V compounds is that their controlling influence on the stoichiometry of II-VI compounds is well established.

The phenomena of self-compensation is well understood in II-VI compounds. Self-compensation is self-doping by vacancies to make it p- or n-type. It is enhanced by doping with the carrier which is oppositely charged to the vacancy. For example, in ZnS doping with Cl (group VII) donors upsets the equilibrium of vacancy concentrations which is governed by the well-known equation: (Ref. 29)

[VZn] [Vs] = K(T) (8'3)

(the product of the vacancy concentrations is a constant K at a constant temperature. The notation V for vacancy is commonly used but could easily be confused with the notation for group V elements so some authors use O for vacancy.)

The Cl atoms act as simple donors so they sit substitutionally on the S sites. Therefore at large Cl doping levels the vacancy concentration |Vg| is reduced and this means that because of equation (8.3) the Zn vacancy concentration must increase to compensate for this reduction. This causes self-compensation


since the Zn vacancy is an acceptor. There is a coulombic attraction between these Zn vacancy acceptors and the Cl donors so it is reasonable to postulate that the stable localised centre Vzn "

cls i s formed in the crystal lattice. The localised electron transition from the excited state to the ground state of this molecular-like centre produces what is called "self-activated" luminescence because it is associated with vacancies which are uniquely characterised by the crystal lattice structure and the atomic weights of the atoms which make up the semiconducting compound ZnS.

(b) Gallium vacancy complexes. In the luminescence spectra of melt-grown n-type GaAs there is almost always a broad peak in the region of 1.2 eV. This peak can be shown to be associated both with variations from stoichiometrv and with the donor-dopincr level. (Ref. 2)

Stoichiometric is defined as the condition in the crystal lattice when the number of Ga atoms on Ga lattice sites is equal to the number of As atoms on As lattice sites. Also the Ga vacancy concentration, Vga, can be derived from the equilibrium equation (8.3) for II-VI compounds, since: (Ref. 29)

[VGa] [VAs] = K(T) ( 8 · 4 )

This means that if the growth conditions produce a variation from stoichio-metry both vacancy concentrations will change. For example, growing in As-rich conditions will reduce [VASJ anc^ then equation (8.4) shows that [yGaJ must increase in order to keep K(T) constant at the growth temperature T.

The 1.2 eV level has indeed been shown to be related to the growth conditions. This is shown by the three photoluminescence spectra in Fig. 8.16 which were recorded at 20 K from melt-grown, solution epitaxy and vapour epitaxy samples. (Ref. 12) All of the samples were lightly doped n-type in the region of 1 to 2 x 10^6 n et donors (ND - N^) per cnW. The vapour-grown sample was grown under As-rich conditions so it follows from equation (8.4) that there would be an excess of Ga vacancies. The solution-grown sample was prepared from a Ga solution under Ga-rich conditions so that equation (8.4) shows that the sample contained a relatively small number of Ga vacancies and a correspondingly larger number of As vacancies. The presence of the 1.2 eV peak in all vapour-grown material produced in As-rich conditions and similarly, the absence of the peak in solution-grown samples grown under Ga-rich conditions is strong evidence that the 1.2 eV line is associated with Ga vacancies in some way.


1 i i i 1

VAPOUR EPITAXIAL n = 2 x IO,6/CC

SOLUTION EPITAXIAL n = 1 x IOl6/CC

- · - MELT-GROWN n s 1 x IOl6/CC

0 B

/ ' ^ \ x 2 5 0 2°°K X'f vf\ \ / \ \ c Π

/ X 3 4 0 \ \ / \ \\ \ X \ i Λ' \ / \ N X20/.rlJ

X 340

r τ" - / ;w

^ τ ι

il ι ν 1 2 Ι · 3 Ι · 4

ENERGY(eV)

Ι·5

Fig. 8.16 Comparison of growth techniques for undoped and lightly doped n-type GaAs at 20 K. Solid curves: vapour epitaxial, n = 2 x lO1^ cm"1; dashed curves: solution epitaxial, n = 1 x 1016 cm"3; dot-dashed curves: melt grown, n = 1 x 101 6 cm"3. (After Williams and Blacknall, 1967 (2).)

The presence of a 1.2 eV peak in the melt-grown sample shown in Fig. 8.16 is also not surprising if it is associated with Ga vacancies since it is well known that the total vacancy concentration in melt-grown GaAs can be as high as 101 9 per cm3. For the epitaxial material where the growth temperature would be several hundred degrees lower then the total vacancy concentration is two or three orders less than for the melt-grown crystals and will be lO1^ to 101 7 per cm3. Consequently, variations from stoichiometry for the epitaxial material will be much more effective in reducing the number of one of the vacancy species to a negligible quantity than in melt-grown crystals. The observed strong dependence of the 1.2 eV line with variations from stoichio-metry in epitaxial material further confirm that it is associated with Ga vacancies. (Ref. 2)

Further evidence for this associated of the 1.2 eV line from detailed luminescence measurements has been reported in detail elsewhere as has the similar work that has helped to identify similar As vacancy self-activated luminescence lines in GaAs. (Ref. 2)

8.2.7 Other Defects and Impurities

Many other luminescence peaks have been observed in measurements on GaAs but only the transition metals have been investigated in any detail. These transition metal results have been discussed elsewhere (Ref. 2) and the


activation energies determined from photoluminescence, electroluminescence and Hall measurements are compared in Table 8.5. (Ref. 2)

Table 8.5 Transition Metals in Gallium Arsenide

Dopant

Cr

Mn

Fe

Co

Ni

Cu

Ag

Activation

Photoluminescence at 20 K or 4 K

0.85

0.11

Approx 0.5 and 0.2

0.58

-

0.155, 0.165, 0.170

0.239

energy (eV)

Electroluminescence at 77 K

-

-

0.36

0.345

0.35

0.18

-

Electrical

0.79

0.094

0.37 and 0.52

0.16

0.21

0.145

0.235

Of all the transition metals, Cu is by far the most important because it is often present as a contaminating impurity. Figure 8.16 shows how a lightly doped n-type vapour epitaxial sample is contaminated with Cu acceptors (peak C). However, for samples grown from a Ga solution, as Fig. 8.16 shows, the photoluminescence shows that Cu is never observed. The absence of copper, Cuca/ in solution-grown samples is presumably due to either the reduction in the Ga vacancy concentration or the leaching out of the Cu by the gallium or more likely both factors are contributing. There is no doubt that the absence of Cu also contributes to the high efficiency of the solution-grown diodes as well as the reduced Ga vacancy concentration. Morgan et al. (Ref. 31) have clearly shown that Cu contamination produces degraded diodes because a large amount of non-radiative recombination is associated with the presence of Cu. They also observed three diode emission peaks which they attributed to Cu: (a) interstitial Cu donors with an activation energy of 0.07 eV; (b) first ionisation energy level of the Cu acceptor, CuGa, with ΔΕΑ = 0.18 (see Table 8.5); (c) the second ionisation energy level of Cu with ΔΕ& = 0.41 eV.

Deep levels in the region of 1.0 eV are often observed in photoluminescence (Refs. 12, 32-34) and cathodoluminescence (Ref. 35) and it has been speculated that one of the levels is a VG aV A s divacancy (Refs. 33-35) and that another is a complex associated with Si. (Refs. 12,32) Surface state density has also recently been measured by Mettler who analysed photoluminescence at 300 K in both n and p-type GaAs, grown by liquid phase epitaxy (Ref. 35).

8.3 IMPURITIES IN GALLIUM PHOSPHIDE

8.3.1 Introduction

Just as with GaAs luminescence studies have enabled the role of various impurities, impurity pairs and vacancy complexes in GaP LEDs to be understood to a large extent. Once again, as with GaAs, it has been shown that both Cu


(Refs. 36-38) and gallium vacancies (Refs. 39-43) produce degradation of the diodes. Also photoluminescence (Ref. 44) and cathodoluminescence (Ref. 45) have confirmed that in the ZnO-doped red LED the majority of the emission is on the p-side of the junction. (Ref. 46)

Although the degradation and the emission on the p-side of the junction are similar to GaAs the impurities used to produce the emission in the diodes act in a very different way. In fact in GaP the impurities are used to overcome the inherent disadvantage of an indirect-gap material. Referring back to Chapter 5, the theoretical recombination probability, B, for band-to-band recombination shown in Table 5.1 was four orders of magnitude lower than for GaAs. However, by restricting all the visible radiative recombination to selected impurity bands, recombination losses due to indirect band-to-band transitions can be avoided. In this way efficient red diodes doped with Zn and 0 on the p-side of junction and efficient green diodes doped with nitrogen have been produced. The theory of the recombination of Zn-0 donor-acceptor pairs and nitrogen isoelectronic traps has taken a long time to develop and is still not completely understood. Consequently only a relatively brief description of the luminescence results will be given here and the reader is referred to the more extensive reviews given elsewhere. (Refs. 47-52).

8.3.2 Donor-acceptor Pairs

The electron on the neutral donor impurity atom is attracted by the hole on the neutral acceptor impurity atom to form a "dipole" stable donor-acceptor (D-A) pair. A schematic diagram showing a Znca-Op D-A pair in the zinc blende GaP crystal lattice is shown in Fig. 8.17. r is the separation between the centres of the impurity atoms. The |111| crystal direction is shown and the sign of the charge on the impurity atoms is indicated.

Fig. 8.17 A Zn-0 donor-acceptor (D-A) pair in the GaP crystal lattice, r is the separation between the centres of the impurity atoms.


nvDA* t n e energy of the luminescence associated with the recombination of the electron with the hole to leave an ionised donor and acceptor is given by the equation: (Ref. 53)

hvDA = E G - (ΔΕΑ + ΔΕο) + q2/r . (8.5)

The term q^/r is the coulombic interaction between the electron and the hole and this reduces the binding energy of the donor and the electron slightly. The actual change in this binding energy will depend on the value of r. For substitutional centres like Znga or Cdça and Sp or Op the impurity atoms sit substitutionally on lattice sites so the separation r takes discrete values which can be related simply to the interatomic spacing. The distribution of r values will depend on the random distribution of donors and acceptors in the crystal lattice. For each r value a sharp luminescence line would be expected and for GaP at helium temperatures as many as 300 lines have been observed for some donor-acceptor pairs. (Ref. 53) The r values observed for these sharp lines are just for distant D-A pairs which are separated by 10 to 40 A and are not nearest neighbours. There are two basic types of sharp line spectra:

Type I : IVp - VIp, e.g. Cp - 0 p

Type II : 1 1 ^ - VIp, e.g. ZnGa - 0 p

A type I spectrum in which both impurities in the pair are on the phosphorus lattice site is compared to two type II spectra in Fig. 8.18. (Refs. 47,54). Fig. 8.18(a) shows a part of the infrared luminescence Type I spectrum of GaP doped with 0 donors and C acceptors at 1.6 K. The sum of the activation energies, EA + ED, is 941 meV. The type II spectra shown in (b) and (c) are for Zn-0 and Cd-0 pairs respectively. Once again the temperature of measurement was 1.6 K. For (b), for Zn-0,EA + E D is 956.6 meV and for (c), for Cd-0, E A + E D is 989 meV. The bracketed integers are an index of the substitutional sites allowed for a given acceptor around the 0 donor (or vice versa) and the other numbers denote the degeneracy of equivalent pairs for a given scalar separation. Insets in (b) and (c) show the isotope shift when some of the 0^6 atoms are replaced by 0^ ones.

This isotope addition definitely proved that oxygen was present as one of the pair members. Each pair line is shown to be replicated by a similar line 0.71 ± 0.02 meV higher in energy corresponding to the 0^° pairing with the acceptors.


PHOTON ENEMY (*V>

Fig. 8.18 (a) A type I DA C+O pair spectrum in which both impurities in the pair are on the phosphorus lattice site is compared to (b) a type II Zn+O and (c) a type II Cd+O spectrum where the impurities in the pair are on the Ga and P lattice sites respectively. The photoluminescence spectra were all measured at 1.6 K. (After Dean et al. (54).)

Although the importance of distant D-A pairs is undisputed at 1.6 K which was the temperature of the measurements shown in Fig. 8.18 at higher temperatures other mechanisms dominate. In the case of the red luminescence band that produce the efficient electroluminescence and photoluminescence in GaP diodes it has now been shown that nearest-neighbour Zn-O pairs (or Cd-O) and excitons bound to nearest-neighbour pairs dominate right up to room temperature. At around 10 K for Cd-O pairs a zero phonon line is observed at 1.907 eV (Ref. 55) followed at lower energies by vibrational structure just as was predicted for the acceptor-VAs and Vca-donorAs pairs by the configurational coordinate model. (Ref. 2) This zero phonon line is also observed in the excitation spectra of


the Cd-O-doped sample. A similar shaped emission and excitation spectrum was also observed for Zn-0 pair luminescence at lO K but no phonon structure could be resolved. (Ref. 55)

The exact mechanism of the Zn-0 luminescence at room temperature is still not understood but there is no doubt that nearest-neighbour Zn-0 centres do play an important role. A fairly recent discussion of the various models for Zn-0 has been given by Bergh and Dean. (Ref. 47)

Photoluminescence efficiency measurements on Zn-0 pair luminescence showed that 22% efficiency could be achieved for the red luminescence band for lightly doped samples. (Ref. 56) This high efficiency means that the diode power efficiency could eventually reach well above the 15% so far achieved and make these red diodes almost as efficient as the best GaAs diodes. (Ref. 57).

8.3.3 Nitrogen and Bismuth Isoelectronic Traps

When group V elements such as N or Bi are added to GaP they form isoelectronic traps. They are called isoelectronic because they have the same number of valence electrons as the atom they replace when they sit substitutionally on the lattice site. However, they cause a large lattice distortion which changes the crystal potential to such an extent that a potential "well" forms at the nitrogen or bismuth centre and this "well" acts as a trap for an electron or hole. (Ref. 49) As soon as the electron (or hole) is bound to the N atom another hole (or electron) can subsequently be trapped so that an exciton is bound to the site. These excitons are readily observed in the low-temperature photoluminescence spectra as is shown in Fig. 8.19. (Ref. 58) The N concentration for this sample was estimated from its absorption spectrum to be about 5 x lO·^ N atoms/cm . The zero-phonon lines A and B are seen followed by various optical and acoustic phonon replicas, which can be clearly resolved at 4.2 K the temperature at which the photoluminescence spectrum was recorded. Magnetic field splitting of the A- and the B-lines is shown in the insert of the left hand side of the diagram. The nitrogen ion is represented by two concentric circles and the Zeeman spectra showed that the A line split into three lines and the B line into five.

In green and yellow N doped LEDs a range of nitrogen doping levels is used. In every case, however, the diode doping level is far greater than the doping levels used to observe the A and the B zero-phonon exciton lines and their phonon replicas described above. For this reason we must now discuss heavier doping levels.

Luminescence of Commercial LED Mater ia l s 167

100

90

80

.t 70

I 60 C

+- 50 c <v o v) 40 Q> k.

o Ξ 30

L 7 ( 7 V Ï O M B H

B , 5 Lines

< § > - —

L·^

}A

|__ Magnet ic analysis

M z r — -

Γ / —Ï—<r-J

A , 3 Lines f ~ 0 . l

(a) UBNC I 0 3 Y 4 . 2 ° K

- 5 x I0 I 6 N / C C

A - X

A-LO1

B-LO 1

A-TA*

A-TO

2.22 2.24 2,26 2.28

Photon energy (eV)

Fig. 8.19 Excitons in the luminescence spectrum at 4.2 K in GaP lightly doped with 5 x 1016 nitrogen atoms per cm3. A and B are the zero phonon exciton lines and the phonon replicas of the lines are also seen in the spectra. Magnetic field splitting of the A- and B-lines is shown in the insert in the left-hand side of the diagram. (After Thomas and Hopfield, (58).)

When the concentration of N is increased above 5 x 10 atoms/cm3 N-N pair lines appear and at very high concentrations they are observed on their own and the A and B lines are absent. These pair lines are clearly observed in both fluorescence and absorption as a series of sharp lines. (Ref. 58) The strength of the A line in absorption has been used to calculate the concen-tration of N in the crystal. (Refs. 58,59) Proton-activation analysis (Ref. 60) can also be used to determine the N concentration, but, like absorption, it is not suitable for rapid analysis of N concentrations. Stringfellow et al. (Ref. 61) have shown that for the concentration of N used for green GaP LEDs 1 to 16 x 1018 atoms/cm3 the ratio of the intensity of the N pair line to the zero phonon A line as determined from the photo-luminescence spectrum was directly proportional to the N concentration.

The same type of zero phonon lines due to the decay of excitons bound to N have also been observed for Bi. (Ref. 62) In addition, however, Bi-group VI donor pairs have been observed with Te, Se and S implying that Bi is effec-tively a hole trap. (Ref. 63)

It appears that isoelectronic traps cannot be formed by elements like As which form alloys with GaP. (Ref. 49) Only when no alloying occurs is an iso-


electronic trap possible presumably because with alloying at low concentra-tions close to the GaP phase boundary there is insufficient perturbation of the crystal lattice by the added element to product a potential "well" which can trap carriers.

8.3.4 Gallium Vacancy Complexes

The study of Ga vacancy complexes in GaP is still in its infancy compared to the work on vacancy complexes in GaAs.

Luminescence at 1.5 K revealed at sharp zero-phonon line A at 2.177 eV followed by vibrational structure as shown in Fig. 8.20. (Ref. 40) This zero-phonon line and the vibrational structure is characteristic of the lumines-cence predicted by the configurational coordinate model for a localised centre like a Ga vacancy complex. (Ref. 2) This "orange" line shown in Fig. 8.20 was associated with 0, first, because it was observed in GaP crystals grown by the water-vapour transport method, (Ref. 64) and second, because a shift in the zero-phonon line A was observed for different 0 isotopes. (Ref. 43) Gallium vacancies were also associated with the transition because the line diminished when samples were annealed in the presence of Li (Ref. 43) and Zn (Ref. 40) In one case Li-0 complexes were formed and in the other the familiar Zn-0 pair luminescence occurred.

z D >-< cc t m ce <

Z

JL 2 2 0 2 1 5 2 I O

PHOTON ENERGY (eV)

205

Fig. 8.20 Luminescence spectra at 77 and 1.5 K for the proposed VQa-0p complex in GaP. A is the zero-phonon line corresponding to a transition between the lowest energy state in the ground state and excited state of the molecular-like configurational coordinate levels. Phonon replicas are also observed in the 1.5 K spectra. (After Bhargava and Kurtz, (40).)

As with the Ga vacancy complex in GaAs this complex also acts as a non-


radiative centre at 300 K (Ref. 40) but more work needs to be done to positively confirm that nearest-neighbour Ga vacancy-0 donor complexes are responsible for the luminescence. In particular luminescence and excitation temperature measurements and controlled annealing experiments should be carried out.

8.3.5 Conclusion

In conclusion the donor and acceptor impurity ionisation energies for GaP that have been estimated from luminescence studies are summarised in Table 8.6. (Ref. 47) Determination of ionisation energies from pair spectra and bound exciton spectra is particularly accurate and the majority of the ionisation energies shown in Table 8.5 were determined in this way. (Refs. 47,65)

Table 8.6 Donor and Acceptor Ionisation Energies in GaP

Donors Acceptors

Donor

0

S

Se

Te

Si

Sn

Energy I

895.5

104.1

102.7

89.8

82.1

65

:meV) Acceptor

Be

Mg

Zn

Cd

C

Si

Ge

Energy (i

50

53.5

64

96.5

48

203

300

8.4 GALLIUM ARSENIDE PHOSPHIDE ALLOYS

8.4.1 Direct-band-gap Material

The major development of red-emitting GaAsi_xPx layers is concentrated on producing layers with a composition where the mole percent phosphorus is in the range 36.6 - 42% (i.e. x = 0.37 to 0.42). The photoluminescent spectra of the material at 300 K exhibits a strong emission line whose peak occurs at a wavelength dependent on the composition, as shown in Fig. 8.21. This provides a convenient non-destructive method of determining material composition. This correlation between emission peak and composition is valid provided that the incident power density from the excitation source is not high enough to cause local heating of the specimen. Black (Ref. 66) has used a He-Ne laser focused to give a spot size of 0.1 mm to investigate the variation of photolumines-cence over the single wafer and hence to determine the variation in compos-ition.


670

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£ 6 6 0

LU υ z ιυ u

£ 650 5 Ζ> -J

O I û- 640

1

— ei

-

-

-

1

1 1 1

λ = 670nm X = 0-3662

\

Λ = 640nm X = 0 -4202

1 1 1

1 1 1

H

--

-

GL

1 1 0-36 0 3 8 0 - 4 0

GaAs|-x Px COMPOSITION-

0·42

—*X

Fig. 8.21 Photoluminescence peak at 300 K as a function of the composition in GaAs^_xPx alloys over the composition range x = 0.366 to 0.42.

Photoluminescence (Ref. 67) (PL) and cathodoluminescence (Ref. 68) studies in addition to electroluminescent (EL) emission from diodes (Refs. 69-71) have generated information about the luminescent processes which occur in GaAs^_xP . Figure 8.22 shows normalised spectral distribution at 300 K of (i) PL(n) spectra of an n-type sample, (ii) PL(p) spectra from the same sample after Zn diffusion, (iii) EL spectra made from this material. The line width of the n-type sample curve (i) was measured (Ref. 71) for various carrier concen-trations. As shown in Fig. 8.23, below 2 x 101 7 cm"3 the line width is fairly uniform but it increases rapidly for higher carrier concentrations, as for GaAs.

Luminescence of Commercial LED Materials

£ r

2.00 1.90 1.85

hi/ (eV) 1.80 1.75

Fig. 8.22 Normalized 300 K luminescence spectra of

GaAso.62pO.38· Photoluminescence PL(n) from the n-region

and photoluminescence PL(p) from the p-region of the

diode are compared to the electroluminescence (EL) from

the same diode. (After Lorenz and Blakeslee. (71).)

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Z 50 σ

£ 40

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-. '

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1 1 1

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Electron concentration (cm-3) I0,s

Fig. 8.23 Linewidth at half-intensity for PL(n) as a function of electron concentration. Full curve,

GaAs0eg2p0.38 anc^ broken curve for GaAs cathodolumines-

cence spectra. (After Lorenz and Blakeslee, (71).)

171


The PL spectra of the low doped n-type samples are skewed, the lower energy side of the spectra falling rapidly, whereas the higher energy side decreases more gradually. This shape is characteristic of band-to-band transition BB, and Lorenz (Ref. 71) was able to obtain a good fit to the high-energy side of curve (i) using the theoretical calculations of Mooradian and Fan. (Ref. 72) Lorenz found the same agreement at 77 K but at 2 K two closely overlapping emission bands were observed indicating the recombination mechanism at very low temperature had changed.

As compared to the photoluminescence of the n-type sample the photo-luminescence of the p-type Zn diffused sample is found to be broader, shifted to lower energy and generally a higher efficiency process. The shift in peak to a lower energy by about 25 meV suggests that the dominant emission corresponds to conduction band to acceptor-state transitions BA. The considerably larger half-width emission from the p-type material is attributed to come from residuai BB emission. The EL emission from the diffused material shown as curve (iii) in Fig. 8.22 is seen to peak at a wavelength slightly larger than even the photoluminescence of the p-layer. It has been shown in Chapter 5 that the emission peak is moved to longer wavelength for deep junctions because of absorption in the diffused layer. This complicates the interpretation of the electroluminescent spectra. However, it is reasonably evident that the emission is due primarily to BA recombination. The narrower emission spectra for electroluminescence may be attributed to the lower p-type doping in the region of the junction.

Since the electroluminescent spectra corresponds most closely to the photo-luminescence of the p-type material this implies that the electroluminescence originates by injection of electrons into the p-region as was shown to be the case for GaAs and GaP.

The typical emission spectra for diodes at 77 K that are shown in Fig. 8.24 exhibit two lines, PI and P2, with energy in the vicinity of the band gap and the energy of these diode emission maxima at 77 K has been plotted as a function of the material composition in Fig. 8.25. (Ref. 70). For the direct-band-gap material the emission peak PI lies within 10 meV of the band gap and is ascribed either to donor-to-band DA or band-to-band BB recombination. The band located ^30 meV below band edge, P2, is attributed to BA transitions from conduction band to the zinc acceptor.

Luminescence of Commercial LED Mater ia l s 173

l.555eV L543eV 2.0l7eV

1.992 eV

7800 8000 λ(Α)

6100 6300

Fig. 8.24 Emission spectra of two GaAsi„xPx diodes at 77 K in the direct-band-gap region. The separation between the two emission bands is greater in the higher phosphorus alloy diode. (After Craford et al. (70).)

> α>

2 . 3

2 .2

CP ι_ Φ c Φ

2.1 h-

σ û_

2 . 0

1.9

1.8

Peak emission energy ' vs. alloy composition /

"forGaAsi-χΡχ / without nitrogen /

T

T=77eK E r /

/ /

/ /

/ jr

SLLL

/?

■ s/ / ^y £ * * S Donor-acceptor

, ' · s pair band

Γ h

J_ J L JL 0.2 0.3 0.4 0.7 0.8 0.9 1.0 0.5 0.6

X Fig. 8.25 Peak emission energy against alloy composition for GaAs1_xPx diodes without nitrogen at 77 K. The diode current was 4 mA. (After Craford et al. (70).)


8.4.2 Indirect-band-gap Material

When the mole fraction of phosphorus is further increased to produce material with an indirect band gap, with x greater than 0.45 for à diode with a composition GaAso.45Po.54 t n e emission spectra at 77 K shows three peaks as shown in Fig. 8.26. (Ref. 70) The energy of the emission peaks were shown in Fig. 8.25 as a function of alloy composition. The highest peak PI is approximately 10 meV below the band edge and is attributed to the recombination of free excitons. (It is interesting to note that the zero-phonon E g x

emission which is not allowed in pure GaP, due to momentum conservation requirements, is allowed in the ternary system since momentum can be conserved through scattering at arsenic impurities.)

c

0>

2.l06eV

X=0 .54 77eK

Hl-sw

6200

Fig. 8.26 Emission spectra of a GaAso.46Pç). 54 diode at 77 K, in the indirect-gap region, at various driving currents. The near-band-edge peaks P^ and P2 dominate the emission uu Align «.ιθναβ currents. AUG intensity of P"3 saturates at high currents and the position shifts to higher energy. (After Craford et al. (7θ).)


The emission peak P2 for indirect-band-gap material for values of x up to 0.9 is attributed to LA phonon-assisted exciton recombination, with energy Eqx -LA. As the value of x is increased and the arsenic scattering is reduced three lines corresponding to TA, LA and TO phonon assisted exciton recombina-tion can now be observed in place of the single P2 line observed at smaller x values.

The major interest in GaAs;L_xPx materials with composition in the range 0.4 < x < 1 is in the N-doped material because of its increased luminous efficiency over the conventional dopants like Zn and Te. As might be expected the emission spectra depends upon the N concentration. Vapour epitaxy deposition can produce N concentrations up to 1020 atoms cm""3 although lO*9

atoms/cm"3 is the more commonly used material. By comparison for GaP grown by liquid epitaxy it is difficult to obtain N concentrations higher than 2-3 x 1018 atoms cm"3. For compositions where x > 0.9, the photoluminescence (Réf. 73) and electroluminescence (Ref. 70) spectra at 77 K contain three peaks; the A-line recombination at isolated N atoms, and at centres consisting of N pairs ΝΝχ and NN3 (i.e. near-neighbour and third nearest-neighbour sites). The intensity of the NN3 peak increases relative to the A-line as the N concentration is increased, but decreases with either increasing excitation level or temperature. Figure 8.27 (Ref. 73) shows the typical photo-luminescent emission spectra at 77 K for GaAs0#1Po.9 for two levels of excitation. Shown in Fig. 8.28 (Ref. 73) is the dependence of the energy of the emission peaks over the composition range 0.2 < x < 1.0. For compositions where x < 0.9 separate NN3- and ΝΝχ-pair bands do not seem to exist. It is likely that as x decreases the increase in the As:P ratio and the increased alloy disorder cause these two bands to merge into a single NN-pair band.

Energy (eV) 2.0 2.1

1 ' ' GaAs,_xP (x=0.90)

77°K

5 x l 0 4 W / c m 2 ^

500 ^ ^ " ~ ^ Ν Ν , \ ^

1 1 1

2 .2 1

1

A

2.3 1

\ 1 6.2 6.0 5.8 5.6

Wavelength ( I0 3 A) 5.4

Fig. 8.27 Photoluminescence spectra (77 K) of indirect GaAsi_xP : N (x = 0.90). At low excitation levels, distinct NNl* NN3 a n d A-line emission peaks are observed. At high excitation levels (>104 W/cm2) the recombination shifts to the faster, higher-energy N transitions, and the A-line predominates. (After Holonyak et al. (73).)


Fig. 8.28 Energies of the A-line peak (circles) and NN-pair (squares) photoluminescence in GaAsi-xPx. The energies of the Γ and X band minima are also shown. For crystals with x > 0.90 distinct A-line, NN3 and ΝΝχ emission peaks are observed while at compositions x < 0.80 the NN3 and NN^ peaks merge into a single NN-band. (After Holonyak et al. (73).)


The temperature dependence of luminescence spectra as well as providing information on the basic recombination mechanisms also is of considerable importance from the practical viewpoint of the electroluminescence from LEDs. From this aspect we need to know both the temperature variation of the luminescence efficiency as well as the wavelength dependence of the emission spectra.

The temperature dependence of the electroluminescence of diodes both with and without N doping have been studied. (Ref. 70) The spectra of diodes fabri-cated from nitrogen-free GaAso.4iPo.59 a r^ shown at three different tempera-tures in Fig. 8.29. (Ref. 70) In this Figure the spectra have been shifted so that EgX is aligned vertically. It is seen that the near-band-edge emission processes dominate as the temperature is increased similar to that previously observed for GaP. As the temperature is increased the strength of the donor-acceptor pair band decreases rapidly. The precise transitions which occur at 300 K cannot be resolved - although both free exciton and band-to-

176

Luminescence of Commercial LED Materials

band transitions are likely.

c 0)

Electroluminescence forGaAs0.4,P0.59 without nitrogen S.W.HI- Eqx-LA

T=300eK E..+ LA

I I I I I

J _ J I I I L

6200 5800 Wavelength (Â)

Fig-. 8.29 Electroluminescence of a nitrogen-free GaAsQ.4iPo.59 diode as a function of temperature. The spectra have been shifted so that Egx is aligned vertically, assuming that the temperature dependence of the band gap is the same as for GaP. The diode current is 5 mA. (After Craford et al. (70).)

The electroluminescence spectra of N-doped GaAs0.lPo.9 at three different temperatures is shown in Fig. 8.30. (Ref. 70) In this case the A-line and NN3 increase relative to the NN^ line as the temperature is increased from 77 K to 300 K. However, for diodes with composition in which x < 0.8 or with higher N concentrations, the NN^ band is still significant at room temperature. The

177

http://GaAsQ.4iPo.59


domination of room-temperature emission by near-band-edge processes is related to their shorter recombination lifetimes, and the thermal deionisation of carriers from states involved in deeper transitions.

NN3

J I I i I l I i i 5900 5500

Wavelength( Â)

Fig. 8.30 Electroluminescence of a nitrogen-doped GaAs0.iPo.9 diode as a function of temperature. The spectra have been shifted so that the A-line is aligned vertically, assuming that the temperature dependence of the band gap is the same as for GaP. The diode current is 5 mA. (After Craford et al. (70).)


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180

Luminescence of Commercial LED Materials 18·^

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56. K. Maeda, M. Naito and A. Kasami, Japan, J. Appl. Phys. 8_, 817 (1969).

57. R. Solomon and D. De Fevre, Appl. Phys. Letters, 21, 257 (1972).

58. D. G. Thomas and J. J. Hopfield, Phys. Rev. 150, 680 (1966). See also: D. G. Thomas, J. J. Hopfield and C. J. Frosch, Phys. Rev. Letters, 15, 508 (1965) and for a recent review see D. R. Wight, J. Phys D, lO, 431 (1977).

59. J. Shah, R. F. Leheny and P. D. Dapkus, J. Appl. Phys. 46, 5244 (1975).

60. E. C. Lightowlers, J. Electron. Mater. , 39 (1972). See also: E. C. Lightowlers, J. C. Norton and 0. G. Lorimer, J. Appl. Phys. 45, 2191 (1974).

61. G. B. Stringfellow, M. E. Weiner and R. A. Burmeister, J. Electron Mater. 4, 363 (1975).


62. P. J. Dean and R. A. Faulkner, Phys. Rev. 185, 1064 (1969).

63. P. J. Dean, J. D. Cuthbert and R. T. Lynch, Phys. Rev. 179, 754 (1969).

64. C. J. Frosch, J. Electrochem. Soc. Ill, 180 (1964); also L. C. Luther, ibid, 116, 374 (1969) and A. T. Vink and R. C. Peters, J. Luminescence, 2, 209 (1970).

65. P. J. Dean, C. J. Frosch and C. H. Henry, J. Appl. Phys. 39, 5631 (1968).

66. J. F. Black, T. Sentementes and G. Duggan, J. Electrochem. Soc. 119, No.3 369-372 (1972).

67. J. A. Gaj, A. Majerfield and G. L. Pearson, Phys. Stat. Solidi, B48, 513-518 (1971).

68. C. F. E. Stewart, J. Cryst. Growth, 8_, 259-268 (1971).

69. A. H. Herzog, W. 0. Groves and M. G. Craford, J. Appl. Phys. 40, No.4, 1830-1838 (1969).

70. M. G. Craford, R. W. Shaw, A. H. Herzog and W. 0. Groves, J. Appl. Phys. 43, No.10, 4075-4083 (1972).

71. M. R. Lorenz and A. E. Blackeslee, 4th Int. Conf. on GaAs 1972, pp.106-117.

72. A. Mooradian and H. Y. Fan, Phys. Rev. 148, 873-875 (1966).

73. N. Holonyak, Jr., R. D. Dupuis and H. M. Macksey, J. Appl. Phys. 43, 4148 (1972). See also R. J. Nelson and N. Holonyak, Appl. Phys. Letters, 29, 615 (1976) and M. Gal, T. Gorog and A. Kevesztmry, Solid State Communications, 21, 491 (1977).

9 LIGHT-EMITTING DIODE SURVEY

9.1 INTRODUCTION

Probably the main impetus which has stimulated work on the development of the light-emitting diode is the high conversion efficiencies which have been obtained from GaAs p-n junctions. As early as 1965, a power efficiency of 40% had been achieved (Ref. 1) at low temperatures with a hemispherical-shaped junction. As a result, considerable effort has been made into improving the room-temperature efficiency of GaAs and in particular other materials with wider band gaps to achieve efficient electroluminescence in the visible spectrum. To date the highest efficiency at room temperature for a visible emitting LED is 15% for red emission (Ref. 2) and 0.7% for green emission. (Ref. 3)

In the next two sections of this chapter, the relationship between aspects of LED design and the electro-optic characteristics are discussed. The remainder of the chapter reviews the progress achieved in producing LEDs from different materials. Most emphasis is given to those materials which are used for commercially available devices. However, no attempt is made to cover the detailed characteristics of these lamps. Readers are referred to manufac-turers' catalogues for such information.

9.2 ELECTRICAL CHARACTERISTICS

As discussed in Chapter 5, the electrical characteristics of an LED are similar to that of a non-emitting p-n junction diode. With normal processing the I-V characteristics in the forward direction is dominated by the flow of diffusion current and in the reverse direction by space charge generated and surface leakage currents. Since LEDs are not intended for operation in the reverse direction little attention is paid to obtaining a high reverse breakdown voltage. Manufacturers normally find it sufficient to specify a maximum leakage current at a low reverse voltage (typically ^5 volts). This ensures that there is no surface damage on the dice which might degrade the operation of the LED. By not having to achieve a high breakdown voltage, the device designer can freely choose the impurity concentrations at the junction to give maximum luminescent efficiency. However, if new applications arose where the LED were to replace an existing silicon diode then further develop-ment of the reverse characteristic would be required.

The maximum continuous forward current is governed by the area of the dice, the contact geometry and thermal impedance of the dice to the ambient. The active area of the dice is normally chosen to limit the mean current density through the junction to a given value. As well as determining the surface luminescence, the current density is the major factor in determining the degradation of emission. Although degradation can be caused by surface effects, the major cause is attributed to bulk effects. It has been found

183


(Ref. 4) that at a fixed junction temperature, the current density rather than the ambient temperature is the dominant factor in degradation. For red-emitting GaAsr LEDs the relationship between the lifetime τ (defined as the time taken until the light is reduced to half its initial value) and the current density J is given by

τ « i . (9.1) J

This relationship has been found to hold over the normal operating range. However, for abnormally high currents the lifetime can drop more rapidly.

The effect of the area of the top contact in the forward characteristics of an LED is illustrated in Fig. 9.1. Ideally the contact should obscure a minimal

top contact

- p layer

r-n layer

contact area 'a' contact area Ä

A surface luminance A surface luminance

a) Schematic diagram showing lines of current flow and variation of surface luminance.

voltage

b) Forward biased electrical characteristics.

Fig. 9.1 Effect of top contact area.

Light-emitting Diode Survey 185

fraction of the top emitting face of the LED and still provide a uniform density of current flow across the junction. When the contact area is too small ("a") current crowding occurs around the contact which gives rise both to a high spreading resistance and to a non-uniform surface luminance. In particular, a bright ring of radiation occurs at the periphery of the contact. If, on the other hand, the contact area is too large (contact "A" in Fig. 9.1(a)), uniform luminance may be achieved but total emission reduced. In most commercial devices a more complex contact pattern is employed to obtain a satisfactory compromise between these two extremes.

The effect of series resistance on the I-V characteristic is shown in Fig. 9.1(b). The total resistance rt can be divided into two components: (Ref. 5) (a) a constant resistance r^ and (b) a variable spreading resistance r2 so that rt = r^ + r2· For the particular contact geometry in Ref. 5, it was found that r2 αΙ""η where n = 0.6 - 0.7. Obviously the contact contributes mainly to the spreading resistance.

The extra impedance caused by spreading resistance is of particular importance when the dice is operated in a pulsed mode, as is often the case in alpha-numeric displays (or as discussed in Chapter 6 when used in a laser mode). In this mode of operation, peak currents up to 20 times the maximum continuous current are normal. It is standard practice to ensure that the mean power dissipated does not exceed that corresponding to continuous-current operation. A limit on the series resistance of commercial devices is generally covered by specifying a maximum forward voltage under pulsed operation.

9.2.1 Effect of Temperature

The thermal impedance of an LED determines the rise in temperature of the emitting junction above that of the ambient when the LED is energized. For LEDs of the axial lead frame type described in Chapter 4, a typical value of the thermal impedance between the junction and the leads is 200-300°C/watt. Thus when operating an LED the I-V characteristic is determined not only by the temperature of the ambient, but on the temperature rise due to the power being dissipated at the junction.

An approximate expression for the electrical characteristics of a p-n junction was derived in Chapter 5 to be

J = J d o e x p ( S ) + Jrgoe xp(ST)· (9·2>

This expression includes the diffusion and space charge currents but neglects any series resistance effects. Over the normal operating temperature range -40 to 100°C, both components of current must be taken into account. This is illustrated in Fig. 9.2 in which current J is plotted on a logarithmic scale.


a V aV Total current. J « Jdo exp -*— + Jrgo exp -!—

kT 2kT

T, > T0

forward bias

Fig. 9.2 Effect of temperature on the forward characteristic of an LED.

The temperature dependence of the terms J^Q and JrgQ is mainly determined by

their dependence on the intrinsic density, ni, i.e. J^0 « n^2 and Jrgo α n±,

since intrinsic density is strongly temperature sensitive. This is illus-trated in Fig. 9.2 by plotting the I-V characteristic for two different temperatures. It is evident that the forward voltage drop at the p-n junction decreases as the temperature is increased.

9.3 OPTICAL CHARACTERISTICS

9.3.1 Encapsulation

Although we have described the techniques for encapsulation and its importance in LED fabrication in Chapter 4, its contribution to the optical character-istics will be dealt with in this section. From this aspect, encapsulation of the light-emitting dice serves two main purposes in addition to its protective role: (i) it increases the amount of radiation which escapes from the chip and (ii) it acts as a lens and projects the radiation in the direction required.

Firstly let us consider radiation recombination which occurs in the vicinity of a p-n junction. Before the emission can be observed it must travel through the crystal, during which inevitably some of the radiation is absorbed, and then it must escape. This process of transmission depends upon the refractive index mismatch and the angle which the incident ray strikes the surface. As


illustrated in Fig. 9.3, only rays which are contained within a cone of half-angle q^ can escape. This critical angle qi is given by the Snell relation-ship

n, sin q (9.3)

where n^ is the refractive index of the semiconductor and n2 is the refractive index of the surrounding media. For an encapsulated dice, for which the refractive index is in the range 3-4, the critical angle is very small, 15-20° arc. For the planar junction shown in Fig. 9.3, this means that only a small fraction F(^.25ni~2) o f t h e Ü 9 h t generated does not undergo total internal reflection. (Ref. 6)

n re f rac t ive index = n-

ref ract ive index s n,

Fig. 9.3 Emergence of radiation from the top face of a planar p-n junction.

The two obvious ways of increasing this fraction are demonstrated in Fig. 9.4. The first technique aims at keeping the angle of incidence within q^ by shaping the semiconductor dice into an approximate hemispherical shape. Although some high-power GaAs devices are produced this way, it is an expensive and slow process. The alternative technique is to encapsulate the dice in a transparent media of high refractive index, thus reducing the mis-match. Fischer and Nuese (Ref. 7) have developed a series of arsenic-chalcogen-halogen glasses with refractive indices between 2.4 and 2.9 for use with GaP, GaAsP and AlGaAs diodes. Although by using domes of this type Ladany (Ref. 8) has produced GaAs diodes with a power efficiency of 32% at room temperature, most commercial devices use a transparent plastic with a refractive index of approximately 1.5.


transparent medium ith high

refractive ndex

contact / contact to p-type to n-type

/ contact

to n-type contact to p-type

Fig. 9.4 Techniques for reducing total internal reflection.

The main reason for accepting a lower refractive index material is the ease of manufacture and in particular the importance of moulding the encapsulation to obtain the desired radial distribution of intensity. The standard encap-sulation for the majority of commercial lamps consists of the chip being at the base of a transparent plastic cylindrical body which has a hemispherical lens as shown in Fig. 9.5. Rays which are incident on the walls may either be

MAGNIFIED IMAGE OF DICE

IMAGE PLANE

Fig. 9.5 Standard encapsulation lens of an LED.

internally reflected or transmitted. In general these do not have a signifi-cant effect on the radial distribution. As the dome height, s, is increased the rays incident on the hemispherical dome are refracted more and more into the forward direction. This has the effect of producing a magnified virtual image of the dice and restricting the viewing angle. (Ref. 9) It may be shown that the surface brightness of the dice is independent of its magnifi-cation, M, but that the axial intensity is directly proportional to M. Thus increasing the dome height increases the axial intensity at the expense of restricted viewing.


9.3.2 Effect of Absorption

Up to now we have ignored the effect of absorption on the optical character-istics. As expected it reduces the total light output, although the relative importance of the absorption coefficient of the p- and n-type material may not be immediately obvious. Their effect is demonstrated in Fig. 9.6 which shows the variation of total light output for two different GaAsi-xPx alloys and GaP:N as a function of the junction depth. (Ref. 10) For the GaAs0.6

p0.4 alloy which is grown on to a GaAs absorbing substrate, a gradual decrease in emission occurs as the junction depth is increased. This is attributed to the relatively high absorption coefficient in the diffused p-type layer. It was found that for these LEDs 90% of the radiation was emitted from the top face. However, for the other alloy GaAsQ#15P0.85

:N a n d GaP:N grown on to GaP substrates which are relatively transparent to the generated spectrum, only 30% of this radiation is emitted by the top face. Thus absorption in the top p-layer has considerably less effect and the efficiency is relatively independent of diffusion depth.

*> 3

ex

o σ»

N

"5 i o z

IO

•Θ

•6

4

2

O

Γ /T—

■7f i 1

1 1

GaAs^ Ό

1 1

"ir

s. ~*"N

\ \

\ X

GeA,«p«Î\\ \ \ .

v . _

» 1 1 1 1 1

8 IO 12 14 18 20

Junction depth (microns)

Fig. 9.6 Effect of junction depth on emission from a p-n junction. (After Odell (10).)

Absorption can have a dominant, and if care is not taken, a misleading effect on the emission spectra. This is particularly true for direct-band-gap materials in which the radiative recombination is band-to-band. For such materials there is an overlap of the emission spectra with a rapidly increasing absorption edge. The path length for top face emission is just the junction depth, so that attenuation and shift in emission peak increases with junction depth. The edge emission spectra which is relatively insensitive to junction depth corresponds to a much longer path length so that attenuation and peak shift is dependent upon absorption in the n-type layer. For visible emitting diodes the effect of different peak shifts can manifest itself by small differences in the colour between top and edge emission.



The dependence of LED emission on temperature can be attributed to several mechanisms, e.g. band-gap dependence, variation of radiative and non-radiative mechanisms, changes in the injection efficiency at the p-n junction. Even over the limited temperature range -40 to +100°C, there is no single dominant mechanism which can explain the observed characteristics.

The two principal characteristics in which one is interested from a practical viewpoint are (i) the variation in the emission spectra and (ii) the depen-dence of the electroluminescent efficiency on temperature. The negative temperature coefficient of the band gap will result in a similar dependence of the emission peak for crystals in which band-to-band recombination is the principal radiative mechanism. When, however, the mechanism for radiative recombination involves transitions via deep levels, this temperature depen-dence does not necessarily hold. For example, the peak energy of the red emission from Zn-0 doped GaP is essentially independent of temperature over the normal operating temperature range.

The quantum efficiency of the LED normally drops as the temperature is increased, although it has been found (Ref. 11) that for yellow-emitting GaAso.i5Po.85:N the reverse is the case. More details of the performance of individual devices is given in the following sections.

9.4 GALLIUM ARSENIDE

9.4.1 The Efficient Silicon-doped Diode

The amphoterically silicon-doped diode is the most efficient LED that has been made. It is produced by solution growth using the techniques described in Chapter 3. The change from the p- to n-type occurs during a single epitaxial growth run. At high growth temperatures the silicon acts as an n-type impurity and then as the melt is cooled the epitaxial layer gradually changes over to p-type. Depending upon the growth conditions the emission peak occurs from about 910 to 1020 nm, as compared to 900 nm for the band-to-band recombination which occurs for zinc diffused junctions.

The 32% efficient diodes reported by Ladany (Ref. 8) were achieved by using a low-melting-point glass dome in a diode which had an emission peak at 970 nm. Silicon amphoterically doped solution-grown diodes are also available commercially. For currents up to 100 mA TO-18 can housing is normal. Higher power devices are available using a domed construction in a purpose-built housing. (Ref. 12) This more complex fabrication is reflected in increased cost. The power output of these special devices ranges up to 200 mW.

9.4.2 Other Diodes

Diodes grown with vapour-growth techniques using selenium and zinc to dope the n- and p-type layers (Ref. 13) have power efficiencies in the range 0.1 to 0.5% at room temperature. These show little improvement above the cheapest of all GaAs diodes which are produced by the diffusion of zinc into pulled GaAs crystal, provided that the zinc diffusion is optimised. (Ref. 14) Comparison of the characteristics of silicon-doped and zinc-diffused GaAs LEDs is shown in Fig. 9.7.


~ 4 0

£ 3 0

cu

rre

nt

O

O

Go As : Si and Ga A s *· Z n

■ "χ^ι 1

■ 1 U

■Jl

Ga As:Zn Ga As:Si

10 1-2 voltage (volts)

(a) V - l characteristic

Θ50 9 0 0 950 wavelength (nm)

(b) Emission spectra

GaAs :Si and Ga As

(0

IO 20 30 40 current (mA)

Variation of intensity with current

(d)

■25 O 25 5 0 75 IOO junction temperature (C)

Temperature dependence of output power

9.5

Fig. 9.7 Comparison of electro-optic characteristics of Si-doped and Zn-diffused GaAs LEDs, (b) and (c) are measured at room temperature.

GALLIUM PHOSPHIDE

Liquid-phase epitaxy of GaP has produced the most efficient visible emitting LEDs. Table 9.1 compares these devices with LEDs produced from other materials, and indicates their commercial availability. (Ref. 15) Although not indicated in this table, the GaP LEDs produced by liquid-phase epitaxy are more expensive to produce than those fabricated from epitaxial layers deposited vapour-phase techniques. However, efforts are being directed to reducing cost by (i) more economic use of melt, (ii) improved deposition efficiency, (iii) increasing production capacity and (iv) improving repro-ducibility.

Table 9.1 Current Status of Visible LEDs

(From R. N. Bhargava, IEE Trans. Electron.

Dev. ED-22, No.9, 691 (1975).)

LEDs

Colour

Peak

Luminous

emission

output

wavelength

(lumens/

(nm)

watt)

External

quantum efficiency

Best

(%)

Commercial

(%)

Best

luminous

efficiency

(lumens/

watt)

Commercially

available

GaP:Zn,0

GaP:N

GaP:NN

GaAs0i6P0#4

GaAs0.35p0.65:N

GaAs0.15p0.85:N

Ga0.7A10.3As

In0.42Ga0.58p

SiC

GaN

GaN

GaAsrSi

with YF3YbEr

GaAs:Si

with YF3:Yb:Tn

ZnSe

Red

Green

Yellow

Red

Orange

Yellow

Red

Amber

Yellow

Blue

Green

Green

Blue

Yellow

699

570

590

649

632

589

675

617

590

440

515

550

470

590

20

610

450

75

190

450

35

284

500

-20

420

660

60

450

15

0.7

0.1

0.5

0.5

-0.2

1.3

O.l

0.003

0.005

0.1

O.l

0.01

0.1

2.0-4.0

0.05-0.1

0.05

0.2

0.2

0.05

3.0

4.2

0.45

0.38

0.95

0.90

0.45

0.28

0.01

0.6

0.006

Yes

Yes

Yes

Yes

Yes

Yes

No

No

No

No

No

No

No

No



9.5.1 Red-emitting Diodes

These diodes are produced by LPE growth of n- and p-type layers in a single deposition run. The n-type layer is normally doped with Te or S and the p-type layer is doped simultaneously with Zn and 0. In order to achieve the best efficiency from a p-n junction, both the luminescent efficiency of the p-type layer and the injection efficiency should be maximized. As discussed in Chapter 5 the energy of the radiative transition is just less than 1.8 eV which corresponds to an emission peak at 690 nm. In comparison the energy of the band gap is 2.26 eV. Since the energy of the radiation is much smaller than that of the band gap, absorption effects are small. This allows the light rays to be reflected many times before escaping or eventually being absorbed. The resultant visual effect is for the whole of the dice to be uniformly illuminated. Since the probability for any ray escaping before being attenuated is high, the transfer efficiency is high. This has significantly contributed to the high efficiency which has been obtained from these diodes.

However, it is anticipated that even higher external efficiencies are feasible. This is based upon estimates (Refs. 16,17) for the bulk internal quantum efficiency of (Ζη,Ο)-doped GaP of 30-50% for a free hole concentration of 1-2 x 1θ17 c m - 3 # However, much lower values of luminescent efficiency have been found to exist on the p-side near the junction. The width of the "dead" layer depends upon the growth conditions.

The mechanism of radiative recombination via the Zn-0 complex results in the light output being a non-linear function of current as shown in Fig. 9.8(c). This shows that the emission tends to saturate as the current increases, so that maximum efficiency occurs at low currents. Also shown in Fig. 9.8 is the dependence of the total radiated power as a function of junction temperature. It is noted that the emission drops rapidly as the temperature is increased. However, the emission spectra is not appreciably sensitive to temperature, hence the luminous efficacy of the spectra remains approximately 20 lumens/watt over the temperature range indicated.


4 0 < ε 30 w

c 2 2 0 (_ 3 υ

IO

O

GaP:N

and \

GaP:Zn.O y

/ I J 1

j 1

1 1 \S 1 1 1

IO 2 0 3 0 voltage (volts)

>» C V

c

> w

O

u

c

GaP:N GaP:Zn,0

(a) V-l characteristic

520 560 6OO 640 68O 720 wavelength (nm)

(b) Emission spectra

GaP: Ζη,Ο

IO 2 0 30 4 0 - 40 -20 O 20 40 6O 8O IOO current (mA) junction temperature (C)

(c) Variation of intensity d) Temperature dependence with current of output power

Fig. 9.8 Comparison of electro-optic characteristics of Zn-0 and N-doped GaP diodes. (a), (b) and (c) are measured at room temperature.

9.5.2 Green-emitting Diodes

Efficient emission in GaP occurs by exciton recombination at an N-isoelec-tronic centre. N concentrations of 1-2 x IO19 atoms/cm3 are used in VPE layers, but the lower temperature of the liquid growth limits the N concen-tration to mid-1018 atoms/cm^. Nevertheless it is the diodes made from LPE layers that have produced the highest quantum efficiencies. (Ref. 3) Most commercially available LEDs, however, are produced from VPE material in which the p-type region is produced by zinc diffusion.

Considerable research effort is being aimed at increasing the efficiency of green emission with the major improvements at the present being confined to liquid-phase growth. Since green light is generated on both sides of the junction optimisation of the doping profile on both the p- and n-side is necessary to achieve optimum results. (Ref. 18) Recombination kinetic studies in GaP:N have shown that luminescent efficiencies are at present


limited by the carrier lifetimes that appear to be associated with centres extrinsic to GaP:N systems. Thermally stimulated current studies by Smith et al. (Ref. 19) have indicated that a donor at 0.42 eV deep may act as an efficient trapping centre in p-type GaP:N.

The electro-optic characteristics of a typical commercially available green-emitting N doped LED are compared with those of a red-emitting (Ζη,Ο)-doped LED in Fig. 9.8.

9.5.3 Yellow-emitting Diodes

Increasing the N concentration in the vapour-phase epitaxial material above 2 x 10-*-9 atoms/cm-^ leads to the luminescent peak shifting to longer wave-lengths. This is attributed to the dominant emission changing from N-exciton recombination to an exciton recombination at nearest-neighbour NN pair. Yellow-emitting LEDs are made by standard zinc diffusion techniques into such material (Ref. 20) with N concentrations of approximately 5 x lO1^ atoms/cm^. Typical encapsulated diodes efficiencies are around 0.05%, the best result being about 0.1%.

9.6 GALLIUM ARSENIDE PHOSPHIDE

9.6.1 Effect of Nitrogen

The variation of the band gap of the GaAsi-xPx alloy system has been discussed in Chapter 2. Early measurements on the efficiency of p-n diodes produced by the diffusion of Zn into this material, (Ref. 21) demonstrated that the external efficiency dropped rapidly as x increased beyond 0.3. This variation, shown in Fig. 9.9, is attributed to the transition from a direct-band gap to indirect band at x = 0.44. The breakthrough in increasing the efficiency of the indirect-band-gap alloys is due to the incorporation of N into the lattice. Although the association of increased recombination efficiency in GaP with the incorporation of N was made by Dean et al. (Ref.22) in 1967, it was not until 1971 (Ref. 23) that N was shown to have a similar effect on the full range of GaAs^_xPx materials.

In GaP, the effect of N is to create a trap level at about 10 meV below the band gap. This produces a small shift in the wavelength of peak emission to longer wavelengths. However, as the value of x decreases to 0.6, the effect is to produce a deeper trap closer to a lOO meV. This causes a greater shift in emission peak. Figure 9.10 shows data taken from Craford and Keune (Ref. 24) on the dependence of the emission peak on the value of x for GaAsi_xPx material grown with and without N doping.


1.0

0.1

0.01

300°K

οοοι' ' ' ' ' ' ' ' 1 ·— 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Go As GaP

Fig. 9.9 Dependence of LED emission efficiency on the mole ratio x for GaAs-^^x alloys. (After Herzog et al. (21).)

6 8 0

6 6 0

« Ü 6 4 °

Peak Emission 6 2 a · Wavelength 6oO|-

5 8 0

56ol·

with nitrogen

without nitrogen

•3 ·4 ·5 ·6 ·7 ·β

Alloy Composition x •9 Ι-Ο

Fig. 9.10 Dependence of the diode emission peak on the value of x for GaAsi_xPx material grown with and without nitrogen doping. (After Craford and Keune (24).)

Light-emitting Diode Survey

9.6.2 Effect of Substrate

197

In order to reduce the dislocations due to lattice mismatch GaAs;L_xPx with

x < 0.5 is normally deposited onto GaAs substrates and when x > 0.5 GaP substrates are used. However, the choice of substrate material has a significant effect upon the efficiency of light extraction from the dice. As discussed in Section 9.3.2 the substrate with low absorption to the emitted radiation will result in a higher extraction efficiency. Thus if all other factors are equal we would expect the GaAsi_xPx LEDs with x >0.5 grown on to GaP substrates to give a higher external efficiency than those corresponding to x < 0.5 grown on to GaAs substrates. This, of course, is contrary to the trend shown in Fig. 9.9 for N-free material. When the increase in efficiency due to N doping is taken into account we obtain a variation of luminous efficacy as shown in Fig. 9.11. This graph is based upon measurements on commercially available LEDs. (Ref. 25) The data is split into two curves. (i) LEDs which are made of N-free material grown on to GaAs substrates. These have a maximum luminous efficacy of approximately 150 m£m/watt and correspond to the standard red-emitting material GaAsQ.6p0.4· (*-*■) LEDs which are made of nitrogen-doped material grown on to GaP substrates. These exhibit a maximum luminous efficacy approaching 1 £m/watt for orange/red emission.

with nitrogen : GaP substrates.

680 660 640 620 6 0 0

Wavelength (nm)

580 560

Fig. 9.11 Luminous efficacy of commercially available GaAsi_xPx lamps. (After Hall (25).)

9.6.3 Characteristics of Commercial Devices

The cheapest visible emitting LEDs are those produced from GaAso.6p0.4· These emit around 660 nm and produce the characteristic deep red which is seen in the numeric display of most personal calculators. The nitrogen-doped material is still considerably more expensive. It is used for the "high brightness"


red/orange emission from GaAs0.35P0.65:N a n d f o r yellow emission from

GaAs0.l5p0.85:N-

red emission

orange emission

yellow emission

GaAs.5 R4

GaAs.3 5 P.6 5 :N

GaAs, 15 * 8 5 :N

yellow and orange

I O 2 · 0 3 0

voltage (volts)

a) V - I characteristic

5 2 0 5 6 0 6OO 6 4 0 6ΘΟ 7 2 0

wavelength (nm)

b) Emission spectra

10 2 0 3 0 4 0 - 4 0 - 2 0 O 2 0 4 0 6 0 8 0 IOO

current (mA) junction temperature (°C)

c) Variation of intensity with d) Temperature dependence current. of output power.

Fig. 9.12 Comparison of the electro-optic characteristics of GaAsi_xPx alloy LEDs. (a), (b) and (c) are measured at room temperature.

The electrical and optical characteristic of LEDs fabricated from these materials is given in Fig. 9.12. The increase in band gap as x is increased results in an increased forward voltage of the I-V characteristic. It is also evident that the series resistance of the red-emitting LEDs is consid-erably lower than that of the orange- and yellow-emitting diodes. This can be directly attributed to the lower resistance of the GaAs substrate.

9.6.4 Colour

Variation of the alloy composition of GaAs^„xPx alloys permits a limited range of colours to be obtained. As shown in Fig. 9.13 all the colours are fully


saturated, i.e. they lie close to the periphery of the chromaticity diagram. Also included in this diagram are the colours of emission obtained from GaP and from an antiStokes phosphor described in Section 9.10. It is noted that LEDs cover the signal red and yellow regions, but not the signal green. In fact neither the performance of the green emission of GaP nor the green

GaP:N

GaAs P :N 15 -85

GaP: Zn.O

3 -4 -5 -6

X-coordinate

Fig. 9.13 Colour coordinates of LED emission in GaP, GaAs^_xPx and Er^

+ phosphor-coated GaAs.

LaF3:Er,Yb Phosphor produce a colour within the signal green domain. From Table 9.2, it is evident that a suitable colour may be obtained from AlxIn^_ with high values of x. Comparison of the chromaticity coordinates of red emission from (Ζη,Ο)-doped GaP with that from GaAsQ.6p0.4 confirms that the GaP emission appears as a more orange-red in spite of the fact that the emission peak occurs at a longer wavelength.

Table 9.2 Direct-indirect-gap Crossover for III-V Alloys at 300 K

Energy gap range Crossover

GaA£l_xPx

AlxGa^_xAs

In!_xGaxP

ΑΙχ^Ι-χΡ

ΑΙχΙη^.χΑε

for LEDs (e

1.43 to 2.26

1.43 to 2.14

1.34 to 2.26

1.34 to 2.43

0.35 to 2.14

iV) Energy

1.99* 1.96 1.90* 1.92

(a) Γ-L

2.17

(b) L-X

2.26

2.23*

2.33

2.05

(eV) Composition x

0.45*

0.45

0.31*

0.37

0.63

0.74

0.39*

0.44

0.68

*Theoretical values. Data taken from references 48-51.


9.7 INDIUM PHOSPHIDE

Indium phosphide has a very slightly smaller band gap than GaAs and it should be possible to make LEDs from it which are as efficient as the best ampho-terically doped GaAs diodes. Unfortunately, the search for an amphoteric dopant in InP has so far failed (Ref. 26) but more work needs to be done before it can be confirmed that amphoteric impurities do not occur.

Most of the work done so far has been on solution-grown diodes because of the success achieved with this growth technique for GaAs. Gallium (Refs. 27,28) and tin (Ref. 29) solutions have been used to produce diodes with power efficiencies in the range 0.74 (Ref. 27) to 1.3% (Ref. 30) for two-photon phosphor and optical communication applications. The most efficient diodes were produced with double epitaxy layers doped with Sn and Zn grown on a Sn-doped n-type substrate. The diode emission spectra for a p-side up and a "flip-chip" n-type substrate side up are compared to a GaAs:Si amohoteric diode in Fia. 9.14. (Ref. 30) AbsorDtion in the n-enitaxial laver and the substrate cause appreciable narrowing in the spectrum for the "flip-chip" diode and shifts the peak to higher energies. The two-photon phosphor absorption peak shown in the diagram will be commented on later in this chapter. The diode characteristics of these gallium solution-grown diodes have been fully discussed elsewhere by Hall et al. (Ref. 5) and compared to diffused diodes made in melt-grown material whose maximum efficiency was 0.7%.

cc <

§o|

Û O o

I 302eV

Ga As

. In P P SIDE UP

125 I 30 135 PHOTON ENERGY (eV)

Fig. 9.14 Spectral emission at room temperature from two types of InP diodes. The "flip-chip" diode spectrum is compared to a GaAs:Si diode. The arrow at 1.281 eV indicates the peak of the excitation curve of the LaF3:Yb; Er two-photon phosphor. (After Williams et al. (30).)


9.8 OTHER III-V COMPOUNDS

LEDs can be made from any III-V compound which shows both p and n behaviour. For example, infra-red-emitting diodes have been made from GaSb, InAs and InSb.

GaSb diodes were first prepared from melt-grown substrates by diffusion (Refs. 31-33) and alloying (Refs. 34-36) but more recently solution-grown epitaxial diodes (Refs. 37,38) have received some attention. Using a melt of antimony in gallium, p-type epitaxial layers 100 to 200 microns thick were produced by doping with zinc. The n-type single crystal GaSb substrates were doped with tellurium. The power efficiency of the diodes was 0.1% at room temperature but the dopants and their concentration were not optimised.

InAs diodes have been characterised as a function of temperature from 300 to 2 K. (Ref. 39) Diodes can be prepared by zinc (Ref. 39) or cadmium (Ref. 40) diffusion into melt grown n-type single crystals.

InSb diodes made from diffused (Refs. 41,42) and alloyed (Ref. 43) p-n junctions in melt-grown material have been studied at temperatures below 77 K. Visible yellow/green AlAs diodes have been prepared by zinc diffusion into vapour-grown epitaxial AlAs but the efficiency was only 0.001% at room temperature. (Ref. 44)

Finally, there are three other compounds which should be mentioned from which point contact light-emitting structures or electroluminescent devices have been fabricated but p-n junctions have not been produced. Of these three GaN has received the most attention because of the hope that with such a large energy gap blue, green and red diodes could be fabricated by doping with different impurities. Unfortunately, so far this hope has not been realised as only n-type material has been produced. However, Pankove has succeeded in producing metal-insulator-semiconductor (MIS) structures which emit light in a range of colours with a power efficiency of up to 3 x 10~2% and work at relatively low voltages and currents. (Ref. 45)

Two other compounds in which point contact electroluminescence has been reported are AIP (Ref. 46) and AIN (Ref. 47) although little is known of the detailed properties.

9.9 OTHER III-V ALLOY SYSTEMS

9.9.1 Direct, Indirect-gap Alloys

The search for an alternative to GaAsi_xPx diodes has led to the investigation of four other alloy systems in which a direct-gap semiconducting compound is mixed with an indirect-gap one. These four alloy systems are compared with GaAsi_xPx in Table 9.2. The peak emission energy range for the LEDs made from the alloys is just equal to the difference in the energy gaps of the two compounds and is listed in the first column of the Table. After this the experimental cross-over composition and energy is compared to theoretically calculated ones of Archer. (Ref. 48) These latter values were calculated from the following equation:

E(x) = E + (E.-E )x + 0.3 (χ2-χ)/(θ.5(ΕΛ + ΕΊ fl*5 (9.4)

O 1 O L- O 1 J

where x is the mole fraction of the indirect compound and the indices 0 and 1 are the direct and indirect binary compounds.


The crossover composition was much disputed in the alloy system In^_xGaxP and the various references have been well documented by White. (Ref. 52) This dispute continued until the publication of Pitt et al. (Ref. 50) on some high-pressure Hall effect measurements on vapour epitaxial crystals which show that there are two crossover points because the L conduction minimum crosses both the X and the Γ minimum. Their proposed band structure for In^_xGaxP is shown in Fig. 9.15 (Ref. 50) and the crossover points are given in Table 9.2.

3.0

« 2.5

ö 2.0

0 I n P

-

>

X

L ^ '

Ys^

1

-^Γ' / \ x=0.74

x = 0.63

300K

1 1 1

τ

L

~x

0.2 0.4 0.6 0.8

C o m p o s i t i o n (x ) I n ,_x G a x P

1.0 GaP

Fig. 9.15 Proposed band-gap variation for Ini-jâxP alloys. (After Pitt et al. (50).)

LEDs have been constructed from both direct and indirect gap Ini_xGaxP alloys but the power efficiencies so far achieved are about 0.1% (Refs. 51,53,54) This poor efficiency is undoubtedly due to the difficulties of preparing Ini-jcGaxP because of the phase diagram of the alloy and the lattice mismatch of the layer with the GaP or InP substrate. Grading of the layer before growing a uniform composition alleviates dislocations produced by lattice mismatch but the problem of the large two-phase field of the phase diagram means that a uniform composition throughout the diode is very hard to achieve. In spite of these difficulties lasing action at 2.10 eV at 77 K has been achieved. (Ref. 55)

AlxGai_xAs has received considerable attention because of the single and double heterojunction lasers that have been made with the AlxGai-xAs:GaAs system (see Chapter 6). The material has been developed to such an extent that LEDs made from AlxGai-xAs have been shown to have external quantum efficiencies as high as 13.5% provided that a semiconductor AlxGa^_xAs dome structure of the type shown in Fig. 9.4 is used. (Ref. 56) LEDs using hetero-junctions of GaAs and AlxGa^_xAs have also been made and these have external quantum efficiencies ranging up to 10% when the diodes are coated with hemi-spherical epoxy resin domes. (Refs. 57-60)


Heterojunctions and p-n homojunctions in the GalnAsP, (Ref. 61) GaAlAs-GalnP, (Ref. 62) and GaAlAs-GaAsP (Ref. 63) systems have also been fabricated. In the first of these systems diodes with an external quantum efficiency of 4.7% have been achieved at 300 K in the spectral range 1.0 to 1.2 ym (1 to 1.1 eV (Ref. 61).)

9.9.2 Direct-gap Alloys

Efficiencies of 1% have been achieved in InxGa^_xAs LEDs grown from gallium melts in the standard solution growth way. (Refs. 62,64) The diodes emission was in the region of 1 to 1.1 ym for these InxGa;L_xAs diodes. Similarly diodes emitting at 1.06 ym with an efficiency of about 1% have been fabricated from the GaAsxSbi_x alloy system using one step solution growth with silicon doping. (Ref. 65)

Finally, injection laser diodes emitting at 1.06 ym have been fabricated from the InAsxPi_x alloys. (Refs. 66,67)

9.9.3 Indirect-gap Alloys

Only one alloy system, AlxGa^_xP, has so far been used to make LEDs and very little data was obtained. Over the composition range x = 0 to x = 0.25 the red and green emission from the Zn:0-doped gallium solution-grown epitaxial diodes was only very slightly shifted towards higher energies. (Ref. 68)

9.10 TWO-PHOTON PHOSPHOR DIODES

A two- or three-photon pumping process gives up-conversion from infra-red to green, orange or blue. The two-step absorption process is illustrated in a simplified way for green-emitting Yb:Er-doped phosphors in Fig. 9.16. (Ref.69) The infra-red LED pumps the electrons of the Yb ion into an excited state. This electron then recombines to the ground state and provided there are enough rare earth ions in the phosphor the recombination energy can be used to pump an electron on a neighbouring Er ion into its first excited state. The two first excited-state energy levels of the ions are not exactly equal in energy so the electron stays on the Er ion. If the pumping process is repeated the electron can then be excited up into an even higher energy state from which it can recombine to the ground state and give out green light.

Yb3+ Er3 +

Fig. 9.16 Energy band diagram for anti-Stokes phosphor activated with erbium and sensitized by ytterbium. (After Williams and Astles (69) 1973).)


In practice the energy-level picture for Er is more complicated than that shown in Fig. 9.16 and additional levels are present. This means that several sharp green and red lines are observed. In addition weak three- and four-photon processes occur resulting in blue and UV emission. The colour which dominates can be accurately controlled either by controlling the pumping level by varying the current or the intensity of the LED, or by using different rare earth ion doping levels or by changing the phosphor hosts.

4500 5000 5500 6000

W a v e l e n g t h ( Â )

6500

Fig. 9.17 Emission spectra of anti-Stokes phosphor coated on to an InP diode. The eye-response curve and the GaP:N diode emission curve are shown for comparison.

Figure 9.17 compares a GaP:N green diode emission spectrum with an InP-phosphor-coated diode and the eye-response curve. (The eye response curve is shown in more detail in Fig. 5.12). The room temperature emission curve for the InP diode used for the phosphor diodes has been previously shown in Fig. 9.14. In this figure it was shown that the width of the InP diode spectrum was about half that of the GaAs emission. The peak of the phosphor absorption curve for LaF3:Yb:Er is indicated by the arrow at 1.271 eV. This clearly indicates that neither diode gives a perfect match to the phosphor. Although the GaAs:Si spectrum can be made to peak at 1.271 eV by increasing the silicon doping this also increases the half-width so that the net gain is very small.

Although the multi-colour facility make these phosphor-coated diodes sound very attractive the low infra-red to visible conversion efficiencies of 1% for red, 0.1% maximum for green and 0.01% for blue (Geusic et al. (Ref. 69)) that have so far been achieved has prevented wide commercial exploitation. Only a small number of green-emitting phosphor diodes are currently available on the market.

A full review of phosphor-coated diodes has been given recently by Bergh and Dean (Ref. 70) and the reader is referred to this review for the relevant


references on this subject.

9.11 SILICON CARBIDE DIODES

SiC like GaN has a very high band gap and is a very difficult material to make because of its high melting point, unlike GaN, however, it can be doped both p- and n-type so that blue-, green-, yellow- and red-light-emitting diodes can be formed. (Refs. 71-75) Figure 9.18 compares the blue and the yellow SiC diodes with the green GaP:N, the red GaAs;L_xPx, AlxGai_xAs and GaP:Zn,0. All the curves have been normalised for ease of comparison. The top half of the figure shows the eye response. The SiC blue emission was obtained from one of the most common polytypes called the 6H hexagonal phase or aSiC in which there are twelve atoms per unit cell. The n-region is N doped and the p-region is

4000 5000 6000 7000

4000 5000 6000 7000

λ Â

Fig. 9.18 Comparison of blue and yellow SiC diodes compared with the green GaP:N, the red GaAsi_xAlxAs and GaP:Zn,0. The eye response is shown in the upper half of the figure. (After Lorenz (75) 1969).)

Al doped. The yellow diode shown in Fig. 9.18 was also prepared from 6H SiC by diffusing B or B and Al into N doped n-type crystals. By varying the poly-type, and hence the energy gap, but keeping boron and nitrogen as dopants, the


colour of the diodes can be changed as is indicated in Fig. 9.19 for five polytypes. (Ref. 71) The energy gap for these five polytypes is shown in Table 9.3. (Ref. 71) Also shown in the diode peak energy E m a x . Comparison of these two energy values for all the polytypes shows that there is a fixed difference Eg - E m a x = 0.97 eV. This implies that the same recombination mechanism occurs in all polytypes. The exact nature of this mechanism is still unknown.

M 1.00 0.80 0.70 0.60 0.55 0.50 0.45

I

S. °·5

I 2 3

E (eV)

Fig. 9.19 Emission spectrum of SiC diodes made by doping with boron and nitrogen in five different polytypes. (After Lorenz (75) 1969 .)

Table 9.3 Band gap energies, Eg, and diode spectra peak energy, E m a x , of Various SiC Polytypes

Eg Emax Polytype —

3C 2.39 1.42 8H 2.80 1.83 15R 2.986 2.010 6H 3.023 2.050 4H 3.265 2.300

In spite of the low efficiency of the diodes of about 10~J% a brightness of about 100 ft L (342 cd/m2) can be achieved for the yellow diodes using an applied voltage of 4 volts and a current of 50 mA/mm2. This brightness is possible because of the large bandwidth of the emission.

Photoluminescence efficiencies for the yellow emission in boron- and N-doped


SiC are as high as 10% at room temperature (Ref. 76) so this gives the hope for the future that if all the materials problems in this high melting point, high indirect-band-gap compound can be solved some very useful multi-coloured LEDs would become available.

9.12 TERNARY CHALCOPYRITE SEMICONDUCTORS

Although SiC is still in its infancy as an LED, ternary chalcopyrite semi-conductors have only just been born because the first LED has just been reported. Migliorato et al. prepared the first homojunction by using CuInSe2. (Ref. 77) Their diode characteristic at 300 K is shown in Fig. 20. CuInSe2 crystals grown from the melt were used as the n-type substrate. The electron concentration was about 2 x 1017/cm3 with a mobility of 300 cm3/V s. Diodes were formed by annealing the crystals in Se vapour with argon present as a thermal exchange gas for 1 minute at 600-700°C and then quenching to room temperature. The diode emission spectrum at 77 K is shown in Fig. 9.21 and the band gap is indicated by the arrow at 1.038 eV. (Ref. 77) The peak of the emission spectrum was at 1.34 microns and the internal quantum efficiency was 10% at 77 K and 0.1% at room temperature.

|

\~ ^

< E

L. -1

2

1

- 1

/ / CuLnSe2

/ 300° K

/ . 1 2

V ( V o l t s )

Fig. 9.20 CuInSe2 LED electrical I-V characteristic at 300 K. (After Migliorato et al. (77) 1974).)


Wavelength {μ)

1.20 1.30 1.40

C Φ

o «Λ

I a> a> > o O)

1.05 1.00 0.95 0.90 0.85 Photon energy(eV)

Fig. 9.21 Emission spectrum of the CuInSe2 LED at 77 K. The energy gap of 1.038 eV has been indicated by the arrow. (After Migliorato et al. (77) 1974).)

These experiments on CuInSe2 LEDs were subsequently confirmed by other workers who prepared diffused n-layers into p-type substrates using a cadmium atmosphere (Ref. 78) and also by cadmium ion implantation (Ref. 79) into p-type CuInSe2· Once again the internal quantum efficiency was about 0.1% at room temperature.

Electroluminescence has been obtained from, (a) p-type CuGaSe2 single crystals using minority carrier injection from an indium electrode (Ref. 80), (b) single crystals of AgGaS2 (Ref. 81) and (c) CdIn2S4· (Ref. 82)

Heterojunctions of n-type CdS with p-type CuInSe2 have also been prepared with solar photovoltaic cell applications in mind. Although the solar cell power efficiency was quite high at about 5% the electroluminescence from the diodes at 1.4 microns was relatively weak with an external efficiency of 0.01% at room temperature. (Ref. 83)

9.13 CONCLUSION

There is little doubt that many other types of LEDs will be produced especially those based on heterojunctions and ternary semiconductors. The main criteria which will determine the commercial importance of these devices will be the efficiency which can be achieved in the visible spectrum and their cost. Undoubtedly the key to improved LEDs are advances in crystal-growth techniques in conjunction with the development of techniques for defining the

CulnSe2

77°K

E g ( l . 0 3 8 e V )

208


non-radiative mechanisms.

Presently available devices are designed for low current operation where they achieve very long lifetimes. However, many problems need to be solved before they can be operated at the higher current densities which would be required to produce higher lumen output. In particular this would involve operation at much higher junction temperatures, and as we have seen in this chapter, the emission spectra of most commercially available LEDs is fairly sensitive to junction temperature. This is reflected not only in the total emission but also in the colour dependence.

Nevertheless, as we will discuss in the next chapter, many of the applications of the LED depend upon obtaining an adequate brightness for viewing at reading distance. For these applications considerable success has been achieved over the last few years in reducing the required operating current.

12. Texas Instruments Optoelectronic Data Book (1975).

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16. C. van Opdorp, R. C. Peters, R. L. A. van der Heyden, J. H. T. Hengst and R. K. F. H. Oostindie, Abstract T5, LED Conf., Atlanta, Ga. (1974).

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21° Luminescence and the LED

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Light-emitting Diode Survey

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S. K. Brierley and G. G. Fonstad, J. Appl. Phys. 46, 3678 (1975).

B. Ross, Bull. Am. Phys. Soc. 13, 1640 (1968). See also: B. Ross and E. Snitzer, IEEE J. Quant. Elec. QE-6, 361 (1970).

A. G. Thompson and J. W. Wagner, J. Phys. Chem. Solids, 32, 2613 (1971).

H. Kressel and I. Ladany, J. Appl. Phys. 39, 5339 (1968). See also Reference 66.

E. W. Williams and M. G. Astles, New Electronics, 6_, 27 (May 1973). See also: Reference 37 and J. E. Geusic, F. W. Ostermayer, H. M. Marcos, L. G. Van Uitert and J. P. Van der Ziel, J. Appl. Phys. 42, 1958 (1971).

A. A. Bergh and P. J. Dean, Light Emitting Diodes, Monograph in Electrical and Electronic Engineering, Oxford University Press (1976).

G. F. Kholuyanov, Sov. Phys. - Solid State, 6_, 2668 (1965).

R. W. Brander, GEC Sei. Technol. 32, 15 (1965).

R. W. Brander, Proc. IEE, 116, 329 (1969).

V. M. Gusev, K. D. Demakov, M. G. Kosaganova, M. B. Reifman and V. G. Stolyarova, Sov. Phys. Semicond. 9_, 820 (1975) .

M. R. Lorenz, Trans. Metall. Soc. AIME, 245, 539 (1969). See also: M. Y. Altaiski, V. L. Zuev and E. I. Rybina, Sov. Phys. Semicond. lO, 1052 (1976).;

57.

58.

59.

60.

61.

62.

63.

64.

65.

66.

67.

68.

69.

70.

71.

72.

73.

74.

75.

213


A. J. Harris, Phys. Stat. Sol. 35, 697 (1976); and W. Von Muench and W. Kuerzinger, Solid-State Electron. 19, 871 (1976).

76. R. M. Potter and D. A. Cusano, J. Electrochem. Soc. 114, 848 (1967). See also: A. Suzuki, H. Matsunam and T. Tanaka, J. Electrochem. Soc. 124, 241 (1977).

77. P. Migliorato, B. Tell, J. L. Shay and H. M. Kasper, Appl. Phys. Letters 24_, 227 (1974).

78. Phil Won Yu, S. P. Faile and Y. S. Park, Appl. Phys. Letters, 26, 384 (1975).

79. Phil Won Yu, γ. S. Park, S. P. Faile and J. E. Ehret, Appl. Phys. Letters 26, 717 (1975).

80. C. Paorici, N. Romeo, G. Sberveglieri and L. Tarricone, J. Luminescence, L5, 101 (1977).

81. G. D. Guseinov, S. G. Abdullae, I. G. Aksyanov, T. K. Kasumov and R. K. Nani, Materials Res. Bull. 12, 557 (1977).

82. C. Paorici, C. Pelosi, N. Romeo, G. Sberveglieri and L. Tarricone, Phys. Stat. Sol. 36, K33 (1976).

83. S. Wagner, J. L. Shay and P. Migliorato, Appl. Phys. Letters, 25, 434 (1974). See also: S. Wagner, J. L. Shay and L. M. Shiavone, Appl. Phys. Letters, 29, 431 (1976).

1 0 APPLICATION OF LED PRODUCTS

10.1 INTRODUCTION

The term LED (Light Emitting Diode) is somewhat misused in that although it refers to a rectifying junction which emits light, it is often more loosely used to refer to an LED indicator lamp. This product is also known as: a discrete LED, a solid-state lamp, a semiconductor lamp, etc. The reason for this confusion is partly historic in that many research workers use a transis-tor header for the evaluation of LED die and it was in this housing that the first LED indicator lamps were first sold.

LEDs are used in a wide variety of products, most of which have application in the field of displays. However, it seems that rather than replacing existing products, these new devices tend to be used in new applications. In this chapter we shall divide the LED products into three main headings:

(a) LED indicator lamps,

(b) LED displays,

(c) Optically coupled devices.

We shall discuss the use of LEDs in these products and point out the range of applications for which these products are or could be used.

10.2 LED INDICATOR LAMP

LED indicator lamps fall primarily into two types of construction as illus-trated in Fig. 10.1, i.e. the radial and axial lead constructions. The radial lead type which was one of the first lamps introduced by Monsanto (Ref. 1) owes its popularity to its minute size and low cost. It is mainly suited for mounting on printed circuit boards. The axial lead construction is more versatile in that it is available in a range of diameters and heights and the leads can be wire wrapped. It is also generally supplied with a plastic mounting bush so that it can be used either as a panel-mounted lamp or directly on to a printed circuit board. As discussed previously LED indicator lamps are produced with a variety of plastic encapsulations, and this is important because many panel indicators are sold not only for their technical specifications, but because of aesthetic considerations. The viewing angle for a warning light may in one case be required to be very wide so that it can be seen from a wide range of positions, but in another application, such as on a car dashboard, it may only be required to be seen by the driver.

215


r sigg&^qsg

(a) R a d i a l lead const ruc t ion

Fig. 10.1 Standard types of LED indicator lamps. (Monsanto "GaAsLITE" catalogue, 1973.)

The electrical characteristic of most LED indicator lamps is normally just that of the rectifying junction, i.e. they need a series resistance to limit the current. However, some manufacturers do incorporate a current-limiting device in the lamp so that it can be operated directly from a battery, and hence compete as a direct replacement to a subminiature filament indicator lamp.

10.2.1 Range of Applications

The smallest LED indicator lamps are normally designed for mounting directly on printed circuit boards where the compatibility with transistors and integrated circuits make them an obvious choice for circuit status indicators and fault indicators to facilitate servicing. If the circuit board is positioned closely behind a suitably designed contrast enhancement filter then the circuit board can be hidden and the lamp will only be apparent when energized. This is the basis of a splash-proof display with a number of possible consumer applications, e.g. TV channel-selector indicator.

A slight modification of this idea is the display of a hidden legend. As illustrated in Fig. 10.2 this is the illumination of a translucent letter or sign by an LED which has a specially designed radial distribution to give a uniform surface illumination.

(b) Axial lead construction

Application of LED Products 217

Translucent on film

Diffusing wall

ar illuminator indicator lamps

Fig. 10.2 Use of LED indicator lamps to display "hidden" signs.

As might be expected the first types of equipment in which LED indicator lamps have been used are those which are essentially electronic, i.e. computer peripherals, laboratory test equipment. More recently they have been making an impact in the consumer market with incorporation in hi-fi equipment, quality radios, etc. The attractive volume of the car market which is primar-ily an electro-mechanical industry still relies on the filament indicator. However, with the increasing use of electronics in cars, it is anticipated that significant penetration into this market will occur in the 1980s.

Although neon indicators would be expected to be unaffected by the advent of LED indicator lamps, the two types of lamps are similar in that they both have very long lifetimes and are relatively robust, in some mains powered equipment, manufacturers are making circuit modifications which allow LED indicator lamps to be used instead of neon indicator. A typical example, shown in Fig. 10.3, is the circuit arrangement for emergency lighting. In order to ensure that the batteries are fully charged, the initial circuits include a neon indicator across the mains. However, this may give a false assurance if there were a failure in the transformer or bridge rectifier. More recent circuits include a LED indicator lamp in the dc line as shown in the Figure, and this overcomes the possibility of faulty indication.


LED indicator lamp

rechargeable batteries

o

? Ό

" 1

_jf __nnnnnröin , neon

indicator

emergency fluorescent lamp.

Fig. 10.3 Emergency lighting circuit.

10.2.2 Special Indicators

Although the majority of LED indicator lamps consist of a single LED dice, sometimes with a current-limiting device in series, a number of new develop-ments are worth mentioning. The advent of a range of colours of comparable brightness has led to the development of the dual and multicolour lamps. There are five possible combinations for a dual-colour lamp as shown in Fig. 10.4. These depend upon the isolation and polarity required to energize the two dice. With two dice it is not feasible to place both on the axial centre of the lamp housing so that the radial distribution is affected. (Ref. 2)


Fig. 10.4 Possible configurations of the dual-colour lamp. (After Fitzpatrick and Hall (2) 1973)

Two simple applications of the dual-colour lamp using a Wheatstone bridge are illustrated in Fig. 10.5. The battery voltage monitor shown in Fig. 10.5(a) indicates when the battery voltage exceeds 13 volts or is less than 10 volts. The temperature indicator in Fig. 10.5(b) relies upon the variation of the resistance of a thermistor for changing the balance of the bridge circuit. However, probably the most useful configuration is the common anode one, Fig. 10.4(ii), which can be driven from the complementary outputs of standard TTL circuitry.


?z2 5

(a) Battery voltage monitor 4 8 12 16

Battery volts

thermistor

/

¥ où l«>

I (b) Engine temperature indicator

υ 4

2 0 0 4 0 0 6 0 0 8 0 0 R| Resistance (ohms)

Fig. 10.5 Use of dual-colour lamps in Wheatstone bridge circuits. (After Fitzpatrick and Hall (2) 1973)

Another development of the LED indicator lamp is the battery status indicator which contains both an LED dice and integrated circuit in the epoxy encap-sulation of a standard axial lead package. The integrated circuit can be designed to switch on when the supply voltage exceeds a predetermined voltage. One of the applications for which the device has been used is the rapidly growing electronic camera market. Recent developments in this field have resulted in cameras (Ref. 3) featuring a silicon photodiode light-sensing system,an electronically controlled shutter and LED display. In addition to ensure trouble-free operation such cameras include the battery status indi-cator.

10.3 ALPHA-NUMERIC DISPLAYS

It is in this product that LEDs have made their major impact. In the last few years, they have become known world-wide due to their use as numeric displays in pocket calculators and more recently in digital watches. The format normally used for these applications is the seven-segment display which is designed for the display of numbers although the possibility of producing a very limited number of alphabetic characters is used in the watch display.

The more flexible display which is commonly used to produce the full alpha-numeric range is the 35-dot matrix in which LED die are mounted in a 7 x 5 array as shown in Fig. 10.6(a). In the arrangement illustrated, the n-type substrates of the 7-die forming each column are connected to a column pin (C1-C5) and the top p-diffused layers of the 5-die in each row are connected to the row pins (R1-R7). The circuit diagram of the matrix is shown in

220


Fig. 10.6(b). Thus any particular diode can be energized by passing a current via the line pin through the diode and out of the appropriate column pin. However, only one diode can be energized at a time. Thus a specific character is obtained by sequentially addressing the appropriate diodes at a repetition rate which is sufficiently rapid so that the eye does not detect any flicker. Using this scan addressing technique a number of characters can be arranged in parallel as indicated in Fig. 10.6(b). However, to limit the pulse current in each diode to an acceptable value, the arrangement is normally limited to about five characters.

external connections to n-type substrates of LED die.

Contact metallization

LED dice -

external connections yto diffused p-type 'Tayer of LED

die.

(a) Basic Construction of Alpha Numeric

CI C2 C3 C4 C5

Rl ♦ -

R2 - · -

R3 -*-

R4 -^

R5 -^

R6 -^

R7 -^

γψψγγ VvvvV VVVVV ΥΥΥΫΥ γγψΥΥ VVVVV

(b) Circuit diagram illustrating 7 x 5 matrix.

Fig. 10.6 Alpha-numeric character using 35 LEDs in a 7 x 5 array. (a) Shows the basic construction of the display and (b) shows the circuit diagram of the matrix.

The alternative way of connecting a 35-diode matrix which allows the die to be operated under d.c. conditions is to provide individual metallization to the substrates of each dice. A common positive terminal is obtained by bond


stitching a thin wire to the top face metallization of all the dice. This results in thirty-six individual connections and the method by which the number of external contacts is kept to a minimum is to mount a silicon chip on to the ceramic substrate which acts as a memory/decoder. Moreover, further reduction of the number of interconnections could be made whilst still providing effective d.c. operation by use of an LED with a modified negative resistance characteristic. (Ref. 4)

However, because of the relatively high cost and technical limitations of the electrical drive circuitry associated with the 35-LED array, the most common LED display is that of the 7-segment device. This display is fabricated using two alternative approaches: (i) for very small displays typically h inch high or less a monolithic form is preferred; (ii) for larger character heights this approach is too expensive in material, so that a hybrid construction incorporating either bar segments or square die in a reflector housing is used. These types of LED numeric packages are illustrated (Ref. 5) in Fig. 10.7. A refinement of the reflector housing is the light pipe technique which has resulted in a uniformly illuminated display with a character height of 0.6 inch. The electrical configurations of all the 7-segment displays are similar in that they use either common cathode or common anode connections. Individual displays are normally driven under d.c. conditions using a TTL decimal decoder the input to which is in binary coded decimal. This is illustrated in Fig. 10.8. The current drive per segment is typically 25 mA for character heights of 0.3 inch and greater. This can be a considerably current drain for portable displays which utilize a large number of display elements and explains the choice of small characters using a monolithic construction is portable displays where battery life is important. These small displays are normally produced in enstackable packages which contain 3, 4 or 5 digits. The displays are designed for strobing, which is a drive method that allows timesharing so that only 8 + N lead connections need be made to a display cluster for N characters. A more detailed discussion of the circuitry is given in manufacturers catalogues. (Ref. 6)


n-type GaAsP CHIP Circuit board \ \

^Metallization (b)

Plastic overlay Metallized reflector cavities

LED's

Circuit board Plastic overlay

Diffuser plate

LED Effective

bar height

(c) (d)

Fig. 10.7 Schematic drawing of two type., of LEü numeric packages. (After Craford and Groves (5) 1973)


Q . < _ / <

Vcc

RANGE SWITCH j . J_ J__ J . J . _1 ί RBO cj

RB Vcc

A B C D E DECODER/DRIVER

|__ MSDQ47 4

BCD INPUT V E S T 0 " 0 "

MAN 64A

Fig. 10.8 Typical 7-segment display with drive circuitry. (After Monsanto "GaAsLITE" catalogue, 1973.)

10.3.1 Application of LED Displays

The first major application of LED displays has been the personal calculator which uses typically eight or more seven segment displays. For the desk calculator, which has grown enormously in sophistication over the last few years, the 35-diode array is often used especially when alpha-characters are required. The main competitor to these displays has been the plasma display panels. The self-scanning d.c. excited plasma panel (Burroughs Corporation) is normally used for alpha-numeric displays whereas the a.c. excited panel (Owens-Illinois), used mainly for numeric displays, has good threshold characteristics for matrix address and inherent long-term memory. (Ref. 7)

More recently there has been increasing interest in using LED displays in the all electronic watch. Although there is a large commercial potential in a world-wide annual sale of 250 million watches, the electronic watch is having to compete with a well-established industry. By comparison the pocket calculator came along to fill a relatively empty gap, a large portion of the demand was due to its novelty and convenience. The two competing displays for the electronic watch are the LED and liquid crystal 7-segment formats. Although the power available for the pocket calculator is limited by reason-able battery size (normally 9 volts) a mains adaptor is often used. For the digital watch, however, the power available is much less and, of course, there is no possibility of a mains adaptor! Where LED displays are used in watches, they are only energized and illuminated on demand, whereas the liquid crystal display does not produce light, and therefore consumes much less power. However, it cannot be read in the dark since it relies on reflecting back ambient light. A further factor (Ref. 8) in the rivalry between the LED and the liquid crystal displays is the choice of electronic logic (i.e. comple-mentary MOS or injection) which will be used for the watch circuitry.

LED numeric displays are now well established in the test equipment market. Here there has been some reluctance for a change from the analogue to digital


format. However, the increasing accuracy which electronic meters can produce means that the numeric display is the only way of exploiting this accuracy.

10.3.2 Bar Graphs

Although the 7-segment and 7 x 5 array are the major LED display products there is an increasing effort to produce a suitable bar graph format which may be used as a quantitative analogue display. It is argued (Ref. 9) that such displays combine many of the best features of electromechanical analoque meters and electronic digital displays. They are not sensitive to mounting position and offer reasonable resolution and high accuracy.

A typical circuit which uses a 52-LED array is shown in Fig. 10.9. The LED array is fed by a resistor-ladder network and operational amplifiers which act as comparators. Competing for this market are the gas plasma displays which require simpler electronics and liquid crystals with their low power consump-tion.

330 Ω I0kü| 1 f6V ^ Ψ WNr-i-

+ 6VC

Ι Ν 7 5 2 Δ Τ adjust ^200 ß

♦ 5V reference

Common"

Over-^range

100

H98

psÇseon

\^-e^^ -I ^1 + 6 ν 560ΩΦ

Under-range

Fig. 10.9 Bar graph circuit using 52 LEDs (Simpson Electric Circuit). (After Santoni (9) 1976)

10.4 Optically Coupled Devices

The compatibility of the emission from GaAs (and GaAsP) LEDs with silicon p-n junction detectors has resulted in a number of new devices. Probably the most well established is the optically coupled isolator which has a basic


circuit shown in Fig. 10.10(a). It consists of a LED dice which is mounted in a dual in-line package so that its emission falls upon a silicon phototrans-istor (Fig. 10.10(b)). This combination has high speed and reliability and provides electrical isolation of typically up to 2.5 kV.

detector output

rêi m m m

!

,, ,ΓΗ, ,, LU LJU LaJ LU Input

(a) Basic Circuit of Optically Coupled Isolator

LED emitter

(b) Physical construction

Fig. 10.10 Dual-in-line construction of optically coupled isolator.

The device offers the facility of providing circuit isolation in order to prevent ground loops and common mode signals. Before the advent of LEDs this had been achieved by the use of relays and transformers - both low-speed devices and incompatible with modern logic circuits. However, although the optically coupled isolator is well suited for use in high-speed digital logic the non-linearity of its small signal transfer characteristics is far from ideal for analogue applications.

Another device which uses the GaAs LED and Si phototransistor is the opto-electronic read head for punched cards and tape. (Ref. 10) In this applica-tion it is found that the LED emitters have several important advantages over filament lamps - e.g. reliability, stability of radial distribution (filament sag can lead to errors in focusing) and vibration resistance. In addition, LEDs tend to require much lower power than the filament lamp resulting in much less heat dissipation in the head.

A further application of optically coupled devices which has aroused some interest in the auto market is a modification of the car ignition system. As early as 1972 (Ref. 11) trials have been made on a comparison of the conven-tional electromechanical ignition system with a kit which incorporates a GaAs LED emitter and phototransistor. Although worthwhile improvements were found in petrol consumption, easier starting and smoother running, it has not yet been adopted by manufacturers in production models.


10.4.1 Laser Diodes

Many of the applications of p-n junction lasers can be classified under the broad heading of optically coupled devices. In comparison with normal LEDs which operate with spontaneous emission laser diodes exhibit two major advan-tages. Firstly, laser diodes can be modulated to higher frequencies (typically 1 GHz for a double heterojunction diode compared to the best LEDs which are limited to about 200 MHz) and secondly, they inherently produce a narrow emission beam. Although silicon phototransistors or avalanche diodes may be used as detectors, the optimum combination of low noise, linearity and high-frequency response make p-i-n photodiodes the most suitable detector for p-n junction lasers.

Probably the most important application for which laser diodes are being developed is in the field of fibre-optic communications which promises to be a practical alternative to copper wire in the 1980s. Optical fibres offer large bandwidths for multichannel transmission and, with lower losses, repeaters can be spaced more widely than is possible with present systems. Both single and double heterojunction GaxAli_xAs laser diodes are used for fibre-optic communications although only the double heterojunction laser gives continuous or high-duty cycle operation at room temperature. The peak emission wavelength of these laser diodes can be varied from 850 nm to 780 nm by increasing the aluminium content with no significant change in device efficiency (10% typically). This allows the system designed to match the fibre and source for minimum attenuation. (Ref. 12)

As well as the inherent narrow emission beam from a laser diode being used to advantage to obtain good coupling with optical fibres this property makes it suitable for line-of-sight communications. When used in the pulsed mode, emission can be readily detected up to 800 m away. This offers a number of applications such as burglar alarms, safety barriers and so on.

10.5 FUTURE PROSPECTS

In this chapter we have discussed the main products currently available which incorporate the LED. The most important product of the present from a commercial viewpoint is the 7-segment display. It has been the commercial interest in the calculator market which has provided the stimulus and funds for the industrial research which has resulted in the rapid advances in LED technology.

Continuing research in both industry and university should provide

(i) further improvements in the efficiency of existing materials;

(ii) efficient new materials which extend the range of colours with the possibility of blue and white emission;

(iii) devices which incorporate logic, storage and more complex information processing into the display itself.

With the prospects of these developments it is envisaged that LED products will continue to make an impact in a wide range of applications. The penetra-tion into many markets is still at its infancy and it is anticipated that these will grow as the use of sophisticated electronics circuitry is adopted. Typical examples of the market for the LED indicator lamp and the LED alpha-numeric displays are given in Tables 10.1 and 10.2.


Table 10.1 LED Indicator Lamp Applications

1. Car, plane, space station dashboard

2. Mains on/off - domestic and commercial

3. Overload

4. Security monitor

5. Computer P/C board failure

6. Door opener

7. Counting and sizing objects and people

8. Toys

9. Optical communications - short range

10. Charge injection into integrated circuits

11. Opto-isolators

12. Card and tape readers

13. Optical choppers

14. Multi-colour function indicators

15. Record-player track selectors

Table 10.2 LED Numeric and Alphanumeric Display Applications

1. Electronic instruments

2. Calculators

3. Watches and clocks

4. Phones

5. Cars and aeroplanes

6. Airport, railway, bus and petrol stations

7. Newsboard displays

8. Hotels

9. Toys

228

Application of LED Products

REFERENCES

1. Monsanto "GaAsLITE" catalogue (1973).

2. J. R. Fitzpatrick and R. Hall, Proc. Int. Microelectronics Conf., Brighton, England, 1973, publ. Kiver Comm. Ltd. (1973) p.105.

3. R. Schneiderman, Electronics, 48, No.17, 74 (1975).

4. A. R. Peaker, Int. Conf. Alpha Numeric Displays, Paris, 1973, publ. Sitecone, Dieppe, France (1973).

5. M. G. Craford and W. 0. Groves, Proc. IEEE, 61, No.7, 801 (1973).

6. Hewland Packad Components: Appl. Note 937.

7. Eugene I. Gordon, Proc. IEEE, 61, No.7, 807 (1973).

8. G. M. Walker, Electronics, 49, No.12, 91 (1976).

9. A. Santoni, Electronics, 49, No.15, 114 (1976).

10. Texas Instruments Optoelectronics Data Book.

11. Luminitition, 77-85, Newington, Causeway, London.

12. W. S. Boyle, Scientific American, 40 (August 1977).

229

AUTHOR INDEX

Abdullae, S. G. 214 Abrahams, M. S. 180 Adams, I. 212 Adams, M. J. 81, 94 Aksyanov, I. G. 214 Alferov, Zh. I. 104, 213 Allen, J. W. 141, 181 Altaiski, M. Y. 213 Amelek, N. 213 Ambridge, T. 140 Amick, J. A. 46, 50 Andreev, V. M. 104, 213 Anisimova, I. D. 211 Antypas, G. A. 42, 50 Appert, J. R. 210 Archer, R. J. 201, 210, 212 Arx, von H. R. 71 Ashen, D. J. 179 Ashford, A. 140 Astles, M. G. 46, 50, 203, 210,

211, 213

Backrach, R. Z. 210 Backmann, K. J. 211 Bardeen, J. 38 Baroev, T. R. 212 Beattie, A. R. 151, 157, 179 Bebb, H. B. 107, 140, 179 Bell, R. L. 211 Beneking, H. 213 Bennett, H. E. 37 Benoit a la Guillaume, C. 212 Bergh, A. A. 63, 71, 166, 180,

204, 213 Bergstresser, T. K. 11, 12, 37 Berkeyheiser, J. E. 212 Bhargava, R. N. 168, 181, 192, 210 Biard, J. R. 2, 5 Birbeck, J. C. H. 141 Black, J. F. 169, 182 Blackeslee, A. E. 171, 182 Blacknall, D. M. 50, 161, 179 Blakemore, J. S. 151, 179

Blanc, J. 59, 71 Blazey, K. VI. 37 Blom, G. M. 211 Bogardus, E. H. 179 Bois, D. 212 Booker, G. R. 141 Boyle, M. H. 104 Boyle, W. S. 229 Blum, J. M. 63, 64, 71 Brander, R. W. 213 Braslav, N. 71 Braun, S. 141 Braunstein, R. 3, 5, 37 Bresse, J. F. 141 Brierley, S. K. 213 Broder, J. D. 5 Brooks, H. 31, 38 Brown, R. E. 210 Buehler, E. 211 Buiocchi, C. J. 180 Burgess, M. 141 Burmeister, R. A. 38, 181 Burnham, R. D. 212 Burns, G. 5, 104 Burstein, E. 21, 37

Calawa, A. R. 211 Callen, H. B. 38 Calverley, A. 116, 140, 181 Campbell, J. C. 37, 104, 212 Cardona, M. 37, 38 Carlson, R. 0. 5, 104 Carr, W. N? 2, 5, 210 Caruso, R. 181 Casey Jr., H. C. 17, 22, 23, 36, 37,

41, 50, 51, 60, 71, 114, 115, 116, 118, 119, 140, 141, 212

Cauge, T. P. 71 Chang, L. L. 180 Chapman, R. A. 147, 179 Chatterjee, P. K. 179 Chicotka, R. J. 212 Cho, A. Y. 51

231

232

Clark, H. A. 71 Cochran, W. 25, 26, 38 Cohen, M. L. 11, 12, 37 Coleman, J. J. 104, 212, 213 Craford, M. G. 33, 37, 38, 173,

174, 177, 178, 182, 196, 210, 211, 223, 229

Cunnell, F. A. 59, 71 Cusano, D. A. 140, 152, 154, 180,

214 Cuthbert, J. D. 182

D'Asaro, L. A. 104 Dapkus, P. D. 181 Davidyuk, N. Yu. 213 Day, G. F. 71 DeFevre, D. 181, 210 De Winter, J. C. 213 Dean, P. J. 21, 37, 141, 165, 166,

179, 181, 182, 195, 204, 211, 212, 213

Demakov, K. D. 213 Dierschke, E. G. 212 Dill, F. H. 5, 104 Dingle, R. 49, 51, 140 Dittmann, R. 140 Domenico, A. J. 71 Downton, K. J. 71 Duetsch, T. 211 Duggan, G. 182 Dumke, W. P. 5, 104 Dunse, J. U. 179 Dupuise, R. D. 182 Dyment, J. C. 104

Ehret, J. E. 36, 214 Eliseev, P. G. 103, 104, 212 Ellis, Jr., R. C. 211 Enstrom, R. E. 210 Erginsoy, C. 38 Ermanis, F. 179 Esaki, L. 180 Evstropov, V. V. 211 Ettenburg, M. 51, 180, 212 Evans, J. A. 16, 37

Faile, S. P. 36, 214 Faktor, M. M. 140 Fan, H. Y. 172, 182 Faulkner, R. A. 181, 182 Fenner, G. E. 5, 104 Fershenyon, M. 181 Fischer, A. G. 187, 210 Fitzpatrick, J. R. 71, 210, 220,

229 Fonstad, G. G. 213

Author Index

Forbes, L. 141 Foy, P. V7. 104 Frosch, C. J. 37, 181, 182 Fuller, C. S. 179

Gaj, J. A. 182 Gai, M. 182 Gannon, J. J. 210, 212 Garbuzev, D. Z. 104, 213 Gatos, H. C. 108, 140 Gershenzon, M. 211 Geusic, J. E. 204, 213 Gilbert, S. L. 212 Giles, P. 140 Gillett, C. M. 37 Glinchuk, K. D. 180 Goersch, H. 140 Gooch, C. H. 59, 71, 95, 104 Goodman, C. H. L. 3, 5 Goodwin, A. R. 104 Gopinath, A. 140 Gordon, E. I. 229 Gorog, T. 182 Graman, W. 179 Grant, P. R. 113 Greene, P. E. 112, 140, 179 Grimmeis, H. G. 141, 181, 212 Gross, E. F. 179 Grote, N. 203 Groves, W. O. 38, 182, 210, 211, 212,

223, 229 Gunn, J. B. 71 Guseinov, G. D. 214 Gusev, V. M. 213

Hackett, W. H. 94, 141, 181 Haga, E. 21, 37 Hahn, D. 140 Haisty, R. W. 212 Hakki, B. W. 212 Hall, Jr., H. T. 38 Hall, R. N. 5, 104, 197, 210, 211,

219, 220, 229 Harris, A. J. 214 Harris, J. S. 71 Harrison, J. W. 38 Hart, P. B. 210 Hauser, J. R. 38 Hawrylo, F. Z. 179, 180, 213 Hayashi, I. 51, 102, 104, 179 Hayer, T. J. 210 Heinke, W. 140 Herbert, R. A. 4 Henderson, E. 51 Hengst, J. H. T. 210 Henry, C. H. 181, 182, 210

Author Index 233

Herbert, R. A. 5 Herzog, A. H. 38, 152, 179, 182,

196, 210, 211 Heyden, R. L. A. van der, 210 Hill, D. E. 38, 156, 157, 180 Hilsum, C. 3, 5, 36, 50, 140, 179 Hitchens, W. R. 104, 212 Holonyak, N. Jr., 37, 104, 175,

176, 182, 212, 213 Holt, D. B. 140, 141 Hopfield, J. J. 37, 167, 181 HÖrl, E. M. 140 Hsieh, J. J. 104 Hunter, D. R. 122, 123, 141 Hurle, D. T. J. 179 Hwang, C. J. 153, 179, 180

Ikeda, K. 213 Illegems, M. 49, 51 Imenkov, A. N. 211 Ishii, M. 213 Ito, A. 213 Iwata, M. 51

Jacob, G. 212 Jayson, J. S. 94 John, G. 180 Johnson, F. A. 25, 26, 38 Jones, B. F. 71, 210 Jones, G. A. C. 140 Jonscher, A. K. 98, 104

Kadhim, M. A. H. 59, 71 Kaiser, R. H. 114, 115, 116, 118,

119, 140 Kaminskii, A. S. 211 Kaminsky, G. 211 Karnaukhov, V. G. 211 Kasami, A. 181 Kasper, H. M. 36, 50, 214 Kasumov, T. K. 214 Kerps, D. 71, 210 Keune, D. L. 37, 196, 211, 212 Kevesztmry, A. 182 Keyes, R. J. 5, 104 Kholuyanov, G. F. 213 Kimiva, H. 21, 37 Kimura, S. 181 Kingsley, J. D. 5, 104 Kischio, W. 212 Klick, C. C. 180 Klohn, K. L. 62, 71 Koelsch, H. 3, 5 Kohisa, T. 210 Konagai, M. 213 König, V. 180

Kosaganova, M. G. 213 Koyama, S. I. 210 Krag, W. E. 5, 104 Kravchenko, A. F. 213 Kressel, H. 48, 51, 104, 179, 180,

210, 213 Kröger, F. A. 180 Kryukova, I. V. 211 Kudman, I. 180 Kuerzinger, W. 214 Kulymanov, A. V. 211 Kung, J. K. 180 Kurtz, S. K. 168, 181 Kyser, D. F. 141

Lacey, S. D. 23, 24, 37 Ladany, I. 38, 180, 187, 190, 210,

213 Lafeuille, D. 141 Lampert, M. A. 212 Landsberg, P. T. 37, 81, 94, 151,

157, 179 Lang, D. V. 127, 128, 129, 141 Langmann, U. 180 Larionov, V. R. 213 Lasher, G. 5, 104 Lavallard, P. 212 Lavine, M. C. 108, 140 Lax, B. 5, 104 LeFur, P. 179, 180 Lee, M. H. 104, 212 Lefevre, H. 141 Leheny, R. F. 181 Lideikis, T. P. 211 Lightowlers, E. C. 181 Lin, W. N. 140 Lind, T. A. 104 Linden, K. J. 213 Lockwood, H. F. 51 Logan, R. A. 210, 212 Londley, W. T. ,38 Lorenz, M. R. 171, 172, 182, 205,

206, 212, 213 Lorimer, 0. G. 181 Lucovsky, G. 180 Ludowise, M. G. 104, 213 Luther, L. C. 182 Lynch, R. T. 182

Mabbitt, A. W. 212 McFarlane, S. H. 180 Macksey, H. M. 182 McLevige, W. V. 179 McWhorter, A. L. 5, 104 Maeda, K. 110, 140, 181 Majerfield, A. 182

234 Author Index

Makhov, A. F. 140 Marcos, H. M. 213 Maronchuk, Yu. E. 213 Martinez, A. 213 Matsunam, H. 214 Mellichamp, J. W. 212 Melngailis, I. 211 Melta, R. R. 71 Mettler, K. 180 Migliorato, P. 36, 50, 207, 208,

214 Miller, E. A. 212 Milnes, A. G. 141 Minagawa, S. 212 Mischel, P. 213 Monemar, B. 181 Mooradian, A. 172, 182 Mooser, E. 36 Morgan, T. N. 162, 180 Moriijumi, T. 179 Morozov, E. P. 104 MÜgachl, E. 140 Muir, M. D. 113, 140 Mullin, J. B. 179 Murakami, Y. 13, 51

Nag, B. R. 140 Nahory, R. E. 213 Naito, M. 181 Naka, S. 13, 51 Nani, R. K. 214 Nannichi, Y. 71 Nasledov, D. N. 5, 104 Natasuka, H. 63, 71 Nathan, M. I. 5, 104 Nelson, H. 47, 48, 51, 179, 180 Nelson, R. J. 1Q2 Nordheim, L. 14, 37 Norton, J. C. 181 Noyce, R. N. 77, 94 Nuese, C. J. 104, 180, 187, 210,

212

Odell, E. C. 60, 63, 71, 189, 210 Olofsson, G. 141 Olsen, G. H. 104 Onton, A. 212 Ooostindie, R. K. F. H. 210 Opdorp, C. van, 210 Osamura, K. 13, 51 Ostermayer, F. V7. 213

Paduchikh, L. I. 211 Pamplin, B. R. 3, 5, 36, 45, 50 Panish, M. B. 17, 37, 60, 71, 102,

104, 180, 212

Pankove, J. I. 37, 101, 104, 201, 212 Paola, C. R. 63, 71 Paoli, T. L. 104 Paorici, C. 214 Park, Y. S. 179, 214 Pauling, L. 9, 36 Paxman, D. H. 141 Peaker, A. R. 210, 229 Pearson, G. L. 71, 182 Pearson, W. B. 36 Pelosi, C. 214 Penning, D. M. 71 Peters, R. C. 1R2, 210 Phelan, R. J. 211 Philipsborn, H. von 180 Phillips, C. M. 104 Phillips, J. C. 36 Pike, E. R. 141 Pilkuhn, M. H. 102, 104, 154, 180 Pitt, C. W. 51 Pitt, G. D. 202, 212 Pokrovskii, Ya. E. 211 Pollack, M. A. 213 Pollak, F. H. 38 Porteous, P. 140, 141, 210, 211 Portnoi, E. L. 104 Potter, R. M. 214 Prokhorovich, A. V. 180

Oueisser, H. J. 140, 152, 179, 180 Ouist, T. M. 5, 104

Rabenau, A. 212 Read, W. T. 77, 94 Rediker, R. H. 5, 104, 211 Rehn, V. 16, 37 Reifman, M. B. 213 Reinhart, F. K. 104 Reuter, W. 212 Richnoan, D. 212 Rhode, D. L. 33, 38 Ripper, J. E. 104 Rogachev, A. A. 5, 104 Rogers, K. T. 211 Romeo, N. 214 Roosbroeck, W. van 74, 81, 94 Rose-Innes, A. C. 3, 5, 36, 50, 179 Rosenweig, W. 79, 80, 94, 96, 104 Rosier, L. I. 141 Ross, B. 213 Rsoytoczy, F. E. 179 Rumyanster, D. M. 213 Rupprecht, H. 50, 102, 104, 180 Rutz, R. F. 212 Rybina, E. I. 213 Ryvkin, S. M. 5, 104

Author Index 235

Safarov, V. I. 179 Sah, C. T. 77, 78, 94, 141 Santoni, A. 225, 229 Sasaki, A. 51 Sayed, M. M. 141 Sberveglieri, G. 214 Schairer, W. 179 Sehneiderman, R. 229 Schul, G. 213 Schulman, J. H. 180 Schulz, M. 41 Schumaber, N. S. 210 Schwarz, R. F. 180 Schwatz, B. 179 Scifres, D. R. 212 Sedov, V. E. 179 Sell, P. D. 37 Selway, P. R. 104 Sentementes, T. 182 Séraphin, B. 0. 18, 37 Shah, J. 181 Shaklee, K. L. 37, 38 Shaw, R. W. 182 Shay, J. L. 9, 36, 43, 44, 50,

211, 214 Shen, C. C. 104 Shernyakov, Yu. M. 211 Sherstyak, 0. V. A. P. 213 Shiavone, M. 214 Shih, K. K. 63, 64, 71 Shintani, A. 212 Shockley, W. 38, 74, 77, 81, 94,

96, 104 Sigai, A. G. 212 Sinkaev, N. V. 212 Sippel, R. F. 140 Smith, B. A. 43, 50 Smith, B. L. 195, 210 Smith, F. G. H. 50, 211 Smith, R. A. 27, 29, 37 Snitzer, E. 213 Solomon, R. 181, 210 Soltys, R. J. 5, 104 Spitzer, W. G. 141, 180 Starkiewicz, J. 181 Stevens, E. H. 38 Stewart, C. F. E. 182 Steyn, J. B. 140 Stillman, G. E. 32, 33, 38 Stocker, H. J. 179 Stolyarova, V. G. 213 Stone, L. E. 212 Strain, J. R. 63, 71 Straples, J. L. 71 Stringfellow, G. B. 33, 34, 35,

38, 59, 71, 112, 140, 167, 181

Sturge, M. D. 179 Sumski, S. 104 Suzuki, A. 214 Suzuki, S. 210 Svistunova, K. I, 211 Swann, R. C. G. 71

Takagi, T. 51 Takahashi, K. 179, 213 Tanaka, T. 213, 214 Tarricone, L. 214 Tasch, Jr., A. F. 141 Teer, D. G. 51 Tell, B. 36, 50, 214 Thiel, A. 3, 5 Thomas, D. G. 21, 37, 167, 181 Thompson, A. G. 14, 15, 25, 37, 50,

213 Tietjen, J. J. 46, 50 Trofim, V. G. 104 Trumbore, F. A. 36, 41, 50, 181 Trussler, J. W. A. 141 Tsarenkov, B. V. 5, 104, 211 Tsu, R. 180 Tuck, B. 59, 60, 71, 155, 180

Uemura, Y. 51

Van der Ziel, J. P. 213 Van Uitert, L. G. 213 Van Vechten, J. A. 11, 14, 36 Varga, A. J. 180 Varshni, V. P. 75, 94 Vieland, L. J. 180 Vink, A. T. 182 Von Muench, W. 214 Von Neida, A. R. 181 Vorobkalo, F. M. 180 Vovnenko, V. I. 180 Vyas, M. K. R. 212

Wagner, J. W. 50, 213 Wagner, S. 214 Walker, G. M. 229 Wardinger, L. 62, 71 Warner, J. 16, 37 Warschauer, D. M. 211 Wecht, K. W. 37 Weiner, M. E. 181 Weisberg, L. R. 38, 59, 71 Weiber, B. 181 Welker, H. 3, 5 Wernick, J. H. 9, 36, 43, 44, 50 Westgate, C. R. 141 Whelan, J. M. 141 White, A. M. 126, 127, 140, 144, 179

236 Author Index

202, 212 White, H. G. 210, 212 Wiegmann, W. 210, 212 Wight, D. R. 116, 119, 120, 121,

140, 141, 181, 210 Williams, E. W. 16, 36, 37, 50,

107, 140, 147, 152, 158, 161, 179, 180, 200, 203, 210, 211, 213

Williams, F. 181 Witting, D. B. 140 Wittry, D. B. 141 Wolfe, C. M. 32, 33, 38 Wolff, G. A. 3, 5, 212 Woodall, J. M. 211

Woolley, J. C. 14, 16, 36, 37, 43, 50

Wright, P. D. 213

Yakovlev, Yu. P. 211 Yakusheva, N. A. 213 Yamanda, I. 51 Yee, S. S. 38 Young, M. L. 212, 141 Yu, P. W. 36, 179, 214 Yungerman, V. M. 211

Zeiger, H. J. 5, 104 Zhilagaev, Yu. V. 104 Zuev, V. L. 213

SUBJECT INDEX

Absorption 18, 74, 83 direct gap 18-20 doping effects 21-23, 156, 157 impurity 145 in an LED 189

Acceptors 142-145 Activation energy 142-145 AgGaS2 208 AgInTe2 8 AlAs diode 201 AlGaAs band gap variation 199 laser 102, 103 LED 202

AlGaP diode 203 AlInAs 199 AllnP 199 Alloys-III-V 14, 42, 45 diodes 201-203

A1N 201 AIP 201 Alpha-numeric displays 220-228 applications 224-228

Amphoteric Si doped GaAs diode 190 Auger recombination 72, 80, 81,

149-151, 157

Boltzman factor 89 Bonding covalent 8 ionic 8 neutral 8 tetrahedral 6

Brillouin Zone 12

Capacity measurement 124-129 electrical injection 127-129 optical injection 125-127

Capture cross section 78-80 Carrier concentration 27-29 quenching 151-158

Carrier generation 74, 78, 83 Cathodoluminescence 113-124

absorption effects 116 and LED efficiency 119-121 carrier concentration measurement 117

carrier concentration quenching 154 diffusion length 122-123 dislocations 123-124 equipment 113-116 GaAs 115 lifetime 122-123

CdIn2S4 208 CdS-CuInSe2 208 CdSnP2 43 Chromâticity coordinates 136 diagram 137

Colour measurements 135 chromaticity coordinates . 136 1931 CIE functions 136

Compounds, III-V 10, 36, 41, 44 LEDs 201

Continuity equations 82 Critical points 12 Cross-substitution 3, 4 Crystal structure 10 chalcopyrite 6, 7 wurtzite 6, 7 Zinc Blende 6, 7

CuGaSe2 208 CuInSe2 8, 10 LED 207, 208

Current flow equations 82

Detectors 111 Dice mounting 66, 67 Diffusion Fick's second law 58 GaAs 59 GaAsP 61 GaP 59, 60 planar 57, 58 review 58

Diffusion length

237

238 Subject Index

cathodoluminescence 122 Direct gap

absorption 18-20 recombination probability 75

Dislocations cathodoluminescence 123, 124

Donors 142-145 Donor-acceptor pairs GaAs 149, 150 GaP 163-166

Doping 26-28

Efficiency of LED cathodoluminescence 119-121 emission 91 external quantum 92 internal quantum 92 power 92

Electrical properties of semi-conductors 26-35

Electroluminescence 72 Electron beam plasma technique 49 Electroreflectance 15, 25 Encapsulation 68-70, 186-188 Energy gap

elements and compounds 13 temperature dependence 17 theory 11, 12, 14

Etchants 108 Exciton GaAs 145 GaP:N 167

Fabry-Perot cavity 97 mirror faces 98

Fermi level 148 Fick's second law 58

GaAlAs contact resistance 63, 64 laser 102, 103 solution growth 47, 48

GaAs absorption 22, 23 acceptor centres 143 acceptor ionization energy 145 band structure 11 cathodoluminescence 115, 118,

119 crystal structure 8, 10 diffusion 59 donor centres 143 donor ionization energies 145 energy gap 13

etchant 108 excitons 145 laser 95, 99, 101 lattice constant 10 LED 2, 183, 190, 191, 199

carrier concentration quenching 152, 154

substrate 55 mobility 32, 33, 36 molecular beam epitaxy 48, 49 ohmic contact 63 phase diagram 40 phonon spectra 26 photocapacitance 126, 127 photoconductivity 145 photoluminescence 142-162 recombination probability 75 solution growth 48

GaAsP band gap variation 15, 199 contact resistance 63, 64 diffusion 61 LED 195-199 colour 198, 199 emission 172-178 materials specification 54 nitrogen 195-196 substrate effect 197

nitrogen doping 17, 18 phase diagram 42 photoluminescence 169-172

GaAsSb diodes 203 GalnAs band gap variation 16 LED 203

GalnSb band gap variation 16 phase diagram 43

GaN electroluminescence 201 energy gap 13 structure 10

GaP absorption 24 band structure 12 cathodoluminescence and diode

efficiency 120, 212 contact resistance 63, 64 crystal structure 8, 10 diffusion 59, 60 electroreflectance 25 energy gap 13 etchant 108 lattice constant 10 LED

Subject Index 239

colour coordinates 199 dice 61 green 194, 195 red 193, 194 substrate 55 yellow 195

mobility 33-36 nitrogen doping 17, 18 phase diagram 40 recombination probability 75 Zn-O complex 79, 80

GaSb LED 201 Gibbs Phase Rule 39, 40

Hall Effect 29 Hydrogen Atom Model 27

InAs LED 201 InAsP LED 203 InAsSb 16 Indicator lamp 215-220 applications 216-220

Indirect gap absorption 19-21 recombination probability 75

InGaP band gap variation 199, 202

InP LED 200 solution growth 46, 47

InP/GalnAsP/lnP laser 103 InSb LED 201 Intrinsic carrier concentration 27 Ionization energy 142-145 acceptors 27 donors 27 GaAs acceptors and donors 145 GaAs transition metals 162 GaP acceptors and donors 169 hydrogen 144

Isoelectronic trap GaAsP LED 195, 196 GaP 166-168

Kerf patterns 65

Laser 95-104 applications 227 continuous operation 101-103 degradation 103 Fabry-Perot cavity 97 GaAs 3 GaAS/GaAlAs/GaAs 102, 103 gain 97 near and far field pattern 98-101

power output 97 spectra 100 standing waves 99 stripe geometry 99 threshold current 98

LED alloy (III-V) 201-203 carrier concentration quenching

152, 154 colour measurement 135-139 compounds (III-V) 201 current density 88 definition 86-88 dice mounting 66, 67 diffusion 57-61 diffusion current 89 electrical characteristics 91, 183-

186 with temperature 185, 186

emission 129- 139 efficiency 91

encapsulation 68-70 fabrication 53-71

schedule 53 forward current 90, 91 flux measurement 132, 133 GaAs 2, 190, 191

substrates 55 GaAsP 172-178 materials 54

GaP 191-195, 199 substrates 55

indicator lamps 215-220 applications 216-220, 228 optically coupled 225, 226

InP 200 intensity measurement 133-135 I-V characteristic 91 luminous efficacy 93 ohmic contacts 61-64 optical characteristics 186-190

absorption 189 encapsulation 186-188 temperature 190

photometric terms 131 photoresist technique 55-57 polar distribution 135 power efficiency 92 properties 1 quantum efficiency 92 radiant power distribution 92 radiometric terms 131 recombination rate 90 scribing and breaking 65, 66

240 Subject Index

SiC 205-207 survey 183-209 table 192

ternary compound 207, 208 two-photon phosphor 203-205 wire bonding 68-70

Lifetime cathodoluminescence 122, 123 excess carriers 78 minority carriers 75-77, 79

Light sources 111 Liquid encapsulation growth 44 Luminosity function 92, 93 Luminous efficacy 93, 131 excitancy 131 flux 131 incidence (illuminance) 131 intensity (candlepower) 131 sterance (brightness) 131

Luminosity function 92, 93

Melt growth Compounds (III-V) 44 GaAs luminescence 161

Minority carrier density 76 diffusion length 84, 89 electrical injection 85, 86, 99 lifetime 75-77, 79 optical generation 82, 83

Mobility acoustic lattice 31 impurity ion 31 neutral impurity 31 polar optical 30

Molecular beam epitaxy 29, 48

Nitrogen isoelectronic trap 17, 18

Ohmic contacts 61-64 Optically coupled devices 225, 226 Optical reflectivity 24

Phase diagrams 39-41 CdSnP2 43 Ga-As 40 GaAsP 42 Ga-P 41 InGaSb 43

Phonon spectra 25 GaAs 26

Photoconductivity 145 Photoluminescence 142-176

definition 72, 82, 83 detectors 111

equipment 105-111 GaAs band-to-acceptor 144-149 band-to-band 147-149, 157 carrier concentration quenching

151-158 exciton 145 LED emission comparison 2

GaAsP 169-172 GaP 162

donor-acceptor pairs 163-166 N and Bi 166-168

lifetime and timeresolved spectra 112

light sources 106 microphotoluminescence 112

Photometer integrating 132 intensity 134

Photometric terms 131 Photoresist 55, 56 Polar distribution 135 Precipitate 158

Radiant excitance (emittance) 131 flux 131 incidence (irradiance) 131 intensity 131 power distribution 92 sterance (radiance) 131

Radiative recombination direct gap 73, 74 GaP (Zn-0) 79, 80 indirect gap 73 probability 74, 75 rate 74 via defect centres 77, 78

Radiometrie terms 131

Scribing and breaking 65, 66 Kerf patterns 65

Self compensation 159 SiC diodes 205-207 energy gap 13 mobility 36 structure 10

Solution epitaxial growth 46, 47 GaAs 48 GaAs luminescence 161 InP 46, 47

Spectroradiometer 137, 138 Stoichiometric 41, 160 Substitutional impurities 142, 143

Subject Index 241

Ternaries compounds I-III-VI2 10 compounds II-IV-V2 lO, 36 growth 45 LED 207, 208 phase diagrams 43

Tetrahedral radii of atoms 9 Transition metals 162 Tristimulus coordinates 135, 137 method 138, 139

Two-photon phosphor LED 199, 203-205

Vacancy-complex GaAs 158-162 GaP 168-169

Vapour epitaxial growth 45 GaAs luminescence 161 GaAsP 45, 46 GaP 45, 46

Wire bonding 66, 68

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Luminescence and the Light Emitting Diode. The Basics and Technology of LEDS and the Luminescence...

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