A Fundamental Study of Interface Effects in HgCdTe...

A fundamental study of interface effects in HgCdTe materialsand devicesZhang, J. (2015). A fundamental study of interface effects in HgCdTe materials and devices

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A Fundamental Study of Interface Effects in HgCdTe Materials and Devices

by

Jing Zhang BSc, MSc

This thesis is presented for the degree of

Doctor of Philosophy

of

The University of Western Australia

School of Electrical, Electronic and Computer Engineering

The University of Western Australia

2015

Declaration Declaration of Published Work Appearing in this Thesis

This thesis contains published work and/or work prepared for publication, which

has been co-authored. The bibliographic information of the published works and the

details of contribution of the multiple authors to each publication are set out following

this declaration, pages 6 to 8.

Signature: (Candidate)

Jing Zhang

Signature: (Supervisor)

Professor Gilberto A. Umana-Membreno


Professor Jarek Antoszewski


Professor John M. Dell


Winthrop Professor Lorenzo Faraone

i

Abstract

The semiconductor-passivating layer interfaces, as well as the dielectric properties of

the passivating layers, play important and very often dominant roles in determining

HgCdTe device performance. With a narrow bandgap, HgCdTe infrared detectors are

strongly influenced by the quality of the passivation layer(s). The surface band

bending is often of the order of the bandgap energy for HgCdTe materials, even those

used for short-wave and mid-wave infrared detection, and can easily accumulate,

deplete, or invert the surface, drastically affecting device performance. The situation

is worse for long and very long wave infrared detectors. Surface recombination

processes can be enhanced in narrow bandgap materials like HgCdTe, and become the

dominant loss mechanism for photo-generated excess carriers. High-quality

photodiode detectors are limited by generation-recombination within the depletion

region, tunnelling through the depletion region and surface/interface effects. Surface

leakage is another surface-related current mechanism. The 1/f noise is surface related,

and is associated with surface charge tunnelling into and out of the passivation

interface. For ‘n-type/Barrier/n-type’ (nBn) heterostructure HgCdTe detectors, the

absorber is covered with the barrier which consists the passivation layer itself, yet

surface related phenomena impact greatly on the performance of nBn detectors.

Surface passivation technology can greatly improve the HgCdTe/insulator interface,

leading to a reduction of 1/f noise and generation-recombination noise, and an

increase of responsivity and detectivity of HgCdTe IR detectors. Understanding the

fundamental properties of interface states in narrow bandgap semiconductors is

essential to the systematic development of techniques to ameliorate their effects and

improve device performance.

The surface and interface chemistry of II-VI compounds has not been as extensively

studied in the open literature as that of III-V compounds, and there is a lack of

consensus on key questions related to II-VI surfaces and interfaces. Passivation

techniques for HgCdTe have been developed using empirical approaches over a long

time, with detailed information about surface conditioning, passivation material

properties, deposition conditions and annealing processes often retained as proprietary

knowledge. The absence of published work on a rigorous physical understanding of

ii

interfaces in narrow bandgap materials is the motivation for this work, and also the

challenge.

In this thesis the interface effects in HgCdTe materials and devices have been

investigated, concentrating on two passivant materials: CdTe and silicon nitride. The

surface and interface effects in molecular beam epitaxy (MBE) low-temperature

grown CdTe passivated HgCdTe structures have been studied, employing

photoconductive devices and gated photodiode devices. In order to determine the

effectiveness of this low-temperature deposited CdTe passivating film,

photoconductors were utilised to investigate the effectiveness of the passivation by

comparing photoresponsivity between devices with and without sidewall CdTe

passivation. Surface recombination simulations of the photodetectors were performed

to understand the behaviour of the passivation and estimate the surface recombination

velocity at the interfaces of CdTe passivated surfaces. This is a new and effective way

of estimating surface recombination velocities. The gated HgCdTe photodiode,

passivated by MBE low-temperature grown CdTe was used as a tool to investigate

passivation properties and performance, allowing the band bending at the surface to

be controlled by varying bias through the gate. This allowed the magnitudes of dark

current and dynamic resistance to be manipulated by changing the conditions at the

passivant/semiconductor interface in the photodiode, and therefore change the

dominant surface recombination mechanism.

The capabilities of low-temperature processing, good surface insulation and

hydrogenated films make SiNx a suitable choice for passivating HgCdTe. In this thesis

studies have been carried out to investigate SiNx thin films for surface passivation of

HgCdTe epitaxial layers without the need for a CdTe intermediate capping layer.

Conventionally, high-quality SiNx films for surface passivation layers are deposited at

temperatures in the range 200 °C to 750 °C. These temperatures are much higher than

the maximum allowed for HgCdTe processing temperature (typically < 120 °C) that

can be used without a Hg overpressure to prevent dissociation of the HgCdTe.

Inductively-coupled plasma-enhanced chemical vapour deposition (ICPECVD)

systems with a high-density plasma source offer the ability to deposit relatively high

quality SiNx films using a minimal thermal budget. SiNx films in this thesis were

deposited at low temperatures (80 °C - 130 °C) employing a Sentech SI500D

ICPECVD system with a high-density and low ion energy plasma source [1, 2]. The

iii

low ion energy of the plasma source enables the SiNx film to be deposited on the

HgCdTe without significant surface damage. Prior to SiNx films being deposited on

HgCdTe, a series of SiNx films were firstly deposited on CdTe/GaAs and Si substrates

under different deposition conditions to examine the influence of ICP power,

deposition temperature, and NH3/SiH4 flow ratio on the properties of SiNx films

themselves.

The SiNx/HgCdTe metal-insulator-semiconductor structures were utilised as a tool in

studying the interface between SiNx and HgCdTe. Interface trap density, Dit, was

considered as the measure in evaluating surface passivation performance and in

correlating passivation quality with other film properties. The SiNx/n-Hg0.68Cd0.32Te

interface characteristics were investigated employing capacitance-voltage and

conductance-frequency measurements, and the corresponding Dit were extracted from

the high-frequency and low-frequency capacitance-voltage characteristics, and also by

the conductance method. Analysis of the SiNx/n-Hg0.68Cd0.32Te MIS structures

indicated that Si-rich SiNx film deposited at 100 °C by ICPECVD exhibit electrical

characteristics suitable for surface passivation of HgCdTe-based devices. That is,

interface trap densities in the range of mid-1010 cm-2eV-1, and fixed negative interface

charge densities of ~ 1011 cm-2 [1, 2]. In addition, the relationship between different

bond concentrations in the SiNx and surface passivation performance has been

explored using infrared absorbance spectra. The Si-H and N-H bond concentrations

were found to be directly correlated with passivation performance, such that SiNx

films with a combination of high [Si-H] and low [N-H] bond concentrations were

found to be suitable as electrical passivation layers on HgCdTe. This could be a useful

criterion for optimising the passivation quality of SiNx films for HgCdTe-based

devices.

iv

For Jayden and Yingliang

v

Acknowledgements

I would like to express my gratitude to the numerous people who have contributed to

this thesis. First and foremost, I would like to express my gratitude to my supervisors,

Prof. Gilberto A. Umana-Membreno, Prof. Jarek Antoszewski, Prof. John Dell, and

Prof. Laurie Faraone for their guidance and supervision throughout my Ph.D. study.

Without their encouragement and profound knowledge in HgCdTe, this research

would not have been possible. I feel especially grateful how supportive they have

been after the birth of my child.

It has been such an enjoyable and unforgettable experience being a member of the

Microelectronics Research Group (MRG), headed by Prof. Lorenzo Faraone. The

facilities, funding and travel opportunities afforded by the group have given enormous

support throughout the thesis. I am thankful to all group members who have warmly

helped me on my research and also on personal life. I have enjoyed working with all

of my fellow Ph.D. candidates and staff members in the group. Particularly, I would

like to thank Gordon and Ryan for their kind training and assistance to me on the

nanofabrication facilities and MBE. I would also like to thank Richie, Gordon, Imtiaz,

Renjie, Wen and Jarek for their great efforts in running, updating and maintaining the

MBE system. Special thanks to Ms. Sabine Betts and Ms. Karen Kader, for their

warm support over the years in making MRG group a big happy family.

I would like to thank for the financial supports during my study from International

Postgraduate Research Scholarship, the Samaha Research Scholarship,

Microelectronics Research Group, School of Electrical, Electronic and Computer

Engineering, and the University of Western Australia.

I acknowledge the Australian Department of Innovation, Industry, Science and

Research and the International Science Linkages (ISL) for support during the study. I

also acknowledge the support from the Australian Research Council, Western

Australian Node of the Australian National Fabrication Facility, and the Office of

Science of the WA State Government. I would also like to thank the Centre for

Microscopy, Characterisation and Analysis, the University of Western Australia, for

the support with SEM and XRD. I also thank the School of Chemistry and

Biochemistry for XRD.

vi

Let me reserve my final appreciation to my family. I am grateful for the love and

support that my sister, father and mother have provided over the years. Special thanks

to my husband, Yingliang, whose sincerest love has always given me strength in

overcoming difficulties. I am very proud of my son, Jayden, who joined the family

during my Ph.D. study. He has given our family huge pleasure and kept me energised

throughout the final stages of my study.

vii

Contents

Contents

Declaration..................................................................................................................... i

Abstract ......................................................................................................................... ii

Acknowledgements ..................................................................................................... vi

Contents ........................................................................................................................ ii

List of Figures ............................................................................................................... ii

List of Tables ................................................................................................................ ii

1 Introduction ........................................................................................................... 1

1.1 Infrared detection technologies .................................................................. 1

1.2 Material interface limitations to photon detector performance .................. 2

1.3 Research objectives and significance ......................................................... 3

1.4 Thesis structure .......................................................................................... 4

1.5 Publications arising from this thesis .......................................................... 6

2 HgCdTe Passivation Technologies ....................................................................... 9

2.1 HgCdTe as an infrared detector material ................................................... 9

2.1.1 HgCdTe device architectures ........................................................... 10

2.1.2 Measures of device performance ..................................................... 16

2.2 Surface and interface issues with HgCdTe .............................................. 18

2.3 Surface passivation materials and technologies ....................................... 20

2.3.1 Passivation materials ........................................................................ 20

2.3.2 CdTe passivation .............................................................................. 22

2.3.3 ZnS passivation ................................................................................ 22

2.3.4 SiNx passivation ............................................................................... 23

2.3.5 Dual-layer passivation ..................................................................... 24

ii

Contents

2.4 Surface preparation for HgCdTe and CdTe ............................................. 25

2.5 Modification of interface trap density ...................................................... 26

2.6 Summary .................................................................................................. 29

3 Material Characterisation .................................................................................. 30

3.1 Introduction .............................................................................................. 30

3.2 Physical Characterisation ......................................................................... 30

3.2.1 Microscopy ...................................................................................... 30

3.2.2 X-ray diffraction .............................................................................. 32

3.2.3 Reflection high energy electron diffraction ..................................... 34

3.2.4 Energy dispersive X-ray analysis..................................................... 36

3.2.5 Spectroscopic ellipsometry .............................................................. 37

3.2.6 Optical reflection/transmission for structural and compositional characterisation ............................................................................................ 38

3.2.7 Optical reflection/transmission for bonding and detailed characterisation of thin films ....................................................................... 40

3.3 Electrical Characterisation ....................................................................... 40

3.3.1 Magneto-transport measurements .................................................... 40

3.3.2 Current-voltage measurements ........................................................ 44

3.3.3 Capacitance-voltage and capacitance-frequency measurements ..... 46

3.4 Performance of ICPECVD SiNx passivation on other semiconductors .................................................................................................... 49

3.4.1 Surface passivation by hydrogenated silicon nitride ....................... 50

3.4.2 Experimental setup and design ........................................................ 51

3.4.3 Investigation on SiNx film stability over time ................................. 56

3.4.4 Investigation on SiNx film properties influenced by NH3/SiH4 flow ratio ...................................................................................................... 64

3.5 Summary .................................................................................................. 91

4 Surface and Interface Effects in CdTe/HgCdTe Structures ........................... 93

4.1 Introduction .............................................................................................. 93

4.1 Sidewall effects in photoconductive devices ........................................... 93

4.1.1 Experimental procedures ................................................................. 93

4.1.2 Surface and interface recombination in photoconductive devices ... 95

iii

Contents

4.2 Interface effects in ZnS/CdTe/HgCdTe gated photodiodes ..................... 98

4.2.1 Gated photodiode fabrication process .............................................. 99

4.2.2 Dark current as a function of gate bias .......................................... 101

4.3 Summary ................................................................................................ 107

5 Interface Effects in Metal/SiNx/HgCdTe Structures ...................................... 108

5.1 Fabrication of the MIS structures .......................................................... 108

5.2 Study of interface trap density at the SiNx/HgCdTe interface ............... 110

5.2.1 Capacitance-voltage measurements on MIS structures ................. 110

5.2.2 Interface trap density extracted by quasi-static method ................. 116

5.2.3 Conductance-frequency measurements on MIS structures ............ 118

5.2.4 Interface trap density extracted by conductance method ............... 122

5.3 Relationship between SiNx passivation performance and thin film bond concentrations ........................................................................................... 124

5.4 Summary ................................................................................................ 130

6 Conclusions and Future Work ......................................................................... 131

6.1 Summary and Conclusions .................................................................... 131

6.2 Recommendations for future work ........................................................ 134

References ................................................................................................................. 138

Appendix A: HgCdTe Properties ........................................................................... 164

A.1 Bandgap ................................................................................................. 164

A.2 Lattice constant ...................................................................................... 166

A.3 Intrinsic carrier concentration ................................................................ 167

A.4 Mobility.................................................................................................. 167

Appendix B: Deposition Parameters Concerning High-temperature Deposited SiNx Films ................................................................................................................. 169

iv

List of Figures

List of Figures

Figure 2.1 Device cross section of HgCdTe photoconductor and its schematic band

diagram ........................................................................................................................ 10

Figure 2.2 Device cross sections and their schematic energy band diagrams of (a) n+-

on-p planar homojunction and (b) P+-on-n mesa heterojunction photodiodes. ........... 12

Figure 2.3 Device cross section of a HgCdTe nBn detector and its schematic energy

band diagram under bias. ............................................................................................. 15

Figure 2.4 Possible charge centres for a semiconductor surface passivated with a

dielectric, and the resultant interface states and surface band-bending. ...................... 19

Figure 3.1 SEM micrographs corresponding to (a) and (b) 60 nm-thick CdTe film on

HgCdTe, and (c) 300 nm-thick CdTe on HgCdTe. The CdTe layer was deposited in

an MBE system. ........................................................................................................... 31

Figure 3.2 Double crystal X-ray diffraction spectra of MBE grown CdTe layer on

GaAs substrate. ............................................................................................................ 33

Figure 3.3 Double crystal X-ray diffraction spectra of MBE grown HgCdTe layers on

CdZnTe substrate (n-HgCdTe/n+-HgCdTe/CdZnTe). ................................................. 33

Figure 3.4 X-ray diffraction spectra of (a) the LPE HgCdTe before CdTe growth and

(b) the MBE grown CdTe (on LPE HgCdTe). ............................................................. 34

Figure 3.5 The RHEED patterns recorded for HgCdTe sample CMCT042. (a) During

substrate thermal cleaning; (b) Toward the end of thermal cleaning; (c) At the start of

growth of n-HgCdTe layer (x = 0.4); (d) Toward the end of the growth of n-HgCdTe

(x = 0.4) layer; (e) At the start of growth of the MWIR absorber layer (x = 0.316); (f)

Toward the end of the growth of absorber layer. ......................................................... 36

Figure 3.6 (a) Refractive index, n, and (b) extinction coefficient, k, measured by

ellipsometry for a 11.18 μm-thick HgCdTe/CdZnTe sample numbered MCT223

(x = 0.281). ................................................................................................................... 37

Figure 3.7 FTIR transmission spectra for MCT223 (x = 0.281, depilayer = 11.18 μm)

with the air background being subtracted. ................................................................... 38

Figure 3.8 FTIR transmission spectra for MCT225 (x = 0.375, depilayer = 8.9 μm)

before and after wafer annealing, with the annealing being carried out in a saturated

Hg atmosphere at 235 °C for 24 hours. ....................................................................... 39

Figure 3.9 Image of the centre part of a fabricated Greek Cross van der Pauw structure

on HgCdTe taken under an optical microscope. .......................................................... 42

ii

List of Figures

Figure 3.10 Comparison of the electron conductivity - electron mobility spectra

measured before and after MBE CdTe growth. ........................................................... 42

Figure 3.11 Plots showing the electron mobility spectrum measured after the vacancy

filling anneal at liquid nitrogen temperature for (a) MCT231 (x = 0.388, depilayer =

6.4 μm) and (b) MCT240 (x = 0.347, depilayer = 5.23 μm). ........................................... 43

Figure 3.12 A typical current-voltage characteristic of Au/Cr/SiNx/Si MIS structure.45

Figure 3.13 Resistivity variation of D4-100C SiNx film over a period of four months.

The MIS structures were left to age in laboratory atmosphere. ................................... 45

Figure 3.14 The SiNx/Si MIS structure measured at 298 K with variable sweep ranges

from -15 V to 15 V (innermost pair of curves), -20 V to 20 V, -24 V to 24 V and -

30 V to 30 V (outermost pair of curves). ..................................................................... 48

Figure 3.15 The SiNx/Si MIS structure measured at 298 K (red solid line) and 77 K

(blue dashed line) with sweep ranges from -20 V to 20 V. ......................................... 48

Figure 3.16 Deposition rates of silicon nitride films on silicon substrate as a function

of ICP power at a substrate temperature of 80 °C and 100 °C. ................................... 57

Figure 3.17 Deposition rates of silicon nitride films on CdTe/GaAs substrate as a

function of ICP power at a substrate temperature of 80 °C and 100 °C. ..................... 57

Figure 3.18 A typical IR absorbance spectra of low-temperature (80 °C - 100 °C)

deposited silicon nitride film deposited by Sentech SI500D system. .......................... 60

Figure 3.19 The IR absorbance spectra of the as-deposited silicon nitride films by six

different recipes on CdTe/GaAs substrate. .................................................................. 61

Figure 3.20 The IR absorbance spectra of the silicon nitride films by six different

recipes on CdTe/GaAs substrate after six-months exposure to a laboratory atmosphere.

...................................................................................................................................... 62

Figure 3.21 The IR absorbance spectra of the C5-SiNx film on CdTe/GaAs substrate

monitored over a six month time frame. The films were allowed to age in laboratory

atmosphere. .................................................................................................................. 63

Figure 3.22 Measured refractive index at a wavelength of 632.8 nm and [N]/[Si] ratio

estimated by EDS, as a function of NH3/SiH4 flow ratio for samples deposited at

80 °C and 100 °C. ........................................................................................................ 67

Figure 3.23 Measured refractive index at a wavelength of 632.8 nm and [N]/[Si] ratio

estimated by EDS, as a function of substrate temperature........................................... 67

Figure 3.24 Plot illustrating the relationship between n and x for ICPECVD SiNx

deposited at 80 °C -100 °C with varied NH3/SiH4 flow ratio. ..................................... 70

iii

List of Figures

Figure 3.25 [N]/[Si] ratio as a function of NH3/SiH4 flow ratio for ICPECVD SiNx

deposited at 80 °C -100 °C with varied NH3/SiH4 flow ratio. ..................................... 71

Figure 3.26 Plot of film refractive index, n632.8nm, of ICPECVD SiNx deposited at

80 °C -100 °C as a function of the SiH4/NH3 gas ratio................................................ 72

Figure 3.27 The change in film deposition rate versus NH3/SiH4 flow ratio for silicon

nitride films deposited at 80 °C, 90 °C and 100 °C with a fixed SiH4 gas flow of

6.9 sccm. ...................................................................................................................... 74

Figure 3.28 IR absorption coefficient spectra of SiNx films deposited by ICPECVD at

various NH3 flow rates and a fixed SiH4 flow rate at 80 °C. ....................................... 76

Figure 3.29 IR absorption coefficient spectra of SiNx films deposited by ICPECVD at

various NH3 flow rates and a fixed SiH4 flow rate at 100 °C. ..................................... 77

Figure 3.30 Plots showing the Si-H stretching peak shifting to higher frequency as a

function of (a) film composition [N]/[Si] and (b) NH3/SiH4 flow ratio increases. ..... 78

Figure 3.31 Plots showing the main absorption coefficient peak shifting to higher

frequency as a function of (a) film composition [N]/[Si] and (b) NH3/SiH4 flow ratio.

...................................................................................................................................... 79

Figure 3.32 The absorption coefficient as a function of wavenumber for sample B1-

NH8-80C as an illustration of the fitted absorption bands (dashed line) in the range

from 450 cm-1 to 1400 cm-1 with four different Gaussian distributions. ...................... 81

Figure 3.33 [N-H] and [Si-H] bond concentration as a function of film composition

[N]/[Si] and NH3/SiH4 flow ratio. ................................................................................ 86

Figure 3.34 The ratio of H bonded to N over that bonded to Si, [N-H]/[Si-H], as a

function of NH3/SiH4 flow ratio, refractive index and film composition [N]/[Si]. ..... 87

Figure 3.35 The fraction of [N-H] and [Si-H] as a function of SiNx film composition

[N]/[Si]. The indicated temperatures refer to the substrate temperature during

deposition. .................................................................................................................... 88

Figure 3.36 The atomic densities of [Si], [N] and [H] as a function of SiNx film

composition [N]/[Si] and NH3/SiH4 flow ratio. The indicated temperatures refer to the

substrate temperature during deposition. ..................................................................... 89

Figure 3.37 Film density, ρ, as a function of (a) SiNx film composition [N]/[Si] and

(b) NH3/SiH4 flow ratio. The indicated temperatures refer to the substrate temperature

during deposition. ........................................................................................................ 90

Figure 4.1 Schematic of the photoconductive devices showing location of the

unpassivated sidewall surfaces. The passivating CdTe film is approximately 200 nm

iv

List of Figures

thick. (a) Fully passivated structure; (b) Partially passivated structure with no CdTe

film on sidewalls. ......................................................................................................... 95

Figure 4.2 Measured and modelled normalised spectral photoresponse of

Hg0.71Cd0.29Te photoconductive devices, measured at a field of 10 V/cm at 80 K. The

low field minimises the effect of sweepout so that the response should be most

sensitive to surface recombination velocity. ................................................................ 96

Figure 4.3 Simulated photoresponse ratio of all surfaces passivated devices (RF) and

partially passivated devices (RP) versus recombination velocity of the top

CdTe/Hg0.71Cd0.29Te interface (sTop) at 80 K, with recombination velocity of the

unpassivated surfaces sWall = 1×104 cm/s. .................................................................... 97

Figure 4.4 A photomicrograph and cross section of the completed gated photodiodes.

(a) Photo of fabricated gated photodiodes and (b) cross-sectional view. .................. 100

Figure 4.5 Diagrams illustrating the effects of n-type region band-bending on a n-on-p

junction. The gate voltage, Vg, is referenced to the p-type HgCdTe. a) p-type surface

in accumulation; b) Vg = Vfb in p-type, c) p-type surface in depletion or weak

inversion; d) p-type surface in inversion and field-induced junction breakdown occurs

in the p-type region under the gate. ........................................................................... 103

Figure 4.6 Measured dark current at 77 K in a cryostat with a cold shield in the

absence of photocurrent for gated photodiodes with (a) a diameter of 300 μm and (b)

360 μm. The gate voltage is referenced to the p-type substrate. The seven curves from

top to bottom are for various gate biases from -1.5 V to 1.5 V in 0.5 V steps. ......... 104

Figure 4.7 Measured dynamic resistance-area product at 77 K in a cryostat with a cold

shield in the absence of photocurrent for gated photodiodes with a diameter of (a)

300 μm and (b) 360 μm. The seven curves from bottom to top are for varying gate

bias from -1.5 V to 1.5 V in 0.5 V steps. ................................................................... 105

Figure 5.1 C-V curves measured at 1 MHz with the three different sweeping voltage

ranges for each of the four MIS sample for the range of ± 2 V, ± 4 V and ± 6 V ..... 111

Figure 5.2 Illustration on the definitions of ∆V+, ∆V– and ∆VH used in the C-V

analysis. ...................................................................................................................... 113

Figure 5.3 Flat band voltage as a function of (a) substrate temperature, and (b)

[N]/[Si], for the three MIS samples of D1-80C, D1-90C, and D1-100C. ................. 115

Figure 5.4 The change in (a) hysteresis widths VH and (b) slow interface charge

densities as a function of bias extremes for the four MIS structures. ........................ 115

v

List of Figures

Figure 5.5 Comparison of the interface trap densities (Dit) of the SiNx/n-

Hg0.68Cd0.32Te MIS structures extracted by the quasi-static method as a function of the

energy from mid-gap at 77 K. .................................................................................... 117

Figure 5.6 Measured capacitance – log ω characteristics at various biases for the four

MIS structures at 77 K. .............................................................................................. 120

Figure 5.7 Measured and fitted Gp/ω versus log ω characteristics at various gate biases

for the four MIS structures at 77 K. ........................................................................... 121

Figure 5.8 Comparison of the interface trap densities, Dit, of all the SiNx/n-

Hg0.68Cd0.32Te MIS structures extracted by the conductance method as a function of

the energy from mid-gap at 77 K. .............................................................................. 123

Figure 5.9 Comparison of the time constant (τit) of all the SiNx/n-Hg0.68Cd0.32Te MIS

structures extracted by the conductance method as a function of the energy from mid-

gap at 77 K. ................................................................................................................ 123

Figure 5.10 Electron capture cross section as a function of energy for the SiNx/n-

Hg0.68Cd0.32Te MIS structures at 77 K. ...................................................................... 124

Figure 5.11 The IR absorbance spectra of the reference silicon nitride films on Si

substrate under four deposition conditions for the MIS structures. ........................... 126

Figure 5.12 Relationship between Dit and [Si-H] and [N-H] bond concentrations. .. 128

Figure 5.13 Dit at mid-gapwith the variations of (a) [H], [N-H], [Si-H] (a) and (b) [Si-

H]/[N-H]. ................................................................................................................... 129

Figure A1.1 Schematics of energy bandgap of (a) HgTe. (b) HgTe-CdTe transition

(zero bandgap). (c) CdTe. The Г6 and Г8 point refer to the electron band and

light/heavy hole band, respectively. ........................................................................... 165

Figure A1.2 Bandgap of Hg1−xCdxTe as a function of cadmium composition, x. ..... 165

Figure A1.3 Cut-off wavelength of Hg1−xCdxTe as a function of cadmium

composition, x. ........................................................................................................... 166

Figure A1.4 Lattice constant of Hg1−xCdxTe as a function of cadmium composition, x.

.................................................................................................................................... 166

Figure A1.5 Intrinsic carrier concentration of Hg1-xCdxTe as a function of x for

T = 77 K, 150 K, 200 K and 300 K. ........................................................................... 167

Figure A1.6 Electron mobility in Hg1-xCdxTe as a function of temperature for varying

mole fraction, x. ......................................................................................................... 168

vi

List of Tables

List of Tables

Table 3.1 Summary of extracted electron transport parameters for HgCdTe sample

MCT225 before and after the CdTe passivation .......................................................... 43

Table 3.2 Data extracted from C-V analysis on a SiNx/Si MIS capacitor measured at

298 K and 77 K ............................................................................................................ 47

Table 3.3 Summary of deposition conditions of SiNx film on CdTe/GaAs and Si

substrate in the Sentech SI 500D system ..................................................................... 52

Table 3.4 Silicon nitride film deposition procedures used for sample D1-SiNx in the

Sentech SI500D ICPECVD system ............................................................................. 54

Table 3.5 Summary of MBE grown CdTe on GaAs substrate .................................... 56

Table 3.6 Absorption bands observed in the as-deposited SiNx samples .................... 59

Table 3.7 Summary on ICPECVD SiNx/Si wafers with varied SiH4/NH3 ratio and

temperature .................................................................................................................. 66

Table 3.8 IR absorption spectra analysis and calculations for bond and atom

concentrations on SiNx/Si wafers deposited under varied NH3/SiH4 flow ratios at

temperatures between 80 °C - 100 °C .......................................................................... 82

Table 3.9 Summary of bond and atom concentrations calculated for SiNx/Si wafers

deposited under varied NH3/SiH4 flow ratios at temperatures between 80 °C - 100 °C

...................................................................................................................................... 84

Table 5.1 Summary of SiNx/HgCdTe MIS samples with silicon nitride film deposited

under different conditions .......................................................................................... 109

Table 5.2 Summary on flat band voltage, fixed charge density, slow interface trapped

charge density and interface trap density extracted for the four SiNx/HgCdTe MIS

samples for a bias sweep range of ± 2 V ................................................................... 114

Table 5.3 Hysteresis widths VH in the high-frequency C-V curves versus bias

extremes for the four SiNx/HgCdTe MIS structures .................................................. 114

Table 5.4 Summary on bond and atomic concentrations calculated for SiNx/Si

reference wafers of the SiNx/HgCdTe MIS structures ............................................... 127

Table 5.5 Summary of results from C-V and IR absorbance analysis on the four

SiNx/HgCdTe MIS structures ..................................................................................... 128

ii

Chapter 1 Introduction

1 Introduction

1.1 Infrared detection technologies

The infrared (IR) electromagnetic spectrum spans wavelengths from 1 μm to 100 μm.

IR radiation was first discovered by Sir William Herschel in 1800, who built a crude

monochromator that used a thermometer as a detector to measure the distribution of

energy in sunlight [3]. He saw temperature fluctuations where there was no visible

light, which led him to discover an invisible light spectrum, associated with heat

radiation, at wavelengths longer than those of visible light.

Infrared detectors can be classified as either thermal detectors or photon detectors.

The principle of thermal detectors is that there is a measurable change in the electrical

characteristics of the material due to a temperature change after the absorption of IR

radiation. In contrast, photon detectors measure directly generated charged carriers

resulting from the absorption of photons. Thermal and photon detectors have different

dependencies of detectivities on wavelength and temperature [3, 4]. Thermal detectors

are favoured at the very long wavelength IR (VLWIR: 14 - 30 µm) region, whereas

photon detectors are favoured at IR regions of shorter wavelength, such as long-

wavelength IR (LWIR: 8 - 14 µm), due to speed of response, the influence of

fundamentally different types of noise, i.e. generation-recombination (GR) noise in

photon detectors and temperature fluctuation noise in thermal detectors. Generally,

higher operating temperature requirements needed to attain background limited noise

performance (when the noise is limited by the random arrival of photons on the

detector) favour thermal detectors over photon detectors. However, this performance

is speed dependent, and thermal detectors generally exhibit significantly slower

response for background limited performance compared to photon detectors.

Photon detectors can be intrinsic, extrinsic, free-carrier detectors or quantum

detectors, depending on the excitation process that induces a carrier concentration

variation on absorption of a photon in the detector material – i.e. intrinsically

(interband transition), extrinsically (impurity to band transition), or by free carrier

absorption (intraband transition). Intrinsic infrared photon detectors typically have

high optical absorption coefficient, high quantum efficiency, and low thermal

generation rate. The most commonly used intrinsic detectors are photoconductors and

1


photodiodes. The main disadvantages of extrinsic detectors are the significant cooling

requirements, and higher levels of noise.

Hg1-xCdxTe is a widely used material for infrared photon detectors and sensors, and is

of significant importance for IR detection in defence and security, mineral

exploration, environmental monitoring, and biological and chemical sensing for

commercial and defence related uses. Its cut-off wavelength, λc, can be tuned by

changing the mole fraction x of CdTe to HgTe to be in the short-wavelength IR

(SWIR: 0.75 - 3 µm), mid-wavelength IR (MWIR: 3 - 5 µm), and long-wavelength IR

(LWIR: 8 - 14 µm) spectral regions [5], regions in which the atmosphere exhibits low

optical absorption. More information on the cut-off wavelength variation with x-value

and temperature is presented in Appendix A.1. Performance advantages offered by

HgCdTe-based photodetectors include higher sensitivity at a given operating

temperature, fast response times, tuneable cut-off wavelength, and suitability for

integration into focal-plane array configurations. These advantages have been very

well documented in the literature [6, 7].

1.2 Material interface limitations to photon detector performance

Surface (when a solid is in contact with vacuum or the gas phase) and interface (when

the solid is in contact with another solid) effects can dominate the performance of

semiconductor devices. Interfaces present between passivation, insulation and contact

layers and at heterostructures. The semiconductor-passivating layer interfaces, as well

as the dielectric properties of the passivating layers, play important and very often

dominant roles in determining performance of all semiconducting devices. With a

narrow bandgap, HgCdTe IR detectors are particularly sensitive to interface effects,

and are strongly influenced by the quality of the passivation layer(s). For Hg1-xCdxTe

with x-value between 0.2 and 0.3, the band gap at 77 K varies between 0.1 eV and

0.25 eV. Hence, the surface potential band bending can easily be of the order of the

band-gap energy, and easily accumulate, deplete, or invert the surface, thus

significantly affecting device performance [8].

For high-quality HgCdTe, the performance of photoconductive detectors is found to

be limited by the carrier recombination at the surface, which determines the effective

minority carrier lifetime. Surface recombination processes can be enhanced in narrow

2


bandgap material detectors like HgCdTe, and can become the dominant loss

mechanism for photo-generated excess carriers [9]. Surface passivation for HgCdTe

photoconductors is critical in achieving high responsivity, especially for devices with

small areas [10, 11].

Sensitivity of high-quality photodiodes, typically exhibiting high zero-bias dynamic

resistance-effective junction area product (R0A), are usually limited by generation-

recombination within the depletion region, tunnelling through the depletion region

and surface/interface effects [12]. In addition to generation-recombination at the

surface/interface and within surface channels, surface leakage is another surface-

related current mechanism. The noise in HgCdTe photodiodes is observed to vary

with bias and temperature [13]. The 1/f noise is believed to be dominantly surface

related, and is associated with surface charge tunnelling into and out of the

semiconductor/passivation layer interface [14, 15]. Surface passivation technology

can greatly improve the HgCdTe/insulator interface, leading to a reduction of 1/f and

g–r noise, and an increase of responsivity and detectivity.

For ‘n-type/Barrier/n-type’ (nBn) heterostructure HgCdTe detectors in which the

absorber is covered with the barrier that consists of the passivation layer itself, it is

expected that passivation is less critical than for other detector structures. However,

several reports have shown that surface passivation plays a significant role in

improving the performance of nBn detectors [16-18]. HgCdTe nBn structures show

potential to outperform HgCdTe photodiodes and reach background-limited infrared

performance (BLIP) at ~ 207 K for MWIR wavelengths [16], while experimental

results have shown dark currents two orders of magnitude higher than expected,

which can be attributed to surface leakage currents associated with etching induced

defects [17], indicating the passivation process needs to be improved.

1.3 Research objectives and significance

The surface and interface chemistry of II-VI compounds has not been as extensively

studied as that of III-V compounds, and there is a lack of consensus on key questions

related to II-VI surfaces and interfaces. Passivation techniques for HgCdTe have long

been researched, however, detailed information on surface conditioning, passivation

material, deposition conditions and annealing processes are often retained as

3


proprietary knowledge, which has led to a lack of detailed publications. All this adds

to the difficulties and challenges associated with this research project.

The principal objectives of this thesis are to investigate interface effects in HgCdTe

materials and devices, encompassing the following:

Characterise molecular beam epitaxy (MBE) low-temperature grown CdTe

passivation films and their passivation effects using photoconductive devices;

Modelling of surface and interface recombination effects in photoconductive

devices;

Investigate changes in narrow-bandgap HgCdTe photodiode performance

resulting from band bending at the surface;

Develop, optimise and characterise low-temperature (80 °C - 100 °C) deposited

SiNx films for passivating HgCdTe without the need for a CdTe capping layer;

Evaluate SiNx passivation quality by comparing the interface trap densities at the

SiNx/HgCdTe interface for SiNx films deposited under varying deposition

conditions utilising SiNx/n-HgCdTe metal-insulator-semiconductor (MIS)

structures;

Correlate performance of SiNx as a passivation layer on HgCdTe with bond

configurations of SiNx thin films.

1.4 Thesis structure

The organisation of this thesis, exclusive of this introductory chapter, is as follows:

Chapter 2 stresses the importance of surface passivation for HgCdTe detectors in

terms of the device architecture and detector performance, and reviews the available

materials and technologies for HgCdTe passivation. The passivation materials and

technologies used in this thesis are presented. Surface preparation treatment, that is

crucial to HgCdTe and CdTe prior to the passivation, is also reviewed. The possible

approaches to ameliorate the effects of interface states, including annealing,

hydrogenation, and modification of stress in the layers, are discussed.

Chapter 3 covers the techniques used for material characterisation related to this

thesis, divided into two categories - physical and electrical characterisations. The

4


purpose for these characterisations is discussed, and a selection of experimental

results presented. In addition, in order to determine the suitable deposition conditions

of SiNx passivation film for HgCdTe, a series of low-temperature (80 °C - 130 °C)

ICPECVD SiNx films were deposited on other semiconductors - CdTe/GaAs and Si

substrates, under different deposition conditions to investigate the influence of ICP

power, deposition temperature, and NH3/SiH4 flow ratio on properties of the

deposited SiNx films.

Chapter 4 studies the surface and interface effects in CdTe/HgCdTe devices,

including photoconductive devices and gated photodiode devices. The CdTe

passivation used was a low-temperature MBE grown film. In order to determine the


photoconductors were utilised to investigate the influence of the passivation by

comparing photoresponsivity between devices with and without sidewall CdTe

passivation. Surface recombination simulations of the photodetectors were performed

to understand the behaviour of the passivation and estimate the surface recombination

velocity at the interfaces of CdTe passivated surfaces. The gated photodiode was used

as a tool to investigate device performance, allowing the band bending at the surface

to be controlled by varying the bias applied to the gate. This allowed the magnitudes

of dark current and dynamic resistance to be manipulated at the surface of the

photodiode, and therefore change the dominant surface recombination mechanism.

Chapter 5 presents work on SiNx thin films for surface passivation of HgCdTe

epitaxial layers without the need for a CdTe capping layer. The SiNx/HgCdTe MIS

structures were utilised as a tool in studying the interface between SiNx and HgCdTe,

and the interface trap density, Dit, was extracted and examined by analysing high-

frequency and low-frequency capacitance-voltage data, as well as by the conductance

method. The correlation between different bond concentrations in the passivation

layer and surface passivation performance was then studied. The Si-H and N-H bond

concentrations (i.e. [Si-H] and [N-H]) were found to be directly correlated with

passivation performance, such that SiNx films with a combination of high [Si-H] and

low [N-H] being considered to be suitable as electrical passivation layers on HgCdTe.

This could be a useful criteria for optimising the passivation quality of SiNx films for

HgCdTe-based devices.

5


Conclusions of this thesis and suggestions for further work are discussed in Chapter 6.

1.5 Publications arising from this thesis

Refereed Journal Publications

[1] J. Zhang, G. A. Umana-Membreno, R. Gu, W. Lei, J. Antoszewski, J. M. Dell,

and L. Faraone, ‘Investigation of ICPECVD Silicon Nitride Films for HgCdTe

Surface Passivation’, Journal of Electronic Materials, vol. 44 (9), pp. 2990-3001,

2015. (Presented at the The U.S. Workshop on the physics and chemistry of II-VI

materials, 2014)

The percentage contribution of each author is as follows:

J. Zhang 80 %, All, except -

G. A. Umana-Membreno Supervisor

R. Gu 10 % MBE growth and technical discussions

W. Lei 10 % MBE growth and technical discussions

J. Antoszewski Supervisor

J.M. Dell Supervisor

L. Faraone Supervisor

[2] J. Zhang, G. K. O. Tsen, J. Antoszewski, J. M. Dell, L. Faraone, and W. D. Hu,

‘A Study of Sidewall Effects in HgCdTe Photoconductors Passivated with MBE-

Grown CdTe’, Journal of Electronic Materials, vol. 39, pp. 1019-1022, 2010.

(Presented at the The U.S. Workshop on the physics and chemistry of II-VI

materials, 2009)



G. K. O. Tsen 10 % MBE growth and technical discussions




W. D. Hu 5 % Technical discussions

6


[3] W. Hu, X. Chen, Z. Ye, J. Zhang, F. Yin, C. Lin, Z. Li, and W. Lu, ‘Accurate

Simulation of Temperature-Dependent Dark Current in HgCdTe Infrared

Detectors Assisted by Analytical Modeling’, Journal of Electronic Materials, vol.

39, pp. 981-985, 2010. (Presented at the The U.S. Workshop on the physics and

chemistry of II-VI materials, 2009)

The percentage contribution of this author is as follows:

J. Zhang 10 %, Oral presentation at The US Workshop and technical discussions.

Refereed Conference Proceedings

[1] J. Zhang, G. A. Umana-Membreno, R. Gu, W. Lei, J. Antoszewski, J. M. Dell, L.

Faraone, ‘Characterisation of SiNx-HgCdTe Interface in Metal-Insulator-

Semiconductor Structure’, 2014 Conference on Optoelectronic and

Microelectronic Materials and Devices, pp.64-66, 2014. (Presented at Conference

on Optoelectronic and Microelectronic Materials and Devices, 2014)



G. A. Umana-Membreno Supervisor

R. Gu 10 % MBE growth and technical discussions

W. Lei 10 % MBE growth and technical discussions




[2] J. Zhang, R. J. Westerhout, G. K. O. Tsen, J. Antoszewski, Y. Yang, J. M. Dell,

and L. Faraone, ‘Sidewall effects of MBE grown CdTe for MWIR HgCdTe

photoconductors,’ 2008 Conference on Optoelectronic and Microelectronic

Materials and Devices, pp. 82-85, 2008. (Presented at Conference on

Optoelectronic and Microelectronic Materials and Devices, 2008)

7




R. J. Westerhout 10 % Technical discussions

G. K. O. Tsen 10 % MBE growth and technical discussions


Y. Yang 5 % SEM assistance and technical discussions



[3] G. K. O. Tsen, J. Zhang, C. A. Musca, J. M. Dell, J. Antoszewski, and L. Faraone,

‘Various annealing methods for activation of arsenic in Molecular Beam Epitaxy

grown HgCdTe,’ 2008 Conference on Optoelectronic and Microelectronic

Materials and Devices, pp. 125-128, 2008.

The percentage contribution of this author is as follows:

J. Zhang 20 %, MBE growth/annealing assistance, and technical discussions.

8

Chapter 2 HgCdTe Passivation Technologies

2 HgCdTe Passivation Technologies

The importance of surface passivation of HgCdTe detectors will be further discussed

in this chapter from the device architectures and detector performance point of view.

Various passivation materials and technologies are subsequently reviewed. Surface

treatments, being crucial to preparation of HgCdTe and CdTe before formation of the

passivation, are also reviewed.

2.1 HgCdTe as an infrared detector material

The discovery of Hg1-xCdxTe (MCT) by Lawson’s group was first published by them

in 1958, and the publication has been recognised as the earliest known reference to

HgCdTe [19]. HgCdTe is a solid solution of the two binary alloys, CdTe and HgTe.

Several important properties of HgCdTe have rendered it the material-of-choice in the

field of high-performance IR detection. Firstly, it is a direct bandgap semiconductor,

leading to large photon absorption coefficient and high quantum efficiency. It is

suitable for short-wave infrared (SWIR, wavelengths between 0.75 µm and 3 µm),

mid-wave infrared (MWIR, wavelengths between 3 μm and 5 μm), and long-wave

infrared (LWIR, wavelengths between 8 μm and 14 μm) detection, due to its

adjustable direct energy bandgap by tuning the Hg/Cd ratio. The lattice constant

across the entire composition range changes by only 0.3 %, making multilayer

crystalline growth possible. HgCdTe has a small electron effective mass, high

electron mobility (50,000 cm2V-1s-1 at 80 K in Hg0.7Cd0.3Te at 77 K) and hence the

potential for very fast response time. A more detailed description of the properties of

HgCdTe can be found in Appendix A.

Epitaxy techniques such as molecular beam epitaxy (MBE) and metal-organic

chemical vapour deposition (MOCVD) have made available many new bandgap-

engineered materials and device structures. MBE facilitates the growth of

multilayered HgCdTe heterostructures with abrupt changes in alloy mole-fraction, x,

between layers. Bandgap engineered structures can produce infrared sensors with

performance well above that obtained from devices fabricated using a single-layer of

material [20]. Additionally, research results indicate that for some structures

compositionally graded CdTe/Hg1-xCdxTe interfaces for passivation and subsequent

9


annealing can improve lifetime and surface recombination velocity characteristics [21,

22].

2.1.1 HgCdTe device architectures

A variety of HgCdTe device architectures have been developed since the 1970’s, with

the common ones described below. The work on surface passivation was initially

pioneered by Societe Anonyme de Telecommunication (SAT) [23, 24], and

passivation has evolved to be of great importance ever since in these device

architectures.

1) Photoconductors:

Photoconductive devices, as the first generation of HgCdTe devices, entered

production owing to reproducible bulk growth techniques and surface passivation by

anodic oxide. They are fabricated by applying metal electrodes to pure n-type material

(Figure 2.1), and are generally limited to linear arrays with typically fewer than 200

elements [25]. Passivation was found to be a critical step in the fabrication of

photoconductive detectors, especially for small-area devices, and the recombination

of photo-generated carriers at the surfaces/interfaces was found to have a direct

Figure 2.1 Device cross section of HgCdTe photoconductor and its schematic band diagram

hv

CdZnTe substrate

n-HgCdTe

Surface passivation

Contact

CdZnTe substrate

hv

n-HgCdTe

Ec

EF

Ev

hole

electron

10


impact on the performance of detectors [10, 11]. Anodic oxide is an effective

passivation for n-type HgCdTe photoconductive detectors due to its large positive

fixed charge that accumulates the n-type HgCdTe surface and creates a surface field

which drives the minority carrier holes away from the interface, thus separating them

from electrons [9]. Consequently, the carrier lifetime is increased resulting in higher

performance detectors. However, the large positive fixed charge renders anodic oxide

inappropriate for devices that incorporate pn-junctions.

2) Photodiodes:

A variety of HgCdTe photodiode configurations have been proposed, including mesa,

planar and lateral n-p, n+-n-p, p-n, n+-p homojunction and heterojunction structures

[26, 27]. The realization of HgCdTe photodiodes usually relies on the two most

important junction architectures based on n-on-p planar homojunctions or P-on-n

mesa heterojunctions (Figure 2.2) [3, 28]. The p-n junctions can be formed by various

techniques including Hg in- and out-diffusion, impurity diffusion, ion implantation,

electron bombardment, doping during growth, and a variety of other more esoteric

methods [29]. The desired p-type doping can be controlled by the density of acceptor-

like Hg vacancies. Arsenic is also a very useful p-type dopant with stability in the

lattice, low activation energy, and ability to control concentration over a wide range

from 1015 cm−3 to 1018 cm−3 [30]. The n-type doping can be produced by Al, Be, In

and B ion implantation into vacancy doped p-type material [29]. Indium is also

frequently used as a well-controlled n-type dopant due to its high solubility and

moderate diffusion.

n+-HgCdTe

p-HgCdTe

CdZnTe substrate

11


(a) n+-on-p homojunction photodiode

(b) P+-on-n heterojunction photodiode

Figure 2.2 Device cross sections and their schematic energy band diagrams of (a) n+-on-p planar homojunction and (b) P+-on-n mesa heterojunction photodiodes.

P+-HgCdTe

CdZnTe substrate

n-HgCdTe

CdZnTe Substrate

p-Hg1-xCdxTe base layer

n-Hg1-xCdxTe

cap layer

EF hv

Ec

Ev holes

electrons

hv

CdZnTe Substrate

Ec

EF Ev

x < y

P-Hg1-yCdyTe wider bandgap

cap layer

n-Hg1-xCdxTe base layer

holes

electrons

12


Surface passivation is a key technology for reducing surface recombination and

improving the performance of photodiode devices [25]. Commonly CdTe or CdZnTe

are used, deposited by MBE, metal organic chemical vapour deposition (MOCVD),

sputtering or e-beam evaporation.

Homojunction devices suffer from significant surface-related issues, where the excess

thermal generation at the surface results in increased dark current and recombination,

which reduces the photocurrent. Heterojunctions such as N+-p-p+ and P+-n-n+ with

heavily doped contact regions (‘+’ denotes high doping, and the capital letters for

wider bandgap) have demonstrated improved performance over homojunction devices

(such as n-p, n+-p, p+-n) [3]. For heterojunction mesa diodes, the passivation process

can be difficult to control, especially for small-area devices typically used for focal

plane arrays for imaging applications (such devices can be less than 100 µm2 [31,

32]). Double layer heterostructures formed by implantation in planar photodiodes

have less stringent requirements for surface passivation since the junction interface is

buried and protected [27, 33].

The most common open literature architectures used in HgCdTe photodiode

applications by the major focal plane array (FPA) manufactures are listed below [3,

26]:

The n+-on-p planar diode pioneered by SAT [34] has been the most widely

developed and used by Sofradir. It is based on ion implantation into acceptor-

doped p-type liquid phase epitaxy (LPE) HgCdTe grown by Te-solution slider.

The n+-n--p planar diodes used by Rockwell generally have boron implanted

junctions into Hg-vacancy doped p-type HgCdTe with MOCVD CdTe as a buffer

layer and ZnS for surface passivation. To overcome the problem with

hybridisation, Rockwell’s Producible Alternative to CdTe for Epitaxy (PACE)

technology provides intrinsic detector arrays with BLIP performance and a

satisfactory yield.

The p-on-n double-layer planar (buried) heterostructure (DLPH) diode developed

by Rockwell has a wide-bandgap cap layer covering a narrow-bandgap absorber,

which overcomes the sidewall problem [33]. The DLPH structure uses arsenic

implantation into indium-doped N-n or N-n-N material grown by MBE on

CdZnTe substrates.

13


The fabrication technology of mesa-etched photodiodes is based on etching

trenches between devices that define the individual (mesa) diodes. In the p-on-n

double-layer heterojunction (DLHJ) diodes, the n-type absorber layer is

commonly doped with indium sandwiched between the CdZnTe substrate and the

highly arsenic-doped, wider-gap p-type region. One of the critical steps is the use

of CdTe passivant to reduce surface currents, especially for small area devices

found in infrared focal plane detector arrays, preventing surface accumulation or

inversion [35].

The technology of the n+-n−-p vertically integrated photodiode (VIP) used by DRS

Infrared Technologies, also referred to as a high-density vertically integrated

photodiode (HDVIP), is based on Te-solution LPE grown HgCdTe on CdZnTe

substrate followed by the formation of a planar, ion implanted n-on-p junction

[36-38]. The diodes are epoxy hybridized directly to the read out integrated

circuits (ROICs) on 100 mm Si wafers. After epitaxial growth, the substrate is

removed and the HgCdTe layer is passivated on both surfaces. Backside

passivation and frontside passivation with inter-diffused layers of CdTe are

considered critical for high performance and yield. Since the diffusion length in

the absorbing region is typically longer than its thickness, any carriers generated

in the base region can be collected, giving rise to the photocurrent. This effect has

been used in a lateral collection device with a small central contact, called a

‘loophole’ device [39, 40]. For the n-p loophole diodes fabricated by GEC-

Marconi Infra-Red (GMIRL), n-type islands are formed by ion beam milling in a

p-type Hg-vacancy-doped layer grown by Te-solution LPE on a CdZnTe

substrate, and diodes are epoxied onto the silicon ROIC wafer [40].

The impact ionisation that can occur in the high-field region of an avalanche

photodiode multiplies the number of photo excited carriers by the avalanche gain,

leading to increased signal level. The alloy composition of the avalanche gain

layer was tuned to achieve both efficient absorption and low excess-noise

multiplication. A high quality surface passivation process, commonly by CdTe, is

essential in achieving high performance avalanche photodiodes [41, 42].

14


3) nBn (n-type/Barrier/n-type) detectors

There has been growing interest in the ‘n-type/Barrier/n-type’ (nBn) heterostructure

HgCdTe detectors, which consist of an n-type absorbing layer, a wide bandgap barrier

layer, and a thin n-type layer for contact [16-18, 43]. This structure allows for

efficient collection of minority carrier holes whilst creating an efficient barrier to

block majority electrons in the conduction band [44]. Figure 2.3 illustrates the nBn

detector and its energy band diagram. By optimising the band structure, it is, in

principle, possible for the device to operate near flat band conditions. Therefore,

Shockley-Reed-Hall (SRH) generation-recombination processes associated with

depletion regions can be eliminated and noise current can be reduced [16-18, 45-47].

The absorber of an nBn detector is covered with the barrier which also acts as the

passivation layer, thus, it may appear that passivation is less critical than in other

device structures. However, work has shown that surface passivation still plays a

significant role in determining the performance of nBn detectors [16-18]. The

HgCdTe nBn structure shows potential to outperform HgCdTe photodiodes and

achieve BLIP at ~ 207 K (for ≈cλ 6 μm) [16]. However, experimental results showed

dark currents that were two orders of magnitude higher than can be attributed to

surface leakage currents associated with etching induced defects alone [17], thus

indicating that the passivation process needs to be improved.

Figure 2.3 Device cross section of a HgCdTe nBn detector and its schematic energy band diagram under bias.

barrier

hv

n-absorber - n+

+

electron

hole ∆Ec

thermal GR

∆Ev Ec

Ev

15


2.1.2 Measures of device performance

In this section, the main figures of merit for quantifying HgCdTe detector

performance are listed, and the impact of surface passivation on device performance

is discussed.

1) Voltage responsivity:

The voltage responsivity is a commonly used figure of merit for photoconductive

detectors. When a n-type photoconductor is biased by a constant current, the voltage

responsivity Rλ (V/W) can be expressed as [48]

( )

=

lEqr

hcR e

Dτζµhλ

λ V/W (2.1)

where λ is the wavelength, h is Planck’s constant, c is speed of light, η is the quantum

efficiency usually defined as the number of electron-hole pairs generated per incident

photon, q is the electron charge, rD is the detector resistance, and l is the interelectrode

length. The expression in brackets in Eq. (2.1) is referred to as the photoconductive

gain with E the DC bias electric field, μe the electron mobility, τ the excess carrier

lifetime, and ζ represents the reduction in effective carrier lifetime due to

recombination at the contacts.

The voltage responsivity of a photoconductive detector is limited by carrier

recombination at the surface, which affects the minority carrier lifetime. Because

surface recombination processes can be enhanced in narrow bandgap material

detectors, it can become the dominant loss mechanisms for photo-generated excess

carriers [9]. Good surface passivation reduces the minority carrier recombination,

resulting in an increase in the minority carrier lifetime and hence the voltage

responsivity [10, 11]. However, strongly accumulated surfaces that can be found in an

anodic oxide passivated devices can result in a shunting path that reduces the detector

resistance and hence responsivity.

2) Current responsivity, detectivity and noise

The spectral current responsivity is a widely used figure-of-merit for photodiode

16


detectors that represents the number of carriers collected at the contacts for one photo-

generated electron-hole pair. The spectral current responsivity is defined as [12]:

( ) gqhc

WAR λhλ =/ (2.2)

where q is electron charge, and g is the photoelectric gain.

NEP (noise equivalent power) is the incident optical power necessary to produce an

output signal equivalent to the internally generated output noise. The detectivity, D, is

the reciprocal of NEP. As NEP and detectivity are sensitive to the size of the detector

and sampling rate, specific detectivity, D*, that is normalized to the detector area and

sampling rate becomes used more frequently [49]. D* is a primary figure-of-merit for

infrared photodetectors, given by [12]:

( )NEP

fAD

2/10D=∗ (2.3)

where A0 is the optical area of the detector, and ∆f is the frequency bandwidth. The

commonly discussed noise sources for IR detectors include generation-recombination

noise, 1/f noise, Johnson-Nyquist noise, shot noise, and photon noise.

At sufficiently low temperature, the detector thermal noise is negligible, and the

detectivity is limited by randomness in the photon arrival conversion process. For IR

detectors used in imaging, this is referred to a BLIP operation. As the device

temperature increases, the detector thermal noise increases approximately

exponentially, and the detectivity therefore decreases exponentially [12]. The 1/f noise

is found to be dominantly surface related and is associated with surface charge

tunnelling into and out of traps at the semiconductor/passivation layer interface [14,

15].

Surface passivation technology can greatly improve the HgCdTe/insulator interface,

leading to a reduction of both 1/f noise and generation-recombination noise, and

results in an increase of responsivity and detectivity. As an example, Lin et al.

reported on a Hg0.8Cd0.2Te photoconductive detector with a stacked ZnS/photo-

enhanced native oxide passivation that showed higher performance than one

passivated with a single ZnS layer [50]. The stacked passivation had a high quality

17


and stable native-oxide/HgCdTe interface with a low surface state density, a near flat-

band condition, and low leakage current. As a result, the device with such passivation

exhibits an improved noise power spectral density, lower effective surface trap

density and higher detectivity than detectors passivated with only ZnS.

3) R0A product

The R0A product is a commonly used figure of merit for HgCdTe photodiode

detectors, and is the produce of the dynamic resistance at zero bias and the effective

junction area. R0A should be independent of the area of the junction, and represents

the variation of the current density caused by a small variation of the voltage at zero

bias [51]. For photovoltaic devices, R0A product is derived from the diffusion current

and generation current combined in parallel at zero junction bias, and it can expressed

as [52]:

1

0

1

00

−

=

−

=

∂∂

+∂∂

+∂∂

=

∂∂

=bb V

GRhe

V VJ

VJ

VJ

VJAR (2.4)

where J = I /A is the current density, and I is the I-V characteristic of the diode. Je and

Jh are the diffusion current densities of the minority carriers at the edge of the

depletion region in the p-type and n-type materials, respectively. JGR is the current

density due to generation and recombination in the space charge region. The R0A

product is directly proportional to signal-to-noise ratio (SNR). A high R0A

corresponds to high SNR. High-quality photodiodes with high R0A product are

considered to be limited by generation-recombination within the depletion region,

tunnelling through the depletion region and surface/interface effects [12]. Hence,

reducing surface-related current mechanisms can result in an increase in R0A, when

appropriate surface passivation is applied.

2.2 Surface and interface issues with HgCdTe

The performance of HgCdTe detectors is often dominated by the properties of the

interface between the HgCdTe surface and any overlaying passivation layers.

Compared with the bulk, the interface contains a higher defect density. Within the

passivation layer at the interface between the passivation and semiconductor, fixed

charges and interface traps can cause band-banding that depends on the passivation

18


film. For Hg1-xCdxTe with x ≈ 0.2 - 0.3, the bandgap at 77 K varies between 0.1 and

0.25 eV. Hence, the surface potential band bending is often of the order of the

bandgap energy, and can easily accumulate, deplete, or invert the surface. For

example, for HgCdTe photodiodes, passivation of the p-HgCdTe can induce surface

depletion and even surface inversion resulting in surface shunting to adjacent diodes,

while fixed negative charge can accumulate the surface of p-HgCdTe, resulting in

higher electric field across the depletion region and increased tunnelling at the surface

with the N+ contact [14].

Figure 2.4 Possible charge centres for a semiconductor surface passivated with a dielectric, and the resultant interface states and surface band-bending.

Ec

EF Ev

Empty

Filled

Interface Trap States

+

+ +

+

+ + + + +

+-_ +

+-

+

x x x x

x x x x

Fixed Charge

Mobile Ionic Charge

Interface Trapped Charge

HgCdTe Transition Region

Dielectric Passivation

Dielectric Trapped Charge

+ +

19


The characteristics of HgCdTe devices are strongly influenced by the properties of

their surfaces and interfaces. While a review on passivation in narrow-gap II-VI

materials has been given by Nemirovsky et al. [53], there have been several other

reviews of passivation of HgCdTe [8, 9, 53-57], and a number of issues with HgCdTe

have added to the difficulties in studying or improving the properties of HgCdTe

surfaces/interfaces [58]. Firstly, because of the weak bonding of Hg in the lattice,

HgCdTe is sensitive to physical and chemical treatments. Cleaving, lapping, wet

etching, and passivation deposition can all lead to surface damage or a change in

stoichiometry. For example, surface wet etching/cleaning can make the surface Te-

rich [58]. After being exposed to air, the surface oxidises to form TeO2 at the surface.

The process history will affect the density of interface states and surface/interface

recombination. This has led to inconsistencies in the literature regarding HgCdTe

passivation results. The weak bonding of Hg in the lattice means that thermal

instability is an issue for HgCdTe unless special precautions are taken, without which

the processing temperature of HgCdTe cannot exceed 90 °C - 100 °C. As a result, any

passivation approach that involves high-temperature deposition is generally

incompatible with good device performance.

Electrical defect sites associated with the passivation can be attributed to dangling

bonds at the interface, impurities, antisites or interstitial atoms, possibly ionised, at the

interface and within the passivation layer. At the interface these defects introduce

additional energy levels within the bandgap and are called surface states or trapping

sites that act as generation-recombination centres, increasing dark currents and

reducing carrier lifetime. If located just within the passivation layer, electrons can

tunnel in and out of these sites, thus demonstrating the characteristics of a slow trap.

In addition to trapping sites, a passivation layer will contain fixed charge, which may

be distributed throughout the passivation layer [59, 60]. Figure 2.4 illustrates a typical

semiconductor passivation system, showing the possible charge centres in the

passivating dielectric and at the interface.

2.3 Surface passivation materials and technologies

2.3.1 Passivation materials

The passivation film must be thick enough to provide adequate environmental

protection and electrical insulation between the metal interconnect and the underlying

20


substrate. A relatively thick layer may be achieved by deposition of a dielectric

material, either as a capping layer for the native passivation layer or directly onto the

HgCdTe [60].

Passivation technologies for HgCdTe were initiated with anodic oxide growth. The

large positive fixed charge in anodic oxide can be advantageous for n-type MCT

photoconductive detectors. However, the positive charge can invert the surface of p-

type MCT [9]. Other attempts to develop a native passivation include anodic sulfide,

plasma oxide, photochemical oxide and anodic fluor-oxide, often combined with

deposited films such as ZnS, SiN, CdTe, and CdZnTe [60]. Plasma and

photochemically grown oxides offer improved fixed charge levels, and native

sulphides and fluorides may be grown with an even lower fixed charge, providing a

suitable passivation layer for p-n junctions. The drawback of all native layers is the

limited thickness (a few hundred nanometers) to which the layer may be grown, and

their lack of thermal, mechanical and chemical stability.

CdTe has eventually emerged as the leading passivation technology for HgCdTe

devices. Some devices employ dual layer passivation, for example, CdTe and ZnS [2,

34], CdTe and SiNx, and SiOx and SiNx. The CdTe layer provides a good passivation

of the surface, whilst the ZnS provides a higher resistivity insulator, improves the

stability of the CdTe, and can also be used as an anti-reflection (AR) coating.

Passivants for HgCdTe can be classified into three groups summarised below. The

commonly used passivants are a single layer or a combination of the passivants.

1) Native films

• Native oxides, such as anodic oxide, plasma oxide, chemical oxide;

photochemical oxide [8, 53, 54, 56];

• anodic sulfide [8, 61, 62];

• anodic fluor-oxide [63, 64];

2) Deposited films

• ZnS [65];

• CdZnTe [30, 66];

• CdTe [30, 67];

• SiOx [68, 69];

• SiNx [11, 70-72];

• Polymers, such as SU-8 [73].

21


2.3.2 CdTe passivation

There has been an intensive research effort on CdTe and heterojunction-based

passivation over the last 10 years. High quality CdTe is known to have high

resistivity, be transparent in the IR region, is nearly lattice-matched to the important

HgCdTe compositions (lattice mismatch < 0.3%), and is chemically compatible with

HgCdTe. CdTe passivation typically results in good interface properties, low fixed

charge, and low interface trap densities [58]. Drawbacks of using CdTe in the

passivation process for HgCdTe include the lack of a selective etch between CdTe

and HgCdTe, and a relatively high processing temperatures of > 200 °C required to

deposit high quality stoichiometric CdTe films.

Epitaxy techniques, like MBE and metal-organic chemical vapour deposition

(MOCVD), enable the in-situ growth of high-quality CdTe on top of HgCdTe in a

single run. In-situ growth reduces the likelihood of contamination and would be the

ideal method of CdTe deposition [74]. The grading at the CdTe/HgCdTe interface is

found to be beneficial for surface passivation [53], as it shifts the HgCdTe surface

into the wider gap CdTe region, which gives higher thermal stability.

2.3.3 ZnS passivation

ZnS has been a popular choice among passivation materials, especial being used as a

dual layer on top of CdTe. The disadvantages include its hygroscopic nature and

inconsistent interface properties caused by contamination during the deposition

process. The distribution of fixed charge, usually negative, can also cause non-

uniformity of device performance [75, 76].

Research has been reported on the study and improvement of its long-term stability.

For example, Zhang et al. [77] characterised interface electrical characteristics of

ZnS/CdTe passivation films on HgCdTe by C-V characteristics of MIS test structures

(x-value = 0.22, 0.25). MIS devices with a ZnS/CdTe dual layer were shown to be

superior to devices with just ZnS. For the MIS structure with anodic oxide/ZnS/CdTe

triple layer films, the surface fixed charge density was too high (1.59×1012 cm-2) for

good passivation of photodiodes.

22


2.3.4 SiNx passivation

SiNx has been characterised as having high dielectric quality and relatively low

deposition temperature (depending on the deposition technique), which is desirable

for HgCdTe passivation. It is more moisture resistant than ZnS or SiOx, and acts as an

effective barrier to external contaminants. It has excellent film uniformity and high

resistivity. Tuneable near-zero mechanical stress can be obtained by modifying the

deposition conditions.

Most of the published work on SiNx as a passivant has been on silicon. Initial papers

on plasma deposited silicon nitride are by Sinha et al. [78] and Kern and Rosler [79]

in the late 1970’s. Later, more work was carried out on PECVD deposited

hydrogenated films (SixNy:Hz or α-SiNx:H) [80], hereafter referred to as SiNx.

Schorner and Hezel have reported that the density of interface states Dit of SiNx/Si is

much higher than that of the SiO2/Si interface [81]. Various studies have been

undertaken in order to improve the hydrogenated SiNx passivation quality and to

investigate the influence of film composition on electrical and mechanical

properties [82-85]. A more detailed literature review on the deposition parameters

concerning high-temperature deposited SiNx films is in Appendix B. High-

temperature processes are not applicable to HgCdTe as detailed earlier.

There are two different views on the structural properties of as-deposited SiNx films in

the literature, yet the issue still remains unresolved. Some argue that these materials

are made up of phases. The material may be composed of a mixture of amorphous Si

and silicon nitride [86, 87] or a two-phase mixture of a highly structured Si3N4-like

phase with a low level of defects and low H content, and a highly disordered phase

with a high concentration of Si-H and N-H defect states and high H content [88, 89].

Some others have adopted the random bonding model, in which there are five possible

Si tetrahedrons Si-SiNnN4-n (n = 0, 1, 2, 3 or 4) in the films, excluding H. The

probability for the occurrence of each tetrahedron is determined statistically [90].

Information in the open literature on SiNx as a HgCdTe passivant is limited. Two

papers from Fujitsu Laboratories [70, 71] reported electron cyclotron resonance

plasma chemical vapour deposited (ECR PCVD) SiNx passivant for Hg0.7Cd0.3Te n+p

diodes. The maximum temperature was 95 °C during the deposition. Their

23


measurements of flat band voltage shifts after exposure to humidity verified that SiNx

is more moisture resistant than conventional ZnS passivant. Another paper explored

using SiNx for CdZnTe surface passivation [91], and found that sputtered SiNx can be

used to passivate the CdZnTe surface; however, by itself it appears to provide only

modest improvements in surface resistivity; a combination of oxygen plasma and SiNx

coating greatly improved the surface resistivity with likely long-term stability.

Westerhout et al.’s work on PECVD SiNx passivated HgCdTe photodiodes has shown

that there is an increase in the dynamic resistance and decrease in the leakage current

[92], compared to photodiodes passivated by ZnS alone [72].

The atomic hydrogen in the SiNx film can neutralise defects in the interface and the

bulk, which could be enhanced by high-temperature annealing or thermal treatment to

help with diffusion and releasing hydrogen from the Si-H and N-H bonds located in

the SiNx film into the interface and the bulk [82]. For semiconductors or substrates

with high defect densities, such as HgCdTe, hydrogen is expected to work effectively

when diffusing into the bulk material even without any high temperature process step

[93, 94]. More details of related work on SiNx passivation on HgCdTe are discussed

in Chapter 5.

2.3.5 Dual-layer passivation

To improve the resistivity and stability of CdTe passivation, the application of a dual-

layer passivation has been reported, and has been shown to improve the performance

of HgCdTe photodiodes alluded to in the previous sections. The additional ZnS or

SiNx layer improves the insulation characteristics and protects the underlying CdTe

from environmental factors [92, 95-97].

When using CdTe as a passivant, its high index of refraction, which is similar to

HgCdTe, makes it unsuitable as an antireflection (AR) coating. However, with an

additional film of ZnS or SiNx, both with good adhesion to HgCdTe, an AR coating

can be achieved for front-illuminated devices. .

Plasma or evaporated deposited SiO2 and SiOx have shown good insulation and

interface properties [98, 99], however, their porosity can lead to poor moisture

resistance, similar to that of ZnS. For example, absorption of H2O in SiO2 passivated

devices has been shown to induce flatband voltage shifts [99].

24


Westerhout et al. used PECVD SiNx/CdTe dual-layer passivation on gated diodes

[92], and they demonstrated that SiNx can be used as a high resistivity, stable

replacement for ZnS to passivate plasma-type converted HgCdTe photodiodes.

2.4 Surface preparation for HgCdTe and CdTe

Surface cleaning and conditioning is an essential step before any passivation process,

in order that the passivant can be deposited or grown with minimal fixed charge and

interface states. The specific pre-passivation surface treatment used is dependent on

the passivation process, and, in general, the surface is cleaned via wet processes. The

commonly used etchants include Br2/methanol (1-10%), Br2/HBr (1-5%),

Br2/HBr/Ethylene glycol (in various ratios), K2Cr2O7:HNO3:H2O(4 gm:10 ml:20 ml),

photo electrochemical etching in 1M HClO4, or in 1M KCl [100].

One of the advantages of wet etching over dry etching is that it generates lower

structural and electrical damage to the material. Wet etching of HgCdTe nowadays is

generally not used for device processing, and is limited to surface preparation because

of its poorly controlled etch rates and isotropic etching behaviour. Nearly all the

etchants listed below leave the HgCdTe surface Te rich, which leads to increased

surface leakage across p-n junctions that degrades diode characteristics. The Te rich

layer can be controlled by quenching the etching using methanol [20]. The removal of

the Te-rich layer, Te oxides, and HgTe-rich layers can be achieved by soaking in a

10% KCN solution that regains surface stoichiometry without removal of further

HgCdTe [100, 101].

For dual-layer passivation, surface cleaning and conditioning of the CdTe layer is

important. Since CdTe has been widely used in solar cells and room-temperature X-

ray and gamma ray detectors, as well as IR HgCdTe detectors, there is a significant

body of literature on wet etching of CdTe. Almost all of the etchants used for CdTe

are transferable to HgCdTe. Reported etching solutions include [102-104]:

• HCl or H2SO4

• HNO3

• NaOH

• 10% NaOH + Na2SO4

• 2 HF, 1 HNO3, 1 CH3COOH

25


• Br2 + CH3OH (e.g. 0.5% Br2 in CH3OH)

• 50 HNO3 + 10 CH3COOH + 1 HCl + 18 H2SO4

• 7g K2Cr2O7 + 3g H2SO4

• 0.5% Br2 + 10 mg AgNO3 + CH3OH

• 2 H2O2, 3 HF, 1(or 2) H2O

• 2 HNO3, 2 HCl, H2O

• 10 ml HNO3, 20 ml H2O, 4 g (or 2 g) K2CrO7

• HNO3/H3PO4 (NP) (used for solar cells) [105, 106]

• H2O2 + HBr + ethylene glycol(EG) [107]

The surface cleaning and conditioning solutions for HgCdTe and CdTe used in this

thesis were all based on a Br2/methanol (0.05-1 %) wet etch, with different ratios used

according to the specific purpose of the processing.

2.5 Modification of interface trap density

There are a number of possible approaches to ameliorate the effects of interface traps,

including annealing, hydrogenation, and modification of stress in the passivation

layers.

Epitaxy techniques, like MBE and metal-organic chemical vapour deposition

(MOCVD), enable the in-situ growth of high-quality CdTe on HgCdTe. In-situ grown

CdTe layers have been characterised as having low fixed charge density [74]. The

electrical properties of the interface can be significantly influenced by the CdTe

growth temperature, pre-treatment and post-treatments [108]. Thermal annealing in a

Cd/Hg atmosphere has been recognised as a crucial step in passivation. Annealing

leads to the formation of compositional grading across the CdTe/HgCdTe interface,

and the reduction in surface recombination velocity (SRV) from 2 × 104 cm·s-1 to ~

3000 cm/s at 77 K [109]. Diodes fabricated using the compositionally graded layer

were reported to have one order of magnitude higher R0A value in comparison to

those passivated by an abrupt interface between CdTe and HgCdTe [110].

Although annealing at elevated temperature helps to create a graded interface layer

due to interdiffusion across the CdTe/HgCdTe interface, electrical and photoelectrical

properties of the HgCdTe active layer may also be changed. A combination of

26


annealing at 350 °C for 1 hour followed by annealing at 125 °C for 24 hours was

proposed in [111], which resulted in a stable interface as shown by long-term

annealing for two weeks. The CdTe passivation film used in this thesis was grown by

MBE at 100 °C and followed by in-situ annealing at 180 °C in the growth chamber to

achieve a compositionally graded CdTe/HgCdTe interface.

Photon-induced interface modification was reported by Agnihotri et al. [108], which

includes pre-annealing the HgCdTe wafers under ultraviolet photon excitation and in

a Hg environment, as well as the post-deposition annealing of CdTe/HgCdTe under

photon excitation. Pre-annealing of 2 hours and post-annealing of 3 hours were found

to give the lowest fixed charge and interface trap density. This work postulates that

excited Hg atoms and hydrogen radicals are formed by direct collisions in the vapor

phase, which then passivate the wafer surface, reduce the native oxides, and remove

water molecules on the wafer surface.

For the case of ZnS as a passivant, ammonium sulfidation treatment ((NH4)Sx) of the

HgCdTe substrate was reported to be a way of modification of interface traps [112].

Sulfidation results in a decrease in the concentration of contaminants originating from

the native oxide-covered HgCdTe substrates. The fixed charge and slow trap density

were found to be between 2 and 7 times lower in the treated samples compared to

untreated MIS structures.

Most hydrogenation studies of semiconductors have been carried out on GaAs and Si

materials, and has led to the development of the technical application of hydrogenated

amorphous silicon [113]. However, little work has been performed on II-VI

compounds. The principle interest in hydrogenated passivants is the ability to

passivate the electrical activity of dangling or defective bonds.

Importantly, and of less positive impact, hydrogen can also significantly change the

electrical and optical properties of the bulk materials, including passivation of shallow

acceptor and donor impurities in several technologically important semiconductors

[114]. Hg vacancies have been shown to be effectively passivated by atomic

hydrogen. Hydrogen injection and passivation of the residual impurities are also

observed in Hg0.8Cd0.2Te boiled in water [115, 116]. The work in this thesis also

observed the surface and bulk passivation effects induced by ICPECVD hydrogenated

SiNx. Magneto-transport measurements were carried out before and after SiNx

passivation on HgCdTe. Comparing the results analysed using quantitative mobility

27


spectrum analysis (QMSA), there is an increase in electron mobility in HgCdTe after

being passivated by SiNx, which can be explained by hydrogen passivation of

dangling bonds during the ICPECVD process. This is discussed further in Section

3.3.5.

As the thermal stability of hydrogen passivation, most of the defects and impurities in

Si, such as Au, Cu, Fe and Zn, that are passivated by reaction with atomic hydrogen

were found to be regenerated by post-hydrogenation annealing ( > 400 °C) [117].

Similarly, Hirayama and Tatsumi and Iyer et al. have observed remarkable stability of

the hydrogen-passivated surface when being supplied with atomic hydrogen at

temperatures below 400 °C [118, 119], whereas they have found that the hydrogen

passivation was quickly lost at temperatures higher than 400 °C. Also, because of the

greater degree of lattice relaxation associated with deep levels, the passivation of deep

level centres was found to be much more thermally stable than shallow level

passivation [117].

Exposing a ZnS passivation layer to H2/CH4 RIE plasma has been shown to reduce

the fixed charge density, while the interface trap density is unchanged [120]. It was

found that the LWIR photodiode leakage current was reduced after the treatment, and

the junction properties also were improved with hydrogenation [121].

Lattice mismatch between passivation and epilayer and/or between substrate and

epilayers of different mole-fractions can create a variety of distortions and defects in

HgCdTe, affecting the performance of infrared detectors. Therefore, modification of

stress in the layers can result in modification of interface traps. White et al. have

observed an increase in trap-assisted tunneling at the junction surface perimeter,

which is associated with the increased lattice stress [122]. The interdiffusion of Zn

and Hg, due to the deposition of ZnS on HgCdTe and post-baking in a vacuum, has

led to lattice stress due to the smaller lattice constant of ZnTe. CdTe passivation as the

standard approach in infrared detector technology is associated with its nearly lattice

match with HgCdTe, resulting in minimal stress [9]. As to the effects of interfacial

lattice mismatch between HgCdTe epilayers, Sugiura et al. [123] observed that

HgCdTe epilayers can be easily affected by lattice mismatch of less than ± 0.1%. In

addition, they observed that the mismatched HgCdTe epilayers in tension tend to

deteriorate more easily than those in compression, which is associated with the

28


asymmetric dislocation distribution due to excess mercury vacancies, the sign of

strain and lattice structure.

2.6 Summary

Surface passivation is becoming a key technology for reducing surface recombination

and improving the performance of HgCdTe detectors. This chapter reviewed the

importance of surface passivation for HgCdTe detectors in terms of device

architectures and performance. Passivation materials and surface treatments for

HgCdTe were also reviewed. Lastly, this chapter presented surface passivation

materials and technologies used in this thesis.

29

Chapter 3 Material Characterisation

3 Material Characterisation

3.1 Introduction

Characterisation techniques that have been used to investigate the CdTe/HgCdTe

interface include material characterisation probes, optical characterisation studies and

electrical characterisation. There are a number of material analysis techniques that are

applicable to HgCdTe, including high-field low-temperature magneto-optic studies

[124-128], scanning capacitance micro-probe measurements [129], photo-reflectance

and other modulated optical pump-probe techniques [130, 131], reciprocal-space X-

ray topographic imaging measurements [20, 132], magneto-transport studies based on

quantitative mobility spectrum analysis (QMSA) or high-resolution mobility spectrum

analysis (HR-MSA) [133-136], spatial lifetime mapping [137, 138], and scanning

photoluminescence [129, 137, 139].

The characterisation techniques employed in this thesis are discussed below in two

categories: physical characterisation and electrical characterisation. Some of the

techniques were performed as in-situ monitoring characterisations within the MBE

growth chamber, whereas others were undertaken as ex-situ characterisations. The in-

situ diagnostic techniques used to monitor the growth of HgCdTe in the MBE

chamber related to this thesis include reflection high energy electron diffraction

(RHEED) and spectroscopic ellipsometry. The other two commonly used in-situ

diagnostic techniques in the growth of HgCdTe, beam flux monitoring and residual

gas analysis (RGA), will not be discussed here.

3.2 Physical Characterisation

3.2.1 Microscopy

In this thesis, optical microscopy, as an ex-situ characterisation technique, has been

used to inspect defects and morphology. As to the growth of MBE HgCdTe in the

thesis, the Cd0.96Zn0.04Te substrates with a (211)B orientation from the Nippon Mining

and Materials were used. The Cd0.96Zn0.04Te substrate is nearly lattice matched to

Hg0.78Cd0.22Te that helps to minimise the misfit dislocation formations [57]. The CdTe

cell temperature of 525 °C to 540 °C, Te cell temperature of 310 °C to 325 °C and

30


substrate temperature of 185 °C were used, with the beam equivalent pressures (BEP)

of approximately 10-6 Torr. The LPE HgCdTe wafers used in the thesis were

purchased from the Epitech Company, and have a (111) orientation and x value of 0.3.

Figure 3.1 SEM micrographs corresponding to (a) and (b) 60 nm-thick CdTe film on HgCdTe, and (c) 300 nm-thick CdTe on HgCdTe. The CdTe layer was deposited in an MBE system.

(a)

(b)

(c)

200 nm

100 nm

200 nm

31


MBE grown samples undergo inspection by microscopy after unloading from the

MBE growth system and after wafer processing for defect assessment. The Nomarski

contrast imaging mode is particularly useful as it offers high resolution and clarity

[140], providing easily observed information on surface planarity and macro-defects.

Crosshatched patterns are generally observed on the HgCdTe surface even when

samples are grown close to MBE optimal conditions, which is due to strain induced

by lattice mismatch. The crosshatch pattern on the (211)B surface comprises of three

sets of lines in the ]312[ , ]132[ and ]101[ directions [141].

Scanning electron microscopy (SEM) has also been used for imaging to examine

surface morphology in more detail. For example, the low-temperature MBE grown

CdTe passivation films used in this thesis were studied using SEM to investigate their

surface morphology. Smooth grains with flat surfaces were observed for both 60 nm-

thick and 300 nm-thick CdTe films (Figure 3.1 (a) and (c)). An SEM micrograph of

observed surface defects is shown in Figure 3.1 (b).

3.2.2 X-ray diffraction

Double crystal X-ray diffraction (DCXRD) was used to characterise the crystalline

quality of MBE grown CdTe and HgCdTe materials, utilising a Panalytical Empyrean

X-ray diffractometer. The rocking curve measurements allow the crystal perfection

and strain to be characterised. The full-width-at-half maximum (FWHM) of the

DCXRD rocking curve peak was used as a metric of the crystalline quality of the

MBE grown layers. Depending on the MBE growth conditions, HgCdTe samples

used in this thesis typically have a FWHM between 70 - 120 arcsecs, which is an

indication of good single-crystal material [142]. Figure 3.2 shows the DCXRD spectra

of a CdTe layers on a GaAs substrates, grown at 275 °C by MBE, indicating a FWHM

of 66 arcsecs. The samples were used for the SiNx stability studies in Section 3.4.

Figure 3.3 shows the DCXRD spectra of a MBE grown HgCdTe layers on CdZnTe

substrate (n-HgCdTe/n+-HgCdTe/CdZnTe) with a FWHM of 73 arcsecs. These

samples were used for MIS structure fabrication and capacitance-voltage analysis in

Chapter 5.

32


Figure 3.2 Double crystal X-ray diffraction spectra of MBE grown CdTe layer on GaAs substrate.

Figure 3.3 Double crystal X-ray diffraction spectra of MBE grown HgCdTe layers on CdZnTe substrate (n-HgCdTe/n+-HgCdTe/CdZnTe).

37.95 38 38.05 38.1 38.15 38.20

200

400

600

800

ω

Cou

nts

(degrees)

36.1 36.15 36.2 36.25 36.3 36.350

500

1000

1500

2000

2500

ω

Cou

nts

(degrees)

33


Figure 3.4 X-ray diffraction spectra of (a) the LPE HgCdTe before CdTe growth and (b) the MBE grown CdTe (on LPE HgCdTe).

The CdTe passivation films used in Chapter 4 were grown by MBE at a low

temperature of 100 °C, followed by in-situ annealing at 180 °C in the growth chamber

to achieve a compositionally graded CdTe/HgCdTe interface. Because of the 100 °C

growth temperature, the films are found to be polycrystalline. The θ-2θ XRD spectra

were collected before and after CdTe growth. The crystallographic studies of the low-

temperature grown and in-situ annealed CdTe films were carried out by XRD using

Cu Kα radiation at room temperature. Figure 3.4 (a) shows the XRD pattern of the

HgCdTe before CdTe growth, which indicates that structure of LPE grown HgCdTe is

composed of HgCdTe (111) as the preferred orientation. Figure 3.4 (b) shows the

low-temperature deposited CdTe passivation film to be polycrystalline with (100) and

possibly (111) orientations.

3.2.3 Reflection high energy electron diffraction

Reflection high energy electron diffraction (RHEED) is a very useful surface analysis

technique, and its compatibility with the MBE HgCdTe growth process allows the in-

situ monitoring of the diffraction pattern in order to extract information on the atomic

arrangement of atoms near the crystal surface. The technique has the potential to

10 20 30 40 50 60 70 800

5

10

15C

ount

s (x

104 )

10 20 30 40 50 60 70 800

2

4

2θ

Cou

nts

(x10

2 )

(a)

(b)

LPE HgCdTe

200nm MBE CdTe filmCdTe(111)

CdTe(400)

HgCdTe(111)

CdTe(511)

(degrees)

34


allow real-time adjustment of the growth condition during the actual growth for some

material composition and crystal orientation. The main parts of a RHEED system are

an electron gun, deflection plates, the crystalline film to be analysed, and a phosphor

screen with a shutter. When the electron beam is directed onto the sample at grazing

incidence, an interaction with the surface atoms, which diffract the electrons in a

pattern, may be observed on the phosphor screen.

In this thesis, RHEED patterns were monitored periodically during the MBE growth,

so that in addition to the flatness of the substrate, the condition of the buffer layer and

HgCdTe layers could also be monitored. In contrast to the growth on (100) substrates,

HgCdTe growth on (211)B substrates do not show observable RHEED intensity

oscillations, which are determined by the surface atomic flatness [143].

An example of the recorded RHEED patterns is shown in Figure 3.5 for HgCdTe

sample CMCT042, which was later fabricated into MIS structures in order to

investigate SiNx/HgCdTe interface, as detailed in Chapter 5. Figure 3.5 (a) is the

pattern seen during the thermal cleaning of the (211) B CdZnTe substrate, which has a

Te-rich surface due to the etch in a Br/methanol solution used for surface conditioning.

During the thermal cleaning process, which heats the sample to an elevated

temperature of 285 °C - 290 °C for 20 minutes under ultra-high vacuum, the excess

Te becomes mobile on the CdZnTe surface, giving the RHEED pattern the

appearance of an array of bright spots. During thermal cleaning, the spotty pattern

gradually turns into a streaky one, as shown in Figure 3.5 (b), indicating a flat,

crystalline surface.

The first HgCdTe layer grown on the CdZnTe substrate is a 2 μm-thick indium-doped

n-type layer with an x-value of 0.4, which serves both as a low resistance contacting

layer and as a confinement layer to confine photogenerated minority carriers in the

absorber layer. The RHEED patterns monitoring the above n-HgCdTe growth are

shown in Figure 3.5 (c) and Figure 3.5 (d). The pattern with clear and elongated

streaks is an indication of a flat and single crystal surface morphology. Then the

5 μm-thick absorber layer (x = 0.316) was grown, with RHEED patterns shown in

Figure 3.5 (e) and Figure 3.5 (f), still with elongated streaks, showing the absorber

layer has an atomically flat surface morphology.

35


Figure 3.5 The RHEED patterns recorded for HgCdTe sample CMCT042. (a) During substrate thermal cleaning; (b) Toward the end of thermal cleaning; (c) At the start of growth of n-HgCdTe layer (x = 0.4); (d) Toward the end of the growth of n-HgCdTe (x = 0.4) layer; (e) At the start of growth of the MWIR absorber layer (x = 0.316); (f) Toward the end of the growth of absorber layer.

3.2.4 Energy dispersive X-ray analysis

Energy dispersive spectroscopy (EDS) is an electron beam analysis procedure for

examination of the stoichiometry of SiNx films. The FEI Verios SEM system is

equipped with an Oxford energy dispersive X-ray spectrometer with an 80 mm2

silicon drift detector installed. EDS is a technique for quantitative elemental analysis

of a sample by measuring the spectrum of characteristic X-rays emitted from a sample.

To stimulate the emission of characteristic X-rays from a sample, a high-energy beam

of electrons focused onto the sample under study. The number and energy of X-rays

photons emitted from the sample can be measured by an energy-dispersive

spectrometer.

The SiNx/Si samples under study in Section 3.4 had corresponding SiNx films

deposited on GaAs substrates as reference samples for EDS using a beam energy of

5 keV in order to estimate stoichiometry of SiNx films, i.e. [N]/[Si] atomic ratio.

(b) (a)

(e) (d) (f)

(c) 10 mm 10 mm 10 mm

10 mm 10 mm 10 mm

36


GaAs was chosen as the substrate to prevent interference from a Si substrate that

would occur due to the relatively thin SiNx films used. Also, EDS was applied on the

SiNx/GaAs reference samples of SiNx/HgCdTe to estimate the stoichiometry of SiNx

films on HgCdTe, in order to avoid any electron beam damage to the HgCdTe wafers.

3.2.5 Spectroscopic ellipsometry

Spectroscopic ellipsometry measures the change in polarization as light reflects or

transmits from a material structure as a function of optical wavelength. By comparing

the measured wavelength-dependent dielectric function with a pre-stored library file,

film thickness and composition can be extracted. It is also capable of characterising

crystallinity, surface/interface roughness, doping concentration, and other material

properties associated with a change in optical response [144].

Figure 3.6 (a) Refractive index, n, and (b) extinction coefficient, k, measured by ellipsometry for a 11.18 μm-thick HgCdTe/CdZnTe sample numbered MCT223 (x = 0.281).

5 6 7 8 9 103.15

3.2

3.25

3.3

Wavelength (µm)

n

5 6 7 8 9 100.05

0.1

0.15

Wavelength (µm)

k

(a)

(b)

37


The ability to perform in-situ ellipsometry during MBE growth allows real-time

adjustment of growth conditions to improve film quality. While in-situ capability is

now available in our MBE system, during the growth of the samples related to this

thesis the in-situ ellipsometry had not been commissioned, so refractive index and

extinction coefficient of grown HgCdTe layers was measured by ex-situ ellipsometry,

with an example shown in Figure 3.6. Such a thick layer of 11.18 μm was designed

for the sample to be measured by ellipsometry with an increased accuracy in

modelling.

3.2.6 Optical reflection/transmission for structural and compositional characterisation

Transmission measurements were undertaken using Fourier transform infrared (FTIR)

spectroscopy to determine the HgCdTe compositions (x-value) and layer thicknesses.

FTIR absorbance spectra from the SiNx films were used and monitored to examine the

film stability over a six-month time frame. The results of these studies are discussed

in detail in Section 3.4 and Chapter 5. The bonding configurations are discussed in

Section 3.4.4.3.

Figure 3.7 FTIR transmission spectra for MCT223 (x = 0.281, depilayer = 11.18 μm) with the air background being subtracted. Solid line: measured FTIR transmittance; dotted line: modelled transmittance curve.

500 1000 1500 2000 2500 30000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Wavenumber (cm-1)

Tran

smis

sion

ModelMeasured

38


Figure 3.8 FTIR transmission spectra for MCT225 (x = 0.375, depilayer = 8.9 μm) before and after wafer annealing, with the annealing being carried out in a saturated Hg atmosphere at 235 °C for 24 hours. Dotted line: measured transmittance before annealing; Solid line: transmittance after annealing.

Figure 3.7 shows the room-temperature transmission spectra of HgCdTe sample

MCT223 that was characterised by ellipsometry in Section 3.2.2. The abrupt cut-off

indicates the MBE grown layer has good compositional uniformity. The x-value and

film thickness can be extracted from the transmittance curve. The cut-off is a function

of the bandgap and is dependent on x-value. The fringes in the curve show the

interference effects of the infrared radiation reflected between the sample surface and

the epilayers/substrate layer interface, allowing film thickness to be calculated from

the periodicity of the fringes. The extracted x-value for HgCdTe sample MCT223 is

0.281, with a thickness of 11.18 μm. The estimated error in determining the epilayer

thickness and its x-value is ± 0.1 %. The measured transmission is shown in

Figure 3.7 as the solid line and the modelled results as the dotted line. The FTIR

transmission spectra for HgCdTe sample MCT225 before and after a vacancy-filling

anneal are shown in Figure 3.8, with the annealing being carried out in a saturated Hg

atmosphere at 235 °C for 24 hours. There was only a slight change in the mole

fraction indicated after wafer annealing.

1000 2000 3000 40000

0.1

0.2

0.3

0.4

0.5

0.6

Wavenumber (cm-1)

Tran

smis

sion

After annealingBefore annealing

39


3.2.7 Optical reflection/transmission for bonding and detailed characterisation of thin films

An indirect estimate of the [N]/[Si] ratio in thin films of SiNx can be obtained based

on relations between [N]/[Si] and parameters that are [N]/[Si]-sensitive that have a

monotonic variation with [N]/[Si] and are easily measurable. In this thesis, refractive

indices determined by ex-situ spectroscopic ellipsometry have been used to achieve

this [145, 146]. Ex-situ ellipsometry has been carried out by using a Filmetric

spectroscopic thin-film analyser. After data fitting, refractive index was firstly

acquired in the range from 380 nm to 1050 nm, then the refractive index at a single

wavelength (usually at 632.8 nm, n632.8nm) was used for the estimation of film

stoichiometry. A single n value of n632.8nm is useful when comparing the properties of

SiNx films with literature and to other materials, such as SiO2.[147] The relationship

between [N]/[Si] ratio and refractive index are investigated in Section 3.4.

3.3 Electrical Characterisation

3.3.1 Magneto-transport measurements

Hall-effect and resistivity measurements are employed to study carrier transport in

semiconductor materials to gain knowledge of carrier concentrations and mobility,

thus providing information that can be correlated to material quality and device

performance [148]. The principle of the Hall effect can be explained by considering a

slab of conducting material through which a uniform current density flows under the

presence of an applied magnetic field applied in the direction perpendicular to the

current flow [149, 150]. In this session, Greek Cross Van der Pauw test structures

[151] were utilised for the Hall measurements. Figure 3.9 illustrates a

photolithographically defined Greek Cross Van der Pauw structure fabricated on

HgCdTe.

For the Hall-effect measurements, Hall and resistivity voltages were measured as

function of magnetic field intensity [152, 153] in a narrow-gap 2-Tesla electromagnet

Hall-effect measurement system at UWA. In this thesis, the high-resolution mobility

spectrum analysis (HR-MSA) algorithm developed by researchers at the University of

Western Australia was used to analyse the measured magnetic field dependent

resistivity and Hall coeffient, since HR-MSA has demonstrated to be less sensitive to

40


moderate noise levels in the measured data, and with improved robustness and

resolution than commercially available mobility spectrum analysis algorithms [154,

155]. The HR-MSA algorithm, although slow computationally, has the ability to

resolve closely spaced mobility peaks and provides accurate information on the

mobility distribution of each carrier species [1, 2, 133, 135, 156]. Mobility spectrum

analysis of the variable magnetic field Hall data yields a conductivity spectrum as a

function of mobility for the individual hole and electron carrier species present in the

sample, with each conductivity peak representing an individual carrier distribution.

From each conductivity peak an average mobility and carrier concentration can be

obtained, thus enabling the discrimination of carrier transport parameters in samples

exhibiting multiple carrier species [20, 134, 157]. It is important to note that, since all

the samples studied were n-type, only electron mobility spectra are discussed and

presented in this session, since no significant contribution from holes was found in the

HR-MSA extracted mobility spectrum characteristics.

In order to evaluate the effect of the CdTe passivation film, grown by MBE as at

100 °C and in-situ annealed at 180 °C, on the transport parameters of HgCdTe

epitaxial layers, magneto-transport measurements were carried out on the HgCdTe

wafer labelled MCT225 (x = 0.375, epilayer thickness depilayer = 8.9 μm) using the

following procedure:

1. As-grown wafer was annealed under saturated Hg atmosphere at 235 °C for

24 hours. The wafer was then diced into two pieces.

2. After the vacancy filling anneal, one wafer piece was processed into a mesa

isolated Van der Pauw [158] structure with a Greek Cross topology.

3. Magneto-transport measurements were performed on the processed Van der Pauw

structures at liquid nitrogen temperature. The results of the analysis of these

magnetic field-dependent resistivity and Hall-effect data extracted using HR-MSA

are illustrated in Figure 3.10.

4. The surface of the second half of the annealed MCT225 sample was etched in 0.1%

Br2:Methanol solution, and immediately loaded into the MBE chamber where the

CdTe passivation layer (~ 100 nm thick) was deposited. Following CdTe

passivation, the sample was processed into a Van der Pauw structure. For ohmic

contact formation, the CdTe passivation layer was etched in a 1% Br:HBr solution

41


prior to Au/Cr metal deposition. Note that the sidewalls of the structure were left

unpassivated.

5. Magneto-transport measurements were then performed at liquid nitrogen

temperature on the processed CdTe-passivated Van der Pauw structure from Step 4.

Again, the measured data was analysed using HR-MSA. The extracted electron

mobility spectra are presented in Figure 3.10, for comparison with the unpassivated

structure. The extracted transport parameters are summarised in Table 3.1.

Figure 3.9 Image of the centre part of a fabricated Greek Cross van der Pauw structure on HgCdTe taken under an optical microscope.

Figure 3.10 Comparison of the electron conductivity - electron mobility spectra measured before and after MBE CdTe growth.

400 μm

103 104 1050

1

2

x 10-4

Mobility (cm2 V-1 s-1)

She

et C

ondu

ctiv

ity (Ω

/squ

are)

-1

HgCdTeCdTe/HgCdTe

3

42


Table 3.1 Summary of extracted electron transport parameters for HgCdTe sample MCT225 before and after the CdTe passivation

Peak 1 (Bulk) Peak 2 (substrate interface) Peak 3 (surface) Total sheet

conductivity

µb (cm2/V⋅s) Nb (cm-2) σe µsi (cm2/V⋅s) Nsi (cm-2) σsi µs (cm2/V⋅s) Ns (cm-2) σs σxx (0) (Ω/square)-1)

Unpassivated 25597 1.27×1011 0.62 9801 1.14e×1011 0.21 4331 2.01e×1011 0.17 8.47×10-4 Passivated 20176 7.41×1010 0.66 7307 5.75×1010 0.19 2522 1.24×1011 0.14 3.67×10-4

µ (cm2/V⋅s): electron mobility; N (cm-2): electron concentration; σ : conductivity percentage; σxx (0) (Ω/square)-1): total sheet conductivity

Figure 3.11 Plots showing the electron mobility spectrum measured after the vacancy filling anneal at liquid nitrogen temperature for (a) MCT231 (x = 0.388, depilayer = 6.4 μm) and (b) MCT240 (x = 0.347, depilayer = 5.23 μm).

103 104 1050

0.2

0.4

0.6

0.8

1 x 10-4

Mobility (cm2 V-1 s-1)

She

et C

ondu

ctiv

ity (Ω

/squ

are)

-1

(a)

103 104 1050

0.2

0.4

0.6

0.8

1 x 10-4

Mobility (cm2 V-1 s-1)S

heet

Con

duct

ivity

(Ω/s

quar

e)-1

(b)

43

Chapter 3 M

aterial Characterisation

43


From Figure 3.10 and Table 3.1, it is evident that the conductivity of both before and

after CdTe passivation is due to three distinct electron peaks which can be attributed

to the electrons in the bulk of the HgCdTe epilayer (highest mobility), electrons

associated with the substrate/epilayer interface, and the surface or the CdTe/HgCdTe

interface (lowest mobility) [135, 155, 156, 159]. However, the most striking features

of the results presented in Figure 3.10 are: (i) a significant decrease in the total

conductivity of the CdTe passivated sample, and (ii) the reduction in the average

electron mobility for all electron peaks, and (iii) a slight reduction in total

conductivity component associated with the lowest electron peak after CdTe

passivation. While the latter is likely an indication of the effect of surface passivation,

the former two characteristics are most likely an effect of compensation resulting

from Hg-vacancy formation during the in-situ annealing step at 180 °C following the

deposition of CdTe.

Measurements of the electron mobility spectra on other wafers, such as sample

MCT231 (Figure 3.11 (a)) and MCT240 (Figure 3.11 (b)), from the same MBE

growth campaign indicate that after the vacancy filling anneal on MBE grown

HgCdTe wafers, the samples exhibit comparable values of mobility as presented in

the literature [160].

3.3.2 Current-voltage measurements

In the work presented in this thesis, current-voltage (I-V) measurements were carried

out to study directly the insulating properties of the passivant and to investigate the

passivation/semiconductor interface through measurement of the I-V characteristics of

gated diodes as a function of gate bias. All measurements were performed using an

HP4156A semiconductor parameter analyser. In particular, leakage current of the

Au/Cr/SiNx/Si MIS capacitors to evaluate the insulating properties of SiNx; and the

dark I-V versus gated bias measurements were used for on the ZnS/CdTe/HgCdTe

gated diodes to investigate the influence of surface band-bending on the diode

characteristics. This latter will be discussed in more detail in Section 4.3.

Leakage currents were measured at 300 K on circular, 500 μm diameter

Au/Cr/SiNx/Si MIS capacitor structures. Silicon wafer cleaning and buffered oxide

etch were conducted just before SiNx film deposition. The I-V measurements were

44


Figure 3.12 A typical current-voltage characteristic of Au/Cr/SiNx/Si MIS structure.

Figure 3.13 Resistivity variation of D4-100C SiNx film over a period of four months. The MIS structures were left to age in laboratory atmosphere.

0 1 2 3 42

4

6

8

10 x 1010

Months since fabrication

Res

istiv

ity (Ω

cm)

-10 -5 0 5 10-1.5

-1

-0.5

0

0.5

1

1.5 x 10-11

Voltage (V)

Cur

rent

(A)

sweeping from -10V to 10Vsweeping back from 10V to -10V

45


carried out using a voltage sweep from -10 V to 10 V, and back to -10 V, with an

example plotted in Figure 3.12. It can be seen that the I-V characteristics within the

sweep range is essentially linear, indicating a resistive leakage mechanism. Leakage

current can be caused by traps and defects in the hydrogenated SiNx film and at the

SiNx/HgCdTe interface [161]. Traps in the SiNx film and at the interface can be

attributed to dangling bonds, as well as Si-H and N-H bonds in the forbidden

gap [162]. Low-temperature deposited SiNx films can exhibit higher trap densities

[163], therefore the optimisation of film deposition conditions is crucial.

An effective resistivity of the samples was calculated by averaging the calculated

resistivity points (R = dV/dI) in the linear part of the I-V characteristics. The

thickness of the insulating layer used in the resistivity calculations was measured with

a Dektak 150 stylus surface profilometer. Additionally, leakage measurements were

taken over a four month period after fabrication to determine the stability of the SiNx.

Figure 3.13 shows the resistivity of the B4-SiNx film over a four month period, with

the MIS devices left to age in laboratory atmosphere. A gradual decrease in the film

resistivity over time is clearly evident, although the resistivity remains > 1010 Ω cm,

even after four months. Moisture in the air may have contributed to this degradation

of the SiNx films [2].

3.3.3 Capacitance-voltage and capacitance-frequency measurements

Analysing the capacitance-voltage (C-V) characteristics of a MIS structure can

provide information on the insulator thickness, semiconductor doping concentration,

fixed charge within the passivation film and interface trap density. Fixed charge is

thought to be due to structural defects very close to the passivation-semiconductor

interface, which are not in electrical communication with the semiconductor. Interface

traps are attributed to defects or impurities acting as donor or acceptor sites

introducing additional energy levels within the semiconductor bandgap at the

interface. In addition, there are mobile ionic charges within the insulator, and charge

trapped in the bulk of the insulator as well. The electrical properties of the

SiNx/HgCdTe interface can be evaluated from the standard MOS theory based on a

MOS structure [164].

Metal-SiNx-HgCdTe MIS structures allow investigation of the interface between SiNx

and HgCdTe, including the interface trap density, Dit, and were used in this thesis as

46


the primary tool to evaluate surface passivation performance and to correlate

passivation quality with other film properties. To investigate the insulating properties

of the SiNx passivation films, capacitors for test purposes were firstly fabricated on Si

substrates. All C-V measurements were taken with an HP4284A precision LCR meter

with a small-signal frequency of 1 MHz.

As an example of the C-V characteristics and analysis, Figure 3.14 shows the high-

frequency C-V curves measured at 298 K with variable sweep ranges from -15 V to

15 V and then back to -15 V, -20 V to 20 V then back to -20 V, -24 V to 24 V then

back to -24 V and -30 V to 30 V then back to -30 V. The SiNx was deposited by

ICPECVD at 100 °C with a thickness of 192 nm, using the deposition condition of

D4-100C (conditions are detailed in Table 5.1). Figure 3.15 shows another batch of

SiNx/Si MIS devices measured at 298 K (solid line) and at the typical IR

photodetector operating temperature of 80 K (dashed line), also using the same

deposition condition of D4-100C, with a thickness of 220 nm. The C-V analysis and

extracted information are listed in Table 3.2. The interface trap density, Dit, was

extracted by Terman’s method by analysing the high-frequency C-V data measured at

1 MHz [165]. The interface trap density, Dit, can also be extracted from the high-

frequency and low-frequency capacitance-voltage characteristics, and also by the

conductance method, as detailed in Chapter 5.

Table 3.2 Data extracted from C-V analysis on a SiNx/Si MIS capacitor measured at 298 K and 77 K

T = 298 K T = 80 K

Flat band voltage (V) -8.32 -5.47

Fixed charge density ( × 1012 cm-2) 1.51 0.99

Hysteresis width (V) 3.98 3.03

Slow interface states ( × 1011 cm-2) 7.55 5.67

Interface trap density Dit ( × 1012 eV-1cm-2) 3.48 1.74

47


Figure 3.14 The SiNx/Si MIS structure measured at 298 K with variable sweep ranges from -15 V to 15 V (innermost pair of curves), -20 V to 20 V, -24 V to 24 V and -30 V to 30 V (outermost pair of curves). The D4-100C SiNx (deposition conditions detailed in Table 3.3) was deposited by ICPECVD at 100 °C with a thickness of 192 nm.

Figure 3.15 The SiNx/Si MIS structure measured at 298 K (red solid line) and 77 K (blue dashed line) with sweep ranges from -20 V to 20 V. The D4-100C SiNx (deposition conditions detailed in Table 3.3) was deposited at 100 °C with a thickness of 220 nm.

-20 -10 0 10 200

0.5

1

1.5

2

2.5

3

3.5 x 10-8

Applied voltage (V)

Cap

acita

nce/

Are

a (F

/cm

2 )

298 K

80 K

-30 -20 -10 0 100

1

2

3

4 x 10-8

Applied voltage (V)

Cap

acita

nce/

Are

a (F

/cm

2 )

48


3.4 Performance of ICPECVD SiNx passivation on other semiconductors

Epitaxially grown CdTe/HgCdTe heterojunctions have been a leading surface

passivation choice for HgCdTe, however, it is not always feasible to have in-situ

grown CdTe on HgCdTe, due to the requirement of additional processing steps before

the CdTe growth. Also, the relatively high growth temperatures (> 200 °C) required to

deposit stoichiometric CdTe films is well above the desired processing temperature

for HgCdTe. Silicon nitride films of high dielectric quality deposited at low

temperatures are attractive for surface passivation of HgCdTe devices. Hydrogenated

SiNx has demonstrated its capability in passivating the surface, and can improve the

bulk material by hydrogen passivation of defect centers as a consequence of hydrogen

incorporation during the plasma process [84, 166, 167]. The presence of CdTe on

HgCdTe may hinder the diffusion of hydrogen atoms and hence the effect of

passivation. There are few reported studies on SiNx as a passivation layer for HgCdTe

and related compounds. This section aims to develop, optimise and characterise low-

temperature (80 °C - 100 °C) deposited SiNx films for passivating HgCdTe without

the CdTe layer in between [1].

Conventionally, high quality SiNx films for surface passivation layers are typically

deposited at temperatures in the 200 °C - 750 °C range, much higher than the

maximum allowed HgCdTe processing temperature (< 120 °C). The ECR-plasma

CVD process features low-temperature deposition of SiNx films suitable for HgCdTe

passivation [70, 71], however, ECR-plasma CVD systems can suffer from uniformity

limitations [168, 169]. Inductively-coupled plasma-enhanced chemical vapour

deposition (ICPECVD) offers the ability to deposit high quality SiNx films at

temperatures as low as 80 °C [169-171]. Low-temperature (80 °C - 130 °C) SiNx

films deposited in ICPECVD SI500D (SENTECH Instruments GmbH) with a high-

density and low ion energy plasma source have been reported to be characterised by a

low etch rate in wet-chemical etchants, minimal damage to substrate surface during

deposition, low stress and high breakdown voltage [163]. The SiNx films under study

in the thesis were all deposited in the Sentech SI500D ICPECVD system. The low ion

energy of the plasma source in the ICPECVD systems enables SiNx films to be

deposited on HgCdTe without significant surface damage.

49


3.4.1 Surface passivation by hydrogenated silicon nitride

Under amino-saturated conditions, the deposition of SiNx from NH3+SiH4 gas mixture

may be explained as [172]

242Plasma

34 4H )Si(NH 4NH SiH + →+ (3.1)

Then followed by a surface condensation reaction

↑+→ 343Heat

42 8NH NSi )3Si(NH (3.2)

Note that the surface condensation process is facilitated by high temperature and slow

deposition rate. Condensation to stoichiometric Si3N4 cannot be achieved even at a

temperature as high as 530 °C [172]. Hydrogenated SiNx films passivate the bulk and

surface of the semiconductor and minimise surface recombination rate , as discussed

below [93, 173]:

Chemical passivation of the semiconductor surface is achieved by the chemical

bonding of silicon, nitrogen and hydrogen atoms to the atoms at the interface. At the

surface of a semiconductor, there exist unsaturated bonds, referred to as dangling

bonds, which may introduce energy levels within the bandgap of the semiconductor.

Also, surface states can result from dislocations, or chemical residues and metallic

depositions on the surface [83, 172, 174-178]. Chemical passivation terminates the

dangling bonds at the interface, lowers the density of interface states and reduces the

SRH recombination rate.

The polarity of the fixed charge in SiNx films was reported to be negative on HgCdTe

substrates [70]. This has been confirmed in the thesis and is opposite to the one when

deposited on silicon substrates as shown in Section 3.3.7 and in the literature [179,

180].

50


3.4.2 Experimental setup and design

3.4.2.1 Features of the ICPECVD system employed for surface passivation

The SiNx films employed in this chapter were deposited employing a high-density

PECVD system - SI500D (SENTECH Instruments GmbH). The PECVD system

features a planar triple spiral antenna inductively coupled plasma (ICP) source, a He-

cooled substrate electrode and a high-vacuum chamber.

The ICP source is driven by a 13.56 MHz generator. The high-density plasma makes

possible the deposition of high-quality silicon nitride films at much lower temperature

than conventional PECVD [169-171, 181]. The low ion energy of the plasma source

in the ICPECVD systems enables SiNx films to be deposited on HgCdTe without

significant surface damage. The controlled helium gas flow, maintaining a pressure of

1000 Pa to the wafer backside, is for effective wafer temperature control. The high-

vacuum system has a turbo pump and a rotary pump, which are designed for the low

pressure and high flow requirements of the deposition process. During the deposition,

an automatic throttle valve maintains the chamber pressure independent of the gas

flows, and mass flow controllers (MFC) provide precise flow rates. Aiming for higher

hydrogen content in the films, and thus possibly better surface passivation, the SiNx

films employed in this thesis were all deposited employing SiH4+NH3+Ar+He gas

mixtures. Additional H2 could be added into the gas mixtures during deposition to

increase the hydrogen content in the film for future work.

3.4.2.2 Procedures for the development of SiNx deposition conditions for HgCdTe surface passivation

In order to determine ICPECVD SiNx deposition conditions suitable for surface

passivation of HgCdTe, the following procedures were used:

1. A series of low-temperature (80 °C - 100 °C) deposited SiNx films were firstly

deposited on CdTe/GaAs and Si substrates under different deposition

conditions, as discussed in Section 3.4.3. The Si substrates were employed as

reference samples. The influence of ICP power on the quality of the deposited

SiNx films and long-term stability was assessed through the IR absorbance and

film insulating quality. The deposition conditions employed in step 1 are

detailed in Table 3.3.

51

Table 3.3 Summary of deposition conditions of SiNx film on CdTe/GaAs and Si substrate in the Sentech SI 500D system

Sample number Substrate Substrate

temperature (°C)

ICP RF

power (W)

Gas flow

rates (sccm) Film thickness (nm)

Deposition

rate (nm/min)

1 C1-SiNx CdTe/GaAs

80 300 FlowI* -240 (surface 240 nm

under CdTe level) -

A1-SiNx(C1 reference) Si 141 17.230

2 C2-SiNx CdTe/GaAs

80 350 FlowI* 216 26.395

A2-SiNx (C2 reference) Si 153 18.697

3 C3-SiNx CdTe/GaAs

80 450 FlowI* 259 31.650


4 D1-SiNx CdTe/GaAs

100 350 FlowI* 194 23.707

B1-SiNx (D1 reference) Si 134 16.375

5 D4-SiNx CdTe/GaAs

100 350 FlowII** 202 24.684

B4-SiNx (D4 reference) Si 192 23.462

6 C4-SiNx CdTe/GaAs

100 450 FlowI* 219 26.762


7 C5-SiNx CdTe/GaAs

100 600 FlowI* 166 20.285


8 A6-SiNx Si 125 350 FlowI* 126 15.397 *FlowI (sccm): SiH4 6.9, NH3 10.3, Ar 120, He 131.1, SiH4/NH3 = 0.670; NH3/ SiH4 = 1.493

**FlowII (sccm): SiH4 7.5, NH3 9.7, Ar 120, He 131.1, SiH4/NH3 = 0.773; NH3/ SiH4 = 1.293

52

Chapter 3 M


52


2. A series of low-temperature (80 °C - 100 °C) deposited SiNx films were

deposited on Si substrates to investigate the influence of SiH4/NH3 on film

refractive index, film composition, deposition rate and bond/atom densities, as

discussed in Section 3.4.4. The deposition conditions employed in Step 2 are

listed in Table 3.3.

3. SiNx films under four different deposition conditions were then employed on

HgCdTe substrates for capacitance-voltage and conductance-frequency analysis,

as discussed in Section 3.4.1. The deposition conditions for SiNx films in the

four MIS structures are detailed in Table 3.3. For samples labelled D1-80C, D1-

90C and D1-100C, the SiNx films were deposited by varying only the

temperature of the substrate, with samples D1-100C and D4-100C differing

only in the SiH4/NH3 ratio employed for a preliminary assessment of the effect

of N/Si ratio on the quality of surface passivation. The densities of interface

states, Dit, were compared, as it has been considered the key-parameter when

comparing passivation quality [175, 179, 182, 183].

4. Bond density calculations were carried out on the SiNx/Si reference wafers,

where more detailed literature is available, in an attempt to correlate the bond

density with interface states extracted from Step 3. The calculation of the bond/

atom concentrations and their correlation with Dit are summarised in Table 5.4

and Table 5.5. The [Si-H] and [N-H] bond densities in the SiNx film were

considered as indicators of passivation quality.

Surface cleaning and conditioning have been found to be an indispensable part of

surface passivation, in order that the film can be deposited with minimal fixed charge

and interface traps. In this thesis, organic cleaning followed by buffered oxide etch

(BOE) were implemented for Si substrates before silicon nitride deposition, as the

standard RCA clean [184] is not transferable to HgCdTe and CdTe substrates. For

HgCdTe and CdTe substrates, the cleaning procedures used were organic cleaning,

HCl etching followed by Br/Methnol etching, with the substrate left in running DI

water before loading into the ICPECVD system ready for film deposition.

The film deposition procedure in ICPECVD for sample D1-SiNx can be found in

Table 3.4, which gives as an example to show the steps in the film growth. The other

SiNx films in this chapter followed the same procedure.

53


Table 3.4 Silicon nitride film deposition procedures used for sample D1-SiNx in the Sentech SI500D ICPECVD system

Procedure Comments 1 Plasma cleaning of chamber

using gas mixture of CF4 and O2, prior to SiNx film deposition

This step is crucial to have reproducible film properties in different runs.

In general, 20 mins to 40 mins is considered enough to remove any deposited film from the previous run. The duration can be extended depending on the chamber status.

2 Ventilate chamber with N2 3 Load wafer together with its

reference wafer(s) Remember to load reference wafers for film characterisations. The commonly used reference wafers for HgCdTe are as below:

SiNx/CdTe/GaAs for measurements of FTIR in order to check film stability over time. Some obvious damage to CdTe substrate by ion bombardment could also be seen by FTIR if the absorbance fringes of CdTe film changes after the deposition;

SiNx/Si for measurements of refractive index, film thickness, etch rate, FTIR, C-V and I-V;

SiNx/Au/Cr/Si for leakage test; SiNx/GaAs for EDS purpose to estimate film

composition, [N]/[Si].

4 Evacuate chamber 5 Insert wafer(s) and pump the

deposition chamber to high vacuum

6 Waiting period 25 sec 7 He Pressure for wafer

backside cooling to 1000.0 Pa

8 Waiting period 20 sec 9 Increase electrode temperature

to 100°C

54


10 Waiting period of 900 seconds for HgCdTe substrates after the set-point temperature is reached

The longer waiting period of 900 sec is due to the relatively poor thermal conductivity of HgCdTe and CdZnTe, in order to stabilise the surface temperature. A waiting period of 200 seconds is commonly used for silicon and GaAs substrates.

11 Waiting period 10 sec 12 Start gas

sources MFC 1 for NH3 = 10.3 sccm

Purge gas of N2 must be open before or at the starting point of this step for safety purpose when using the gas of SiH4. MFC 6 for Ar =

120 sccm MFC 7 for SiH4 = 6.9 sccm MFC 8 for He = 131.1 sccm

13 Waiting period 15 sec 14 Set 10 Pa for pressure reactor 15 Waiting period 45 sec 16 ICP power source of 350W on Starting of film deposition 17 Deposition duration 491 sec Aiming at ~ 200 nm film 18 ICP power source off End of film deposition 19 Stop gas

sources MFC 1 for NH3 = 0 sccm

MFC 7 for SiH4 = 0 sccm

MFC 8 for He = 0 sccm

20 Waiting period 45 sec Stop purging N2 gas after this step. 21 Set 0 Pa for pressure reactor 22 Stop gas of MFC 6 for

Ar = 0 sccm

23 Pump the deposition chamber to high vacuum

24 Waiting period 30 sec 25 Retract wafer 26 Ventilate chamber with N2

and collect sample(s)

27 Pump the chamber to vacuum or perform plasma cleaning of chamber

55


3.4.3 Investigation on SiNx film stability over time

In order to determine the ICPECVD SiNx deposition conditions suitable for surface

passivation of HgCdTe, a series of low-temperature (80 °C-100 °C) deposited SiNx

films were deposited on CdTe/GaAs and Si reference substrates under different

deposition conditions. The deposition conditions employed are detailed in Table 3.3,

where the SiNx films with sample number starting with ‘A’ or ‘B’ were deposited on

Si reference substrates, the SiNx films with sample number starting with ‘C’ were on

CdTe/GaAs substrate GACT004, and ‘D’ on CdTe/GaAs substrate GACT006.

Regarding the CdTe/GaAs substrates used in the thesis, the CdTe epilayers with a

thickness of approximately 6 μm were grown in a Riber-32 MBE system on GaAs

substrates at a substrate temperatures of between 270 °C to 285 °C. The information

for the MBE grown CdTe wafers is listed in Table 3.5. During the growth, the

background pressure was 2 × 10-9 Torr, the CdTe flux was 1 × 10-6 Torr and Te flux

was 1.5 × 10-6 Torr, and the growth rate at the above mentioned flux rate was

approximately 1 μm/hour. The films on Si substrates were employed as reference

samples. The Si substrate used was 300 μm-thick phosphorous-doped n-type single

crystal (100) silicon with a resistance of 1-20 Ω⋅cm.

Except for C1-SiNx, the other seven batches of SiNx films demonstrated good film

thickness uniformity. The surface of sample C1-SiNx had a roughened and dull

appearance. The films’ thickness, determined by mechanical step profiler Dektak, was

in the range of 141 nm to 259 nm, as shown in Table 3.3. The duration of each

deposition was 491 seconds that can result in ~ 200 nm-thick SiNx film using

deposition conditions of D4-100C.

Table 3.5 Summary of MBE grown CdTe on GaAs substrate

CdTe/GaAs Sample No.

GaAs substrate lot

No.

Expected Thickness

(μm)

Growth Temperature

(°C)

FWHM from XRD

(arcsec) GACT004 23133-18-4 6 270 170

GACT006 23133-18-6 6 275 66

56


Figure 3.16 Deposition rates of silicon nitride films on silicon substrate as a function of ICP power at a substrate temperature of 80 °C and 100 °C.

Figure 3.17 Deposition rates of silicon nitride films on CdTe/GaAs substrate as a function of ICP power at a substrate temperature of 80 °C and 100 °C.

300 350 400 450 500 550 60016

18

20

22

24

26

28

30

32

ICP Power (W)

Dep

ositi

on R

ate

(nm

/min

)

80oC100oC

300 350 400 450 500 550 60016

18

20

22

24

26

28

30

32

ICP Power (W)

Dep

ositi

on R

ate

(nm

/min

)

80oC100oC

57


Figure 3.16 and Figure 3.17 plot the change of deposition rates as a function of ICP

power for samples listed in Table 3.3. The deposition rates were found to increase

with an increase in ICP power, except for sample C5-SiNx and C1-SiNx. The

deposition rate of C5-SiNx was found to be much lower than expected, which could be

correlated with its porosity, as discussed in the Section 3.4.4, and as indicated by the

observed absorbance curves monitored over a six-month time frame.

Regarding sample C1-SiNx, the CdTe surface had a roughened and dull appearance

after the SiNx deposition process. The surface was found to be etched or reacted with

the substrate, since the underlying CdTe surface was found to be 240 nm lower than

previously found by Dektak. No Si-N related band observed in the SiNx film. The

mechanism of CdTe surface etching is unclear. One possibility is the reaction of CdTe

and NH3 during the film deposition expressed in Eq. (3.3) [185]:

34NH CdTe + −+ ++ e2Te)Cd(NH 243 (3.3)

Regarding the BOE etch rate of the SiNx film, a piece of the B4-SiNx sample was

etched in BOE for 20 minutes, and the BOE etch rate was found to be approximately

4 nm/min, which is relatively low, but similar to what was reported by the

manufacture of SENTECH Instruments [186]. Such a low etch rate is an indication of

good SiNx quality [163].

3.4.3.1 Examination of film stability through infrared absorbance

The influence of ICP power on the quality of the deposited SiNx films was assessed

through IR absorbance of the films. The IR absorbance of each film was measured on

the day of the deposition and was monitored during the following six month period.

The films were allowed to age in laboratory atmosphere.

A typical IR absorbance spectrum of low-temperature (80 °C - 100 °C) deposited

silicon nitride film deposited by the ICPECVD system is shown in Figure 3.18.

Table 3.6 identifies the physical cause behind the observed absorption bands in

Figure 3.18 [183, 187, 188]. The band of Si-O-Si stretching cannot be seen in any of

the as-deposited films, and only appeared in sample C5-SiNx after aging in

atmosphere.

58


Table 3.6 Absorption bands observed in the as-deposited SiNx samples

Absorption bands

Frequency of

absorbance peak (cm-1)

1 Si-N sym. Stretching (Si-N (s)) 480

2 Si-H wag-rocking (Si-H (w-r)) 650

3 Si-N asym. Stretching (Si-N (s)) 840

4 Si-O stretching (Si-O (s)) 1080

5 N-H rocking (N-H (r)) 1180

6 Si-H stretching (Si-H (s)) 2185

7 N-H stretching (N-H (s)) 3350

8 O-H2 stretching (O-H2 (s)) 3640

The absorbance spectrum of each of the six films on CdTe/GaAs substrates was

measured on the day of the deposition (Figure 3.19) and monitored again over a six-

month time frame (Figure 3.20). The SiNx/CdTe/GaAs sample labelled C5-SiNx

deposited using ICP power of 600 W appeared to be porous and more susceptible to

oxidation under conventional ambient conditions, with the Si-O-Si stretching peak

appearing after deposition at 1080 cm-1, as shown in Figure 3.20. The evolution of

absorbance of C5-SiNx with time is shown in Figure 3.21. This Si-O-Si oxidation

peak has been reported to become evident when SiNx films undergo oxidation in air,

as reported by Liao et al. [2], Chang et al. [3] and Westerhout et al. [4]. H2O in the air

(moisture) has been found to be responsible for the oxidation of the SiNx films [2]. It

is noted that although the SiNx/Si reference samples showed good stability over six

months, the results from SiNx/CdTe/GaAs samples suggest that high ICP power

conditions are not suitable for CdTe or HgCdTe substrates. Deposition conditions C2,

C3, C4, D1 and D4 showed good long-term stability in terms of the IR absorbance

peaks associated with exposure to O2 and H2O in the atmosphere.

59


Figure 3.18 A typical IR absorbance spectra of low-temperature (80 °C - 100 °C) deposited silicon nitride film deposited by Sentech SI500D system.

60

Chapter 3 M


5001000150020002500300035004000Wavenumber (cm-1)

Abs

orba

nce

(a.u

.)

O-H2(s) N-H(s) Si-H(s) N-H(r)

Si-N asym.(s)

Si-H (w-r)

Si-N Sym.(s)

60


Figure 3.19 The IR absorbance spectra of the as-deposited silicon nitride films by six different recipes on CdTe/GaAs substrate.

5001000150020002500300035004000Wavenumber (cm-1)

Abs

orba

nce

(a.u

.)

D4-SiNx

D1-SiNx

C5-SiNx

C4-SiNx

C3-SiNx

C2-SiNx

Si-N-Si(s)

61

Chapter 3 M


61


Figure 3.20 The IR absorbance spectra of the silicon nitride films by six different recipes on CdTe/GaAs substrate after six-months exposure to a laboratory atmosphere.

5001000150020002500300035004000Wavenumber (cm-1)

Abs

orba

nce

(a.u

.)

D4-SiNx

D1-SiNx

C5-SiNx

C4-SiNx

C3-SiNx

C2-SiNx

Si-N-Si(s)

Si-O-Si(s)

62

Chapter 3 M


62


Figure 3.21 The IR absorbance spectra of the C5-SiNx film on CdTe/GaAs substrate monitored over a six month time frame. The films were allowed to age in laboratory atmosphere. Text on the left indicate the time after deposition that the measurement was taken.

5001000150020002500300035004000Wavenumber (cm-1)

Abs

orba

nce

(a.u

.)

as-deposited

2 days

4 days

1 week

2 weeks

4 weeks

2 months

3 months

4 months

6 months

Si-O-Si(s)Si-N-Si(s)

63

Chapter 3 M


63


3.4.3.2 Examination of film stability through insulating properties

To investigate the insulating properties of the SiNx passivation films, MIS capacitors

were fabricated first on SiNx/Si wafers, using films that were deposited using

condition of D4-SiNx. The capacitance-voltage measurements were carried out, and

fixed charge density and density of interface states were extracted, which have been

discussed on Section 3.3.7. The polarity of fixed charge was positive for the

Au/Cr/SiNx/Si MIS capacitor, which is in agreement with literature [179, 180].

Leakage currents were measured on Au/Cr/SiNx/n-Si MIS capacitor structures at

298 K on the day of fabrication and were monitored within a four-month period of

time after storage in a laboratory atmosphere. The resistivity of the SiNx films on Si

substrates was calculated from the I-V characteristics. Although there was a gradual

decrease in the film resistivity from 9.24 × 1014 Ω⋅cm to 2.61 × 1014 Ω⋅cm over the

four-month period, the MIS devices retained good stability in terms of film

resistivity (Figure 3.12).

3.4.4 Investigation on SiNx film properties influenced by NH3/SiH4 flow ratio

The flow ratio of NH3/SiH4 is a crucial parameter in determining film properties and

passivation performance, influencing film stoichiometry and semiconductor interface

quality [174, 175, 179, 180, 189, 190]. In this section, the influence of NH3/SiH4 flow

ratio on SiNx film properties is explored.

3.4.4.1 Influence of NH3/SiH4 flow ratio on film refractive index, composition and deposition rate

The stoichiometry of the silicon nitride film is defined by the [N]/[Si] ratio, x. The

value of [N]/[Si] in hydrogenated amorphous silicon nitride films can be measured by

direct measurements involving massive-particle detection, such as Rutherford

backscattering spectroscopy (RBS) and elastic-recoil-detection (ERD). The value can

also be extracted by electronic probing methods, such as, energy dispersive

spectroscopy (EDS), Auger electron spectroscopy (AES), X-ray photoelectron

spectroscopy (XPS) and electron microprobe analysis (EPMA). There are indirect

estimations based on relations between x and physical parameters that are x-sensitive,

64


having a monotonic variation with x and that are easily measurable, such as measured

refractive indices at a given wavelength [145, 146]. In this section, the refractive

index, composition and deposition rate of SiNx under study are discussed, and their

relationship was explored.

To investigate the influence of NH3/SiH4 flow ratio on film properties, a series of

films were deposited at low temperatures (80 °C - 100 °C) on Si substrates with

varying NH3 flow rates and a fixed SiH4 flow rate. The deposition conditions

employed are listed in Table 3.7. The Si substrates used were 300 μm-thick

phosphorous-doped n-type (100) silicon with a resistivity of 1-20 Ω⋅cm. The films

deposited on GaAs substrates were employed as reference samples for EDS purpose

for an estimation of [N]/[Si]. The flow rate of SiH4 was kept the same at 6.9 sccm,

and the various flow rates of NH3 used were 12.4, 10.3, 8.2 and 6.1 sccm for films

deposited at 80 °C and 100 °C. The set of four samples of D1-80°C-Si reference, D1-

90°C-Si reference, D1-100°C-Si reference, and D4-1 0°C-Si re&erence are the Si

reference wafers for the SiNx/HgCdTe MIS structures discussed in Chapter 5.

Measured refractive indexes for each sample disted in Table 3.7 are all given at a

wavelength of 632.8 nm. As an estimate of the film compo3ition, [N]/[Si], the EDS

spectra werE obtained using a beam energy of 5 keV on the SiNx/GaAs instead of on

SiNx/Si wafers in order to avoid interference from the Si substrate due to the relatively

thin film. Except for sample D1-NH12-80°C, the composition of all the other samples

under study are below x = 1.33 (Si-rich). The ICPECVD system was found to form

Si-rich films much more readily than Ni-rich ones, which is likely to be due to the fact

that Si-H and Si-Si bonds can be formed more easily than the N bonds [161].

Measurad refractive index at 632.8 nm, n632.8nm, and [N]/[Si], as a function of

SiH4/NH3 flow rate ratio for samples deposited at 80 °C and 100 °C are 0lotted in

Figure 3.22. Within the investigated range of the SiH4/NH3 flow rate ratio from 0.56

to 1.13, n increases with SiH4/NH3 flow rate ratio for samples deposited at 80 °C and

100 °C, with x following the opposite trend.

Measured n and [N]/[Si] as a function of substrate temperature for the samples with

NH3 of 10.3 sccm and SiH4 of 6.9 sccm are plotted in Figure 3.23. It can be seen that

n632.8nm increases with an increase of substrate temperature and [N]/[Si] decreases

65


Table 3.7 Summary on ICPECVD SiNx/Si wafers with varied SiH4/NH3 ratio and temperature*

SiNx/Si wafer ID Temperature (°C)

NH3 flow rate

(sccm)

SiH4 flow rate

(sccm)

NH3/ SiH4 ratio

Deposition duration

(sec)

SiNx thickness

(nm) Deposition rate (nm/min) n632.8nm [N]/[Si]

D1-NH12-100°C 100 12.4 6.9 1.80 700 227 19.46 1.90 1.24 D1-NH10-100°C

(i.e. D1-100°C-Si reference) 100 10.3 6.9 1.49 700 239 20.49 1.93 1.02

D1-NH8-100°C 100 8.2 6.9 1.19 700 220 18.86 2.12 0.98 D1-NH6-100°C 100 6.1 6.9 0.88 1147 220 11.51 2.41 0.61

D1-NH12-80°C 80 12.4 6.9 1.80 700 254 21.77 1.80 1.37 D1-NH10-80°C


D1-NH8-80°C 80 8.2 6.9 1.19 700 236 20.23 1.97 1.11 D1-NH6-80°C 80 6.1 6.9 0.88 700 151 12.94 2.30 0.64

D1-NH10-80°C (i.e. D1-80°C-Si reference) 80 10.3 6.9 1.49 700 268 22.97 1.86 1.17

D1-90°C-Si reference 90 10.3 6.9 1.49 700 252 21.60 1.90 1.07 D1-NH10-100°C


D4-100°C-Si reference 100 9.7 7.5 1.29 491 202 24.68 1.94 0.90

*Other deposition parameters are the same for all the samples listed in the above table, that is ICP RF power of 350 W, chamber pressure of

10 Pa, Ar flow rate of 120 sccm, and He flow rate of 131.1 sccm.

66

Chapter 3 M


66


Figure 3.22 Measured refractive index at a wavelength of 632.8 nm and [N]/[Si] ratio estimated by EDS, as a function of NH3/SiH4 flow ratio for samples deposited at 80 °C and 100 °C. Solid lines –: n632.8 nm; dashed lines – –: [N]/[Si]; : for samples deposited at 80 °C; : samples deposited at 100 °C.

Figure 3.23 Measured refractive index at a wavelength of 632.8 nm and [N]/[Si] ratio estimated by EDS, as a function of substrate temperature. Solid lines –: n632.8 nm; dashed lines – –: [N]/[Si].

80 85 90 95 1001.86

1.87

1.88

1.89

1.9

1.91

1.92

1.93

1.94

n (λ

= 6

32.8

nm

)

Substrate temperature (oC)80 85 90 95 100

1.02

1.04

1.06

1.08

1.1

1.12

1.14

1.16

1.18

[N]/[

Si]

0.8 1 1.2 1.4 1.6 1.81.8

2

2.2

2.4

2.6

n ( λ

= 6

32.8

nm

)

NH3/SiH4

0.8 1 1.2 1.4 1.6 1.80.6

0.8

1

1.2

1.4

0.8 1 1.2 1.4 1.6 1.80.6

0.8

1

1.2

1.4

[N]/[

Si]

67


with increasing temperature, in agreement with the trends reported in the literature

[93, 170, 172].

The five different expressions found in the literature for the relationship of [N]/[Si]

and n632.8nm [146, 175, 183, 191, 192] are discussed as below. The expressions for

[Si]/[N] (i.e. 1/x) and n, includingl several linear ones, are shown in Figure 3.24 (a),

whereas the expressions for [N]/[S)] and n are shown in Figure 3.24 (b).

Makino [193 developed an expression for n in the Si-rich films deposited by

LPCVD, assuming that the films are a bonding-density-weighted linear combination

of NSi3N4 and NSi, ignoring the small amounts of hydrogen bonding of NN-H and NSi-H.

The n to x relation was expressed as

4/31)2)(4/3(

43

xnnxn

n SiNSiSi

+

−+= (3.4)

Bustarret et al. [146] applied Makino’s bonding-density-weighted linear combination

assumption [193] to the case of PECVD α- SiNx:H films, and gave the relation

between n and x as:

4/31)2)(4/3( :: 43

xnnxn

n HSiNSiHSi

+

−+= −−− ααα (3.5)

or

5.03.3

34

234

][][

43:

:

−−

=−+−

==−−

−

nn

nnnnn

SiNx

NSiaHSia

HSia (3.6)

where the refractive index of 3.3: =− HSian is for HSia :− , and 9.143=− NSian is for

nearly stoichiometric 43NSia − nitride film.

The above equation was then modified by Mäckel and Lüdemann [183] as

5.03.3

43

243

][][

43:

:

−−

=−+−

==−−

−

nn

nnnnn

SiNx

NSiaHSia

HSia (3.7)

Claassen et al. [193] gave an expression between n and x by applying an empirical fit

to their data for three different process gases of SiH4+NH3+N2, SiH4+NH3+Ar,

68


and SiH4+NH3+H2, as given below

39.17.0

−=

nx (3.8)

Dauwe et al. [192] also gave an empirical fit to experimental data as the following

35.174.0

−=

nx

(3.9)

In addition, Lelievre et al. [175] gave an empirical fit for their SiNx:H film by LF-

PECVD (Low Frequency PECVD) shown below

22.161.0

−=

nx (3.10)

The values of n in the fit used are at a wavelength of 605 nm, as opposed to 623.8 nm

in the other n and x expressions discussed in this chapter. However, this expression is

listed and plotted together with other expressions in Figure 3.24 to illustrate the trend.

A linear least-squares fit to the measured data of this thesis listed in Table 3.7

between n and 1/x was found to be

37.161.0+=

xn (3.11)

where the norm of residuals is 0.19564.

or

37.161.0

−=

nx (3.12)

It is interesting to note that the linear least-squares fitted line is parallel to what is

given by Lelievre et al. [175]. The lack of data points in the N-rich range decreases

the accuracy of the linear fit, which will require more experimental work on low-

temperaure SiNx in the future. It can also be seen from Figure 3.24 (b) that Si-rich

SiNx is more sensitive to variations in processing conditions than stoichiometric Si3N4.

Small changes in the gas ratio were found to give relatively large variations in the

refractive index and mechanical stress [194]. Gardeniers et al. have noted that

changing the processing gas flow rate ratio can result in relatively large variations in

SiNx refractive index and mechanical stress as compared to those of stoichiometric

Si3N4 [194]. The sensitivity of Si-rich SiNx could be advantageous since it makes it

possible to adjust the properties of the Si-rich SiNx by moderately adjusting

69


(a)

(b)

Figure 3.24 Plot illustrating the relationship between n and x for ICPECVD SiNx deposited at 80 °C -100 °C with varied NH3/SiH4 flow ratio.

0.5 1 1.5 2 2.5 31

1.5

2

2.5

3

3.5

4

[Si]/[N]

n (λ

= 6

32.8

nm

)

T = 80oCT = 90oCT = 100oCBased on Dauwe et al.Based on Claasen et al.Based on Mackel et al.Based on Lelievre et al.Based on Bustarret et al.Linear fit of data points

0.5 1 1.5 21

1.5

2

2.5

3

3.5

4

4.5

5

[N]/[Si]

n (λ

= 6

32.8

nm

)

T = 80oCT = 90oCT = 100oCBased on Dauwe et al.Based on Claasen et al.Based on Mackel et al.Based on Lelievre et al.Based on Bustarret et al.Linear fit of data points

70


(a)

(b)

Figure 3.25 [N]/[Si] ratio as a function of NH3/SiH4 flow ratio for ICPECVD SiNx deposited at 80 °C -100 °C with varied NH3/SiH4 flow ratio. (a) The line is a linear least-squares fit to the measured data as [N]/[Si] = 0.68 × NH3/SiH4 + 0.09. (b) log-log scale plot of [N]/[Si] versus NH3/SiH4 flow ratio. The lines have slopes of β = 1/2 and 1.

0.5 1 1.5 2

0.4

0.6

0.8

1

1.2

1.4

1.6

NH3/SiH4

[N]/[

Si]

T = 80oCT = 90oCT = 100oC

flow ratio

0.5 1.0 2.00.4

0.6

0.8

1.0

2.0

NH3/SiH4 ratio

[N]/[

Si] r

atio

T = 80oCT = 90oCT = 100oCβ = 1/2β = 1

flow ratio

71


Figure 3.26 Plot of film refractive index, n632.8nm, of ICPECVD SiNx deposited at 80 °C -100 °C as a function of the SiH4/NH3 gas ratio. The line is a linear least-squares fit to the measured data.

deposition parameters and hence [N]/[Si] and n632.8nm to match a certain application

requirement.

Figure 3.25 and Figure 3.26 illustrate the relationship between film composition

[N]/[Si] and refractive index n632.8nm of ICPECVD SiNx deposited at 80 °C - 100 °C as

a function of the gas flow ratio. It can be seen that [N]/[Si] increases with the

NH3/SiH4 flow ratio nearly linearly, with the linear least-squares fit as [N]/[Si] = 0.68

× NH3/SiH4 + 0.09, while n632.8nm increases with the SiH4/NH3 ratio with the linear

least-squares fit as n632.8nm = 0.91 × SiH4/NH3 + 1.30, which is also an indication of

the linear relationship between [Si]/[N] and n632.8nm as expressed in Eq (3.11) and

Eq (3.12). The dependence of the silicon nitride composition on the gas ratio could be

explained by the kinetics of the dissociation processes and the free radical sticking

coefficient values [195].

The linear part of the log-log plot of Figure 3.25 (b) yields a square-root dependence

(β ≈ 1/2) for the [N]/[Si] when the NH3/SiH4 flow ratio is in the 1.2 to 1.8 range,

where [N]/[Si] = (NH3/SiH4) β + a0. The dependence becomes stronger (β ≈ 1) at the

0.2 0.4 0.6 0.8 1 1.2 1.41.4

1.6

1.8

2

2.2

2.4

2.6

SiH4/NH3 flow ratio

n (λ

= 6

32.8

nm

)

T = 80oCT = 90oCT = 100oC

72


low gas ratio of 0.88, which is in agreement with the behaviour of PECVD SiNx

reported by Bustarret et al. [146] and Giorgis et al. [196].

The two regions with different values of β were found to be associated with the

polyamine concentration change in the plasma [146, 172]. The (SiH4+NH3) plasma

contains mainly two species, the disilane Si2H6 and the aminosilane Si[(NH2)]m. Si2H6

is formed from SiHm (m < 4) radicals, whereas Si[(NH2)]m is formed from a

combination of SiHm (m < 4) and NHm (m < 3) radicals. The PECVD parameters

determine the ratio and nature of the above mentioned species which, in turn,

determine the composition of the film [197].

The first region with a slope of β ≈ 1/2 in Figure 3.25 (b) is considered a characteristic

of the dissociation-controlled equilibrium between polyaminosilanes (Si[NH2]m,

m = 3, 4) and silicon and nitrogen atoms absorbed at the surface [146]. Disilane Si2H6

dominates the contribution to growth in the second region with β ≈ 1. The minimum

NH3 percentage required to reach a transition to aminosilanes formation is associated

with the conditions under which a change in β is observed [196], as seen in

Figure 3.25 (b).

In addition to film composition, film deposition rate has also been found to be an

indicator of polyamine concentration in the plasma [161, 172, 195, 196]. Figure 3.27

illustrates the change in film deposition rates versus NH3/SiH4 flow ratio for films

deposited at 80 °C, 90 °C and 100 °C. The trend of deposition rate versus NH3/SiH4

flow ratio is non-linear. A linear dependence of the deposition rate on the SiH4 flow

rate has been reported [163, 198], and complex deposition rate variations as a function

of NH3/SiH4 flow ratio have also been reported [161, 196, 197, 199, 200].

The relationship between both [N]/[Si] and deposition rate with NH3/SiH4 flow ratio

are in agreement with those reported by Giorgis with undiluted gas mixtures of

NH3+SiH4 [196]. At very low NH3 flow rate and NH3/SiH4 flow ratio, the

composition of the plasma is very poor in NHm (m = 1, 2) free radicals, as the NH3

dissociation rate is much lower than the SiH4 one [172]. Disilane Si2H6 rises and

Si[NH2]3 is suppressed in the plasma composition. Si and SiHm (m = 1, 2, 3) free

radicals dominate the contribution to growth, and silicon atoms are expected to bond

to more than one hydrogen atom. For NH3/SiH4 flow ratios from 0.88 to 1.19 in

Figure 3.27, there was a dramatic increase in the deposition rate, in the [N]/[Si] ratio

73


(Figure 3.25 (b)), and also in the area under the Si-H (s) peak (~ 2180 cm-1) for films

deposited at 80 °C and 100 °C (Figure 3.28 and Figure 3.29), possibly suggesting a

change in the dominant radicals from disilane to polyaminosilanes.

With the increase of NH3 flow rate and NH3/SiH4 flow ratio, the concentration of NHn

free radicals in the plasma (mainly Si[NH2]3), increases and that of Si2H6 reduces,

resulting in an increase of [N]/[Si] and [N-H]/[Si-H] [172]. In addition, with

increasing of NH3 flow rate, the chamber pressure increases, contributing to an

increase in deposition rate and enhanced disilane elimination [172].

From the data in Figure 3.27, either the conversion efficiency reaches a maximum at

an NH3/SiH4 flow ratio of around 1.49 or the competing absorption-desorption surface

reactions impede the growth rate of films deposited at NH3/SiH4 flow ratios ≳ 1.49

[200], thus resulting is a drop in deposition rate at a NH3/SiH4 flow ratio of 1.80 for

films deposited at 80 °C and 100 °C. At an NH3/SiH4 flow ratio of 1.80, films will be

deposited with Si[NH2]3 as the principal film precursors and suppressed Si2H6 in the

plasma resulting in little or no Si-H bonding, as confirmed by the decrease of Si-H (s)

peaks ( ~ 660 cm-1 and 2180 cm-1) and increase of N-H peaks (1180 cm-1 and

3200 cm-1) in the IR absorption curves.

Figure 3.27 The change in film deposition rate versus NH3/SiH4 flow ratio for silicon nitride films deposited at 80 °C, 90 °C and 100 °C with a fixed SiH4 gas flow of 6.9 sccm.

0.5 1 1.5 210

15

20

25

NH3/SiH4 ratio

Dep

ositi

on ra

te (n

m/m

in)

T = 80oCT = 90oCT = 100oC

flow ratio

74


3.4.4.2 Influence of NH3/SiH4 flow ratio on film IR absorbance

Figure 3.28 and Figure 3.29 plot the IR absorption coefficient spectra of SiNx films

deposited by ICPECVD at varied NH3 flow rates and a fixed SiH4 flow rate at 80 °C

and 100 °C, respectively. The silicon substrate absorption was subtracted and a

baseline correction procedure was applied to the spectra. The data analysis on the IR

absorption spectra is listed in Table 3.8, together with the calculations of bond and

atomic densities. The general trends in Figure 3.28 and Figure 3.29 are that the area of

N-H bonding related peaks increase with NH3 flow rate and NH3/SiH4 flow ratio,

whereas the area of Si-H peaks decrease with NH3 flow rate and NH3/SiH4 flow ratio

for films deposited at 80 °C and 100 °C. This is in agreement with the characteristics

reported by Knolle and Osenbach [201]. The line in Figure 3.30 (a) is the least-

squares fit to the measured Si-H stretching peak frequency versus [N]/[Si] of the

films, expressed as

×=− 163)( 1cmυ [N]/[Si] 2054+ (3.13)

Similarly, the line in Figure 3.30 (b) is the least-squares fit to the measured Si-H

stretching peak frequency versus NH3/SiH4 flow ratio:

×=− 126)( 1cmυ NH3/SiH4 2048+ (3.14)

The Si-H (w-r) vibration was also found to shift to higher frequency with higher

[N]/[Si] and higher NH3/SiH4 flow ratio, similar to the Si-H (s) vibration. The Si-H

(w-r) peak for higher N content films deposited with higher NH3/SiH4 flow ratios of

1.80 and 1.49 are less obvious in the IR absorption spectra than for the Si-rich films,

because there is less amorphous Si in the film.

In addition, the N-H (r) vibration at ~ 2180 cm-1 was found to shift to higher

frequency with higher [N]/[Si] and higher NH3/SiH4 flow ratio, similar to the

behaviour of the Si-H (s) and Si-H (w-r) peaks. The main absorption coefficient peak

also shifts to higher frequency with an increase of [N]/[Si] (Figure 3.31 (a)) and

NH3/SiH4 flow ratio (Figure 3.31 (b)).

75


Figure 3.28 IR absorption coefficient spectra of SiNx films deposited by ICPECVD at various NH3 flow rates and a fixed SiH4 flow rate at 80 °C.

5001000150020002500300035000

0.5

1

1.5

2

2.5 x 104

Wavenumber (cm-1)

Abs

orpt

ion

Coe

ffici

ent (

cm-1

)

D1-NH6-80CD1-NH8-80CD1-NH10-80CD1-NH12-80C

Si-N (sym.s)Si-H (w-r)

Si-N (asym. s)

N-H (r)Si-H (s) N-H (s)

76

Chapter 3 M


76


Figure 3.29 IR absorption coefficient spectra of SiNx films deposited by ICPECVD at various NH3 flow rates and a fixed SiH4 flow rate at 100 °C.

5001000150020002500300035000

0.5

1

1.5

2

2.5 x 104

Wavenumber (cm-1)

Abs

orpt

ion

Coe

ffici

ent (

cm-1

)

D1-NH6-100CD1-NH8-100CD1-NH10-100CD1-NH12-100C

Si-N (sym.s)

Si-H (w-r)

Si-N (asym. s)


77

Chapter 3 M


77


(a)

(b)

Figure 3.30 Plots showing the Si-H stretching peak shifting to higher frequency as a function of (a) film composition [N]/[Si] and (b) NH3/SiH4 flow ratio increases. The least-squares fitted lines in (a) and (b) are 2054]/[][163)( 1 +×=− SiNcmυ , and 2048/126)( 43

1 +×=− SiHNHcmυ , respectively.

0.5 1 1.52100

2150

2200

2250

2300

[N]/[Si]

[Si-H

] pea

k fre

quen

cy (c

m-1

) T = 80oCT = 90oCT = 100oC

0.5 1 1.5 22100

2150

2200

2250

2300

NH3/SiH4 ratio

[Si-H

] pea

k fre

quen

cy (c

m-1

) T = 80oCT = 90oCT = 100oC

flow ratio

78


(a)

(b)

Figure 3.31 Plots showing the main absorption coefficient peak shifting to higher frequency as a function of (a) film composition [N]/[Si] and (b) NH3/SiH4 flow ratio.

0.5 1 1.5820

830

840

850

860

870

[N]/[Si]

Pea

k fre

quen

cy (c

m-1

)

T = 80oCT = 90oCT = 100oC

0.5 1 1.5 2820

830

840

850

860

870

NH3/SiH4 ratio

[Si-N

] Pea

k fre

quen

cy (c

m-1

) T = 80oCT = 90oCT = 100oC

flow ratio

79


3.4.4.3 Calculations of bond and atom concentrations

The concentration of Si-N, N-H and Si-H bonds in the layers can be extracted from

the IR absorption band areas in the optical absorption coefficient curve, by the method

introduced by Lanford and Rand, using the expression [80, 183, 187]:.

∫−=− ννα dYXKYX )()(][ (3.15)

where ][ YX − is the concentration of YX − bonds, )( YXK − is the IR absorption

cross section, and the band area is the integral of optical absorption coefficient α over

the band in question.

The absorption coefficient α can be calculated from the absorbance, A, obtained from

the FTIR measurements. After correction for reflection losses, α may be obtained

using the following equation [202-204]:

dA303.2)( =να (3.16)

where d is the sample thickness in cm.

The bands used in the calculation are the Si-N (stretching) band near 840 cm-1, the Si-

H (stretching) band near 2185 cm-1, the N-H (rocking) band near 1180 cm-1, and the

N-H (stretching) band near 3350 cm-1. As to the IR absorption cross section for the

vibration of Si-H (s), the values used are proportional to the film refractive index,

n, [205]

K [Si-H] = 161058.2 ××n cm-1 (3.17)

The values of absorption cross section for other vibrations [80, 187, 205] are listed in

Table 3.8.

Spectra deconvolution is a conventional technique in IR absorption analysis [146, 187,

205]. Figure 3.32 shows the absorption coefficient as a function of wavenumber for

one of the samples, B1-NH8-80C, as an illustration of the spectra deconvolution.

80


Figure 3.32 The absorption coefficient as a function of wavenumber for sample B1-NH8-80C as an illustration of the fitted absorption bands (dashed line) in the range from 450 cm-1 to 1400 cm-1 with four different Gaussian distributions. The solid line shows the absorption coefficient acquired from measured film absorbance and thickness.

The solid line shows the absorption coefficient being acquired from measured film

absorbance and thickness. The spectra in the range extending from 600 cm-1 to

1400 cm-1 can be decomposed into four Gaussian distributions [187] (dashed lines in

Figure 3.32), centred near 1180 cm-1, 950 cm-1, 840 cm-1 and 650 cm-1 that are

considered as a N-H rocking mode, two Si-N asymmetric stretching modes, and a Si-

H wag-rocking mode.

There are three other absorption bands outside the figure displayed range of 600 cm-1

to 1400 cm-1 in Figure 3.28 and Figure 3.29, which are N-H stretching centred near

3350 cm-1, Si-H stretching near 2185 cm-1, and Si-N sym. stretching near 480 cm-1.

The calculated bond concentrations of Si-N, Si-H and N-H are shown in Table 3.9.

Si-Si bonds are easily formed in Si-rich SiNx films. In contrast with Si-Si bond

formation in Si-rich films, N-N bonds are rarely formed, even in N-rich films [206].

6008001000120014000

0.5

1

1.5

2x 104

Wavenumber (cm-1)

Abs

orpt

ion

Coe

ffici

ent (

cm-1

)

81


Table 3.8 IR absorption spectra analysis and calculations for bond and atom concentrations on SiNx/Si wafers deposited under varied NH3/SiH4

flow ratios at temperatures between 80 °C - 100 °C

Si-N N-H Si-H

Sample SiH4/ NH3

n [N]/[Si] fmax (cm-1)

α (peak) (cm-1)

f (peak) (cm-1)

α (peak) (cm-1)

∫ αdf (cm-2)

K (1019 cm-2)

f (peak) (cm-1)

α (peak) (cm-1)

∫ αdf (cm-2)

K (1019 cm-2)

f (peak) (cm-1)

α (peak) (cm-1)

∫ αdf (cm-2)

K (1019 cm-2)

D1-NH12-100°C 0.56 1.9 1.24 861.6 20742.6 928.7 12462.0 2.93×106 2.07 3341.0 2546.0 4.58×105 8.20 2274.0 1240.0 4.61×105 4.85

837.7 12180.0 1.82×106 1.82 1181.1 2632.6 4.37×105 2.07 660.0 164.0 1.22×104

495.0 811.0 4.98×104

D1-NH10-100°C 0.67 1.93 1.02 842.4 21327.9 950.4 9266.8 2.04×106 2.07 3325.0 1650.0 3.51×105 8.20 2250.0 1200.0 3.19×105 4.98

(i.e. D1-100°C- Si reference)

831.6 17327.0 2.69×106 1.82 1185.1 1541.9 2.13×105 2.07 660.0 399.0 2.97×104

484.3 1391.9 9.92×104

D1-NH8-100°C 0.84 2.12 0.98 832.8 22531.6 909.1 11759.0 3.05×106 2.07 3290.0 1302.0 5.54×105 8.20 2221.0 2560.0 7.09×105 5.62

821.8 13869.0 1.98×106 1.82 1156.0 356.0 6.82×104 2.07 649.0 440.0 4.03×104

480.0 656.0 3.69×104

D1-NH6-100°C 1.13 2.41 0.62 833.6 20390.5 917.9 12481.0 2.92×106 2.07 2151.3 2885.0 5.28×105 6.40 814.9 12716.0 1.87×106 1.82 1175.0 80.0 5.11×103 2.07 635.6 1058.2 8.32×104

82

Chapter 3 M


82


D1-NH12-80°C 0.56 1.8 1.37 852.1 18681.5 941.7 8608.9 2.20×106 2.07 3325.0 1894.0 3.43×105 8.20 2270.0 701.0 1.93×105 4.64

840.2 13140.0 2.06×106 1.82 1197.0 2070.0 2.86×105 2.07 661.0 157.0 2.61×104

497.0 697.0 4.31×104

D1-NH10-80°C 0.67 1.86 1.17 837.8 19324.7 927.4 9120.1 2.35×106 2.07 3325.0 1699.0 3.53×105 8.20 2226.0 1506.0 3.53×105 4.80

(i.e. D1-80°C- Si reference)

831.5 13086.0 1.94×106 1.82 1192.4 1161.0 1.68×105 2.07 666.9 349.6 2.59×104

485.5 765.0 4.49×104

D1-NH8-80°C 0.84 1.97 1.11 836.6 21162.4 903.7 12754.0 3.39×106 2.07 3330.0 874.0 1.67×105 8.20 2210.0 1410.0 3.00×105 5.08

821.1 11192.0 1.54×106 1.82 1181.5 819.0 1.13×105 2.07 643.0 449.0 3.25×104

490.0 1101.0 7.79×104

D1-NH6-80°C 1.13 2.3 0.64 829.8 18598.8 912.4 10952.0 2.49×106 2.07 3323.0 555.0 1.13×105 8.20 2145.9 2520.0 4.56×105 6.34

812.6 11969.0 1.71×106 1.82 2.07 627.4 1305.2 1.65×105

D1-90°C- Si reference

0.67 1.9 1.07 841.6 20306.5 930.5 9535.1 2.48×106 2.07 3330.0 1690.0 3.60×105 8.20 2223.0 1050.0 1.90×105 4.90

833.3 13837.0 2.10×106 1.82 1194.4 1402.0 1.99×105 2.07 672.4 236.9 7.33×103

D4-100°C- Si reference

0.77 1.94 0.9 834.6 20575.5 906.3 11488.0 2.87×106 2.07 3308.0 1280.0 3.27×105 8.20 2217.0 1496.0 3.50×105 5.01

820.8 12189.0 1.70×106 1.82 1180.0 790.0 9.25×104 2.07 660.0 400.0 2.98×104

480 1100 6.19×104

83

C

hapter 3 Material C

haracterisation

83


Table 3.9 Summary of bond and atom concentrations calculated for SiNx/Si wafers deposited under varied NH3/SiH4 flow ratios at temperatures between 80 °C - 100 °C

Sample SiH4/ NH3

NH3/ SiH4

n632.8nm [N]/[Si] [Si-N] (1022 cm-3)

[N-H] (1022 cm-3)

[Si-H] (1022 cm-3)

[Si-Si] (1022 cm-3)

[Si] (1022 cm-3)

[N] (1022 cm-3)

[H] (1022 cm-3)

Total bond

density (1022 cm-3)

Film density (g/cm3)

D1-NH12-100°C 0.56 1.80 1.90 1.24 9.37 4.66 2.23 1.74 3.77 4.68 6.89 18.01 2.96

D1-NH10-100°C 0.67 1.49 1.93 1.02 9.11 3.32 1.59 2.78 4.06 4.14 4.91 16.80 2.94

D1-NH8-100°C 0.84 1.19 2.12 0.98 9.92 4.69 3.98 2.99 4.97 4.87 8.67 21.57 3.59

D1-NH6-100°C 1.13 0.88 2.41 0.62 9.44 0.01 3.38 3.75 5.08 3.15 3.39 16.58 3.16

D1-NH12-80°C 0.56 1.80 1.80 1.37 8.29 3.40 0.90 1.10 2.85 3.90 4.30 13.69 2.31

D1-NH10-80°C 0.67 1.49 1.86 1.17 8.40 3.24 1.69 1.59 3.32 3.88 4.93 14.92 2.53

D1-NH8-80°C 0.84 1.19 1.97 1.11 9.83 1.61 1.52 1.19 3.43 3.81 3.13 14.15 2.54

D1-NH6-80°C 1.13 0.88 2.30 0.64 8.26 0.93 2.89 3.99 4.79 3.06 3.82 16.08 3.01 D1-90°C- Si

reference 0.67 1.49 1.90 1.07 8.95 3.36 0.93 2.73 3.84 4.10 4.29 15.97 2.81

D4-100°C- Si reference 0.77 1.29 1.94 0.90 9.03 2.87 1.75 3.43 4.41 3.97 4.63 17.09 3.06

84

Chapter 3 M


84


Assuming there are no N-N or H-H bonds in the films, the atom concentration of

N(Si), N(N) and N(H) can be calculated using the following expressions [183, 187,

207]:

[Si] = ([Si-N] + [Si-H]) / 4 + [Si-Si] / 2 (3.18 a)

[N] = ([N-H] + [Si-N]) / 3 (3.18 b)

[H] = [N-H] + [Si-H] (3.18 c)

and

=ρ mSi [Si] + mN [N] + mH [H] (3.19)

where ρ is the film density [208]. mSi, mN and mH are the atomic masses of Si, N and

H, respectively.

The Si-Si bond density in Eq. (7.5) can be obtained from Eq. (7.1) and Eq. (7.7) as

[Si-Si] = 2 × [N] / x – ([Si-N] + [Si-H]) / 2 (3.20)

The total bond density is a sum of [Si-N], [N-H], [Si-H] and [Si-Si] as

Ntotal = [Si-N] + [N-H] + [Si-H] + [Si-Si] (3.21)

The film densities calculated by Eq. (3.19) are from 2.31 g/cm3 to 3.59 g/cm3, as

listed in Table 3.9, which are within the range of previous values reported for PECVD

silicon nitride [80, 145, 172, 191, 208]. The calculated bond/atom densities and film

densities are summarised in Table 3.9.

3.4.4.4 Influence of NH3/SiH4 flow ratio on bond and atom concentrations

The influence of the NH3/SiH4 flow ratio on the bond and atom concentrations is

investigated in this section. With the increase of NH3/SiH4 flow ratio, NHn free

radicals, mainly Si[NH2]3, increases and Si2H6 reduces in the plasma composition

[172], hence the ratio of H bonded to N over that bonded to Si, [N-H]/[Si-H] [191,

209], and [N]/[Si] [210] are expected to increase with increasing NH3/SiH4 flow ratio,

resulting in refractive index, n632.8nm, decreasing with the NH3/SiH4 flow ratio [206,

211], as shown in Figure 3.33 and Figure 3.34.

85


Figure 3.33 [N-H] and [Si-H] bond concentration as a function of film composition [N]/[Si] and NH3/SiH4 flow ratio. The indicated temperatures refer to the substrate temperature during deposition.

0.4 0.6 0.8 1 1.2 1.4 1.60

1

2

3

4

5

[N]/[Si]

[N-H

] bon

d de

nsity

( x

1022

cm

-3)

T = 80oCT = 90oCT = 100oC

0.4 0.6 0.8 1 1.2 1.4 1.60.5

1

1.5

2

2.5

3

3.5

4

[N]/[Si][S

i-H] b

ond

dens

ity (

x 10

22 c

m-3

)

T = 80oCT = 90oCT = 100oC

0.5 1 1.5 20

1

2

3

4

5

NH3/SiH4 ratio

[N-H

] bon

d de

nsity

( x

1022

cm

-3)

T = 80oCT = 90oCT = 100oC

flow ratio

0.5 1 1.5 20

1

2

3

4

5

NH3/SiH4 ratio

[Si-H

] bon

d de

nsity

( x

1022

cm

-3)

T = 80oCT = 90oCT = 100oC

flow ratio

86


(a) (b)

(c)

Figure 3.34 The ratio of H bonded to N over that bonded to Si, [N-H]/[Si-H], as a function of NH3/SiH4 flow ratio, refractive index and film composition [N]/[Si]. The line in (a) is a linear least-squares fit to the data points, as [N-H]/[Si-H] = 3.25 × NH3/SiH4 - 2.62. The indicated temperatures refer to the substrate temperature during deposition.

0.4 0.6 0.8 1 1.2 1.4 1.60

1

2

3

4

[N]/[Si]

[N-H

]/[S

i-H]

T = 80oCT = 90oCT = 100oC

1.8 2 2.2 2.4 2.60

1

2

3

4

n (λ = 632.8 nm)

[N-H

]/[S

i-H]

T = 80oCT = 90oCT = 100oC

0.5 1 1.5 20

1

2

3

4

NH3/SiH4 ratio

[N-H

]/[S

i-H] r

atio

T = 80oCT = 90oCT = 100oC

flow ratio

87


(a)

(b)

Figure 3.35 The fraction of [N-H] and [Si-H] as a function of SiNx film composition [N]/[Si]. The indicated temperatures refer to the substrate temperature during deposition.

0.7 0.8 0.9 1 1.1 1.2 1.320

40

60

80

100

[N]/[Si]

[Si-H

]/([S

i-H]+

[N-H

]) %

T = 80oCT = 90oCT = 100oC

0.7 0.8 0.9 1 1.1 1.2 1.30

20

40

60

80

100

[N]/[Si]

[N-H

]/([S

i-H]+

[N-H

]) %

T = 80oCT = 90oCT = 100oC

88


Figure 3.36 The atomic densities of [Si], [N] and [H] as a function of SiNx film composition [N]/[Si] and NH3/SiH4 flow ratio. The indicated temperatures refer to the substrate temperature during deposition.

0.4 0.6 0.8 1 1.2 1.4 1.62.5

3

3.5

4

4.5

5

5.5

[N]/[Si]

[Si]

bond

den

sity

( x

1022

cm

-3) T = 80oC

T = 90oCT = 100oC

0.4 0.6 0.8 1 1.2 1.4 1.63

3.5

4

4.5

5

[N]/[Si]

[N] b

ond

dens

ity (

x 10

22 c

m-3

)

T = 80oCT = 90oCT = 100oC

0.5 1 1.50.25

0.3

0.35

0.4

0.45

0.5

[N]/[Si]

[H] b

ond

dens

ity (

x 10

2 2 c

m-3

)

T = 80oCT = 90oCT = 100oC

0.5 1 1.5 23

3.5

4

4.5

5

NH3/SiH4 ratio

[N] b

ond

dens

ity (x

1022

cm

-3)

T = 80oCT = 90oCT = 100oC

0.5 1 1.5 2

2.5

3

3.5

4

4.5

5

5.5

NH3/SiH4 ratio

[Si]

bond

den

sity

(x10

22 c

m-3

)

T = 80oCT = 90oCT = 100oC

0.5 1 1.5 2

0.25

0.3

0.35

0.4

0.45

0.5

NH3/SiH4 ratio

[H] b

ond

dens

ity (x

1022

cm-3

)

T = 80oCT = 90oCT = 100oC

flow ratio

flow ratio

flow ratio

89


(a)

(b)

Figure 3.37 Film density, ρ, as a function of (a) SiNx film composition [N]/[Si] and (b) NH3/SiH4 flow ratio. The indicated temperatures refer to the substrate temperature during deposition.

0.4 0.6 0.8 1 1.2 1.4 1.62

2.5

3

3.5

4

[N]/[Si]

ρ (g

/cm

3 )

T = 80oCT = 90oCT = 100oC

0.5 1 1.5 22

2.5

3

3.5

4

NH3/SiH4 ratio

Film

den

sity

, ρ (g

/cm

3 )

T = 80oCT = 90oCT = 100oC

flow ratio

90


Figure 3.35 plots the fraction of [N-H] and [Si-H] as a function of SiNx film

composition [N]/[Si]. As expected, Figure 3.35 shows that at lower [N]/[Si], almost

all the hydrogen atoms are bonded to Si, and with an increase in [N]/[Si] there is an

increase of hydrogen atoms being bonded to N and an decrease of H bonded to Si.

The atom densities of [Si], [N] and [H] as a function of SiNx film composition

[N]/[Si] and NH3/SiH4 flow ratio are shown in Figure 3.36. The general trends are that

[Si] decreases with NH3/SiH4 flow ratio and [N]/[Si], which is as expected, while both

[N] and [H] increase with NH3/SiH4 flow ratio and [N]/[Si].

Film densities as a function of SiNx film composition [N]/[Si] and NH3/SiH4 flow

ratio are shown in Figure 3.37. The film density was seen to increase with elevated

substrate deposition temperature, and the defect density is expected to decrease with

increasing film density [212]. The trend of film density with either [N]/[Si] or

NH3/SiH4 flow ratio is non-linear, since it first increases with [N]/[Si] and then

decrease, which is in agreement with the results reported by Claassen et al. [191, 210].

3.5 Summary

This chapter has described the various experimental techniques used for material

characterisation related to this thesis, divided into two categories - in-situ monitoring

characterisation and ex-situ characterisation. Some selected experimental results have

been included, and the purposes for which these material characterisation techniques

were used to extract device parameters were given for each procedure.

Lastly, in order to determine the suitable deposition conditions of SiNx passivation

film for HgCdTe, a series of low-temperature (80 °C - 130 °C) ICPECVD SiNx films

were deposited on other semiconductors - CdTe/GaAs and Si substrates, under

different deposition conditions to investigate the influence of ICP power, deposition

temperature, and NH3/SiH4 flow ratio on properties of the SiNx films. The influence

of ICP power on the quality of the deposited SiNx films was assessed through the

long-term IR absorbance of the films, which determined that SiNx films deposited at a

high ICP power of 600 W appeared to be porous and more susceptible to oxidation.

Regarding the influence of NH3/SiH4 flow ratio on SiNx film properties, such as

refractive index, film composition, deposition rate, IR absorbance and bonding

91


configuration, a series of SiNx films were deposited on Si substrates with a fixed SiH4

flow rate and various NH3 flow rates. The method used for the calculations of bond

and atom concentrations was introduced based on the band areas in the optical

absorption coefficient curve, and thus the influence of NH3/SiH4 flow ratio on bond

and atom concentrations was studied.

92

Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures

4 Surface and Interface Effects in CdTe/HgCdTe Structures

4.1 Introduction

Following the establishment of reproducible bulk growth techniques and anodic oxide

surface passivation technology, HgCdTe photoconductive detectors entered

production in the late 1970’s as first-generation IR imaging systems [30]. As a result

of these developments, very detailed understanding of photoconductor behaviour has

been established including surface recombination, carrier sweepout effects, backside

shunting effects etc.. Because of the detailed understanding and the relative simplicity

of the structure, photoconductors are useful tool for characterisation of various

physical phenomena in HgCdTe materials. In particular, photoconductors allow very

detailed information to be extracted about minority carrier dynamics that is equally

applicable to other photodetector structures.

In this chapter, photoconductive devices are used as a tool in examining the

effectiveness of low-temperature deposited CdTe passivating films by comparing the

photoresponsivity between devices with and without sidewall CdTe passivation [213].

In addition, these results are augmented using gated HgCdTe photodiode structures,

passivated by the same MBE low-temperature grown CdTe, allowing band bending at

the surface to be controlled by varying the gate bias.

4.1 Sidewall effects in photoconductive devices

The responsivity of a photodetector is dependent on the bulk minority carrier lifetime

and the surface recombination velocities at each the surfaces (front surface, back

surface and sidewalls). By comparing devices with and without sidewall passivation,

the surface recombination at CdTe passivated surfaces can be determined.

4.1.1 Experimental procedures

The experiments were carried out using the following procedure:

Four wafers were cleaved from the same Epitech MWIR n-HgCdTe material (x =

0.29), that has a 13.5 μm-thick vacancy-doped absorber grown by LPE. Figure 4.1

summarises the process flow for each of the wafers. Wafer 1 was first etched using a

93


standard wet etchant (1% Br2/HBr) to form mesa structures that will later be

processed into photoconductors. Then Wafer 1, Wafer 2 and Wafer 3 were

passivated with CdTe by MBE in the same run. To minimise fixed charge and

interface states in the CdTe passivating layer, surface cleaning and conditioning of the

HgCdTe layer before CdTe deposition is essential [100]. A Br2/methanol wet etch

was used after an organic surface clean to reveal a fresh HgCdTe surface. After this

surface conditioning, an approximately 200 nm-thick layer of CdTe was deposited by

MBE in the same run on all three samples. The CdTe passivation film was grown by

MBE at 100 °C. During the growth, the CdTe as measured by beam equivalent

pressure flux was 1 × 10-6 Torr and Te as measured by beam equivalent pressure flux

was 1.5 × 10-6 Torr, which were recorded by a Bayard-Alpert vacuum gauge that

could be rotated into the growth position. The background pressure was 2 × 10-9 Torr.

The resulting growth rate was approximately 10 nm/min. Following growth, an in-situ

anneal at 180 °C in the MBE growth chamber was done to achieve a compositionally

graded CdTe/HgCdTe interface.

Wafer 2 was etched using 1% Br2/HBr to form mesa structures that will later by

processed into photoconductors. It is necessary for the photoconductor structures on

Wafer 2 to have their top surfaces passivated with CdTe in order to reduce total

surface recombination so that the carrier lifetime was not too low, and to ensure that

photoresponsivity measurements could be meaningfully compared with the ones on

Wafer 1. The photoconductor structures on both Wafer 1 and Wafer 2 were 240 μm

× 200 μm × 13.5 μm in dimension.

To reveal the contact regions, both Wafer 1 and Wafer 2 were then patterned by

photolithography followed by a Br2/HBr wet etch. Lastly, the two wafers were

patterned by photolithography again for contact formation, and metal contacts were

formed by thermally evaporating 5 nm of Cr and 200 nm of Au. After metal liftoff in

acetone, photoconductors on Wafer 1 were resulted with both sidewall and surface

passivation, whereas the ones on Wafer 2 with only top surface passivation.

Wafer 3 and Wafer 4 were characterised by XRD in order to examine their X-ray

diffraction spectra before (Wafer 4) and after CdTe passivation (Wafer 3), as

previously detailed in Section 3.2.2.

94


Figure 4.1 Schematic of the photoconductive devices showing location of the unpassivated sidewall surfaces. The passivating CdTe film is approximately 200 nm thick. (a) Fully passivated structure; (b) Partially passivated structure with no CdTe film on sidewalls.

4.1.2 Surface and interface recombination in photoconductive devices

The performance of a photoconductive detector is directly impacted by carrier

recombination at the surface, which affects the effective minority carrier lifetime.

Surface recombination processes in narrow bandgap detectors can become the

dominant loss mechanism for photo-generated excess carriers [9]. Surface passivation

greatly reduces the recombination of photo-generated carriers at the surface/interface,

resulting in an increase in the effective minority carrier lifetime and hence the voltage

responsivity [10, 11].

The responsivity of an infrared detector is defined as the output signal of the detector

divided by the input photon power. When a photoconductor device being exposed to a

monochromatic, modulated source, its responsivity (Volts/Watt) can be written

95


as [214]

d

s

IAVR =λ (4.1)

where Vs (Volts r.m.s.) is the signal output, I (Watts/cm2) is the intensity of the

source, Ad is the size of the optically sensitive area of the detector.

The spectral photoresponsivity of the photoconductors described in the previous

section were measured using a system based on an Optronics Laboratories spectral

radiometer measurement system. During the photoresponsivity measurement, the

source of infrared radiation was calibrated firstly, so that the photoconductor devices

were subjected to a known intensity of infrared radiation at a known wavelength, and

was chopped at a relatively low frequency of 1 kHz. The alternating signal output

from the photoconductor, Vs, was measured using a lock-in technique at a field of

10 V/cm with the sample being held at 80 K in a cryostat with a cold shield. This

relatively low field minimised the effect of sweepout, so that the response should be

most sensitive to surface recombination velocity. As expected, it was found that the

photoconductors with all surfaces passivated have significantly higher responsivity

than those partially passivated, as shown in Figure 4.2.

Figure 4.2 Measured and modelled normalised spectral photoresponse of Hg0.71Cd0.29Te photoconductive devices, measured at a field of 10 V/cm at 80 K. The low field minimises the effect of sweepout so that the response should be most sensitive to surface recombination velocity.

4 4.5 5 5.5 60

0.5

1

1.5

Wavelength(µm)

Nor

mal

ised

Pho

tore

spon

se

with sidewall passivation (experimental data)without sidewall passivation (experimental data)with sidewall passivation (simulation)without sidewall passivation (simulation)

96


Figure 4.3 Simulated photoresponse ratio of all surfaces passivated devices (RF) and partially passivated devices (RP) versus recombination velocity of the top CdTe/Hg0.71Cd0.29Te interface (sTop) at 80 K, with recombination velocity of the unpassivated surfaces sWall = 1×104 cm/s.

In a passivation layer, the fixed charge and interface traps can result in band-bending

in addition to recombination, all of which are dependent on the properties of the

passivating film. In order to evaluate the effectiveness of the sidewall passivation and

estimate the surface recombination velocity of CdTe passivated surfaces, surface

recombination simulations of the photodetectors were performed using Synopsys

TCAD device simulator Synopsis Sentaurus Device [215]. The Sentaurus fitting

parameters of trap ionisation energy value of 0.7Eg was used based on some

experimental evidence that Hg vacancies leave a trap at this level [216, 217], and the

interface trap density, Dit, value of 8 × 1012 cm-2eV-1 has resulted in satisfactory fit to

the experimental data.

An estimate of the surface recombination velocity at the CdTe/HgCdTe interface was

obtained in the following manner: First, the structure with unpassivated sidewalls was

modelled using an ideal interface (sTop = 0 cm/s) for the top CdTe passivated surface.

The HgCdTe/substrate interface was modelled with a recombination velocity of

100 cm/s, using results from previous studies on similar structures [218]. The

unpassivated sidewall surfaces were modelled as having sWall = 104 cm/s. Values

larger than this had little effect on the responsivity. Then the all surfaces passivated

structure and partially passivated structures were modelled by increasing the

102 103 1042

4

6

8

10

12

Surface recombination velocity (cms-1)

RF/R

P

97


CdTe/HgCdTe sTop values until the ratio of peak responsivity of the all surfaces

passivated and partially passivated structures (RF /RP) matched the experimental

results (see Figure 4.2). The best fit value for sTop was 200 cm/s, as shown in Figure

4.3, which is more than an order of magnitude lower than the one at the unpassivated

surface sWall. Very high values of the surface recombination velocity of the

unpassivated HgCdTe of > 1 × 105 cm/s have been reported in literature, and, after

passivation, obvious drop with at least an order of magnitude in the interface

recombination velocities were observed [9, 219]. The interface recombination

velocities at the CdTe/HgCdTe interfaces were reported to be much lower than that at

the ZnS/HgCdTe interfaces, and two order of magnitude lower than that of freshly

etched surfaces [220]. The sharp decreases in photoresponsivity in the theoretical

curves were not observed in the experimental data, possibly because of field effect

[221, 222], Stark effect [223-225], Franz-Keldysh effect [223, 225, 226] and graded

band structure [227].

4.2 Interface effects in ZnS/CdTe/HgCdTe gated photodiodes

HgCdTe photodiodes are one of the most widely used devices for IR detection, since

they provide low power, high sensitivity detection, faster response times, and

improved uniform spatial response in focal plane arrays (FPA). A gated photodiode is

a photodiode structure that incorporates a metal - insulator - semiconductor gate

across the region where the pn junction intersects the surface, and can be used to

investigate how the surface band-bending affects the diode characteristics by applying

a gate voltage. The fact that the band-bending at the surface is localised to the region

under the gate and is controlled by the gate voltage has made the gated diode structure

useful in the study of surface related effects.

The device structure used in this thesis is planar, with a reactive ion etching (RIE)

induced n-on-p homojunction [6, 228, 229]. MBE grown CdTe is used as the first

passivation layer on HgCdTe and the second layer of ZnS serves as a higher

resistivity insulating layer in the gated photodiode. This section investigates the

properties of CdTe passivation by observing and interpreting the change in

photodiode performance when influenced by surface band bending.

98


4.2.1 Gated photodiode fabrication process

Surface cleaning of the semiconductor prior to passivation formation is crucial to

device performance. In this thesis, the LPE grown p-type MWIR HgCdTe (x = 0.31,

depilayer = 13.5 μm) wafer was initially cleaned by soaking in successive baths of

trichloroethylene, acetone, and methanol at 70 °C, followed by a light etch in

Br/methanol solution for surface conditioning before CdTe deposition. The sample

was then kept in running deionised water until being dried just prior to loading into

the MBE chamber for deposition of the CdTe passivation.

An approximately 100 nm-thick CdTe passivation film was grown by MBE at 100 °C,

followed by an in-situ anneal at 180 °C in the MBE growth chamber to achieve a

compositionally graded CdTe/HgCdTe interface, as detailed in Section in 4.1.1.

The third step in the fabrication process is the conversion of regions of the p-HgCdTe

to n-type. The devices fabricated in this thesis were based on a RIE plasma-based p-

to-n type conversion process [6, 228, 229], which is a simpler technique than

traditional ion implantation and ion milling methods. CdTe is used as a surface

passivant and as a mask for the p-to-n type conversion process. Windows for type

conversion were defined photolithographically, and the CdTe was etched in a 1%

Br/HBr solution. The photoresist was then stripped and the samples organically

cleaned before being exposed to a hydrogen-methane plasma in a Plasma Technology

RIE 80 system for two minutes, with RIE power of 120 W, H2 of 54 sccm, CH4 of

10 sccm, and a chamber pressure of 100 mTorr. The entire semiconductor area

beneath the circular RIE type converted n-type region is expected to be uniformly

type converted from p to n type with a junction depth of approximately 1.5 μm [230].

ZnS (approximately 200 nm-thick) was then deposited in a thermal evaporator under a

vacuum of 1×10−6 Torr. The wafer was heated under vacuum prior to deposition,

starting at a temperature of 50 °C, and reaching approximately 70 °C by the end of the

ZnS deposition. The ZnS deposition rate is controlled at a low level of ~ 0.02 nm/s in

order to achieve a denser and higher quality film [92].

The wafer was then patterned by photolithography and etched to reveal the type

converted region, and the ZnS was wet etched in 2:1 HCl: distilled and deionised

water solution. To reveal the shared common p-type contact regions, the wafer was

99


patterned by photolithography again, and the ZnS was etched in 2:1 HCl: distilled and

deionised water solution, followed by the etching of CdTe in 1% Br/HBr.

Lastly, for contact formation, the wafer was patterned by photolithography with a

single photolithographic mask for both n and p contacts, and metal contacts to the p

and n type regions of HgCdTe were formed by thermally evaporating 5 nm of Cr and

200 nm of Au. After metal liftoff in acetone, all photodiodes share a common p-type

contact region, and have individual contacts for the n-type regions. A

photomicrograph and cross section of the completed devices are shown in Figure 4.4.

(a)

(b)

Figure 4.4 A photomicrograph and cross section of the completed gated photodiodes. (a) Photo of fabricated gated photodiodes and (b) cross-sectional view.

Junction Bias

ZnS

p-HgCdTe

contact

CdZnTe

CdTe

contact

n

300 μm

100


4.2.2 Dark current as a function of gate bias

Unlike SiNx passivation, ZnS/CdTe is unsuitable for standard C-V analysis, because of

excessive leakage currents through the passivation layer. To evaluate ZnS/CdTe as a

passivation layer, gate bias swept diode dark current measurements have been used.

This section describes the results of dark current measurements on the gated diodes

used to investigate CdTe passivation performance. Varying the gate bias, Vg, allows

the magnitudes of dark current and dynamic resistance to be manipulated at the surface

of the photodiode, and determination of the dominant generation-recombination

mechanisms.

4.2.2.1 Dark current mechanism

Dark currents in HgCdTe photodiodes primarily include diffusion, generation-

recombination (GR), band-to-band tunnelling (BTB), trap-assisted tunnelling (TAT),

shunt and surface currents [231, 232]. Each dark current component can be treated as a

resistance in parallel with the junction, expressed as:

surfaceshuntTATBTBGRdiff RRRRRRR1111111

+++++= (4.2)

Diffusion current is a fundamental mechanism in p-n junction photodiodes due to the

random thermal generation of carriers within a minority carrier diffusion length of the

depletion region edges. In MWIR HgCdTe photodiodes, diffusion current dominates at

higher temperatures (> 150 K), whereas it is less than other current components at

77 K in the reverse bias region, and dominant in the forward bias region.

SRH type generation-recombination centres in the space charge region can contribute

significantly to the dark current, with impurities or defects within the depletion region

acting as GR centres producing GR current within the diode [233]. GR current can also

occur when impurities or defects are not involved, such as radiative and Auger

recombination [234].

BTB and TAT current components affect the performance of photodiodes greatly due

to the narrow bandgap of HgCdTe, and they can be influenced by depletion width,

bandgap, trap energy level and trap density [235, 236]. BTB current occurs when

carriers tunnel from the valence band to the conduction band across the depletion

region. A decrease in depletion width can lead to an increase of BTB current due to the

decreased width of barrier to tunnelling. Narrowing of the depletion width at the

101


semiconductor surface can also lead to an increase in the BTB component. TAT occurs

via impurities or defects located within the depletion region when carriers travel from

the valence band to the conduction band. Increased trap density leads to the increase in

TAT current.

Shunt current component is considered to be associated with the formation of localised

defects in the vicinity of the junction at the semiconductor surface. Shunt current can

occur when the surface is placed in inversion due to incorrect passivation of the surface

[52]. Surface current component is associated with incorrect termination of bonds at

the surface of the HgCdTe or charges in the passivation layer.

4.2.2.2 Dark current measurement of gated diodes

The field induced junction at the surface will be modified with any change of charge

density in the passivation and at the interface. As shown in Figure 4.5 (a), the p-type

surface is in accumulation when Vg < Vfb. Narrowing of the depletion region of the of

the p-n junction on the p-side caused by accumulation will result in an increase of

electric field across the junction, which can increase the TAT current component,

since TAT is sensitive to changes in the electric field across the junction.

Figure 4.5 (b) illustrates the p-type surface in the flatband condition when Vg = Vfb

(~ -2.7 V), where narrowing of the depletion width of the p-n junction on the n-side

can still be observed. As shown in Figure 4.5 (c), a depletion region will form under

the gate when increasing the gate voltage, which decreases the TAT current at the

surface, since the field induced junction reduces the junction electric field at the

surface. An increase in the RdA is expected to be observed. The p-type surface will be

in inversion and field-induced junction breakdown will occur in the p-type region

under the gate, when Vg is increased beyond the threshold voltage (~ 2 V).

Dark current measurements were made with a HP 4156A semiconductor parameter

analyser. The dark currents were measured on two fabricated gated diodes at 77 K in a

cryostat with a cold shield to prevent generation of photocurrent. One diode had a

diameter of 300 μm and the other 360 μm, as shown in Figure 4.6. For a junction bias

of -50 mV, which is typically used to minimise dark current in HgCdTe photodiodes,

the dark current was observed to decrease by three orders of magnitude as Vg was

increased from -1.5 V to 1.5 V. Dark current was GR limited and then diffusion

102


Figure 4.5 Diagrams illustrating the effects of n-type region band-bending on a n-on-p junction. The gate voltage, Vg, is referenced to the p-type HgCdTe. a) p-type surface in accumulation; b) Vg = Vfb in p-type, c) p-type surface in depletion or weak inversion; d) p-type surface in inversion and field-induced junction breakdown occurs in the p-type region under the gate.

(a)

CdZnTe

Vg

n p

(b)

Vg

n p

CdZnTe

(c)

Vg

n p

CdZnTe

(d)

Vg

n p

CdZnTe

103


(a) d = 300 μm

(b) d = 360 μm

Figure 4.6 Measured dark current at 77 K in a cryostat with a cold shield in the absence of photocurrent for gated photodiodes with (a) a diameter of 300 μm and (b) 360 μm. The gate voltage is referenced to the p-type substrate. The seven curves from top to bottom are for various gate biases from -1.5 V to 1.5 V in 0.5 V steps.

-0.4 -0.2 010-12

10-10

10-8

10-6

10-4

Junction Bias (V)

Cur

rent

(A)

Vg = - 1.5 V

Vg = 1.5 V

-0.4 -0.2 0 0.210-12

10-10

10-8

10-6

10-4

Junction Bias (V)

Cur

rent

(A)

Vg = - 1.5 V

Vg = 1.5 V

104


(a) d = 300 μm

(b) d = 360 μm

Figure 4.7 Measured dynamic resistance-area product at 77 K in a cryostat with a cold shield in the absence of photocurrent for gated photodiodes with a diameter of (a) 300 μm and (b) 360 μm. The seven curves from bottom to top are for varying gate bias from -1.5 V to 1.5 V in 0.5 V steps.

-0.4 -0.2 0100

102

104

106

Junction Bias (V)

Dyn

amic

Res

ista

nce

(Ω.c

m2 )

Vg = 1.5 V

Vg = - 1.5 V

-0.4 -0.2 0100

102

104

106

Junction Bias (V)

Dyn

amic

Res

ista

nce

(Ω.c

m2 )

Vg = 1.5 V

Vg = - 1.5 V

105


limited when being forward biased at the junction. The increase of current in the GR

limited region can be caused by a widening of the depletion region formed under the

gate in the n-type region. A slightly reverse junction biased region is dominated by

TAT in the Vg sweeping range in Figure 4.6 (-1.5 V to 1.5 V), and would be by BTB

tunnelling with even greater negative gate bias (> -4 V). This is due to a narrowing of

the junction depletion region at the surface caused by accumulation of the p-type

surface. The tunnelling currents can often dominate the dark current characteristics in

gated diodes due to the decrease in the depletion width at the surface caused by band-

bending. The BTB and TAT can occur at much lower reverse junction biases in gated

diodes than with an accumulated surface in comparison to what would occur in non-

gated diodes.

Within a given range of Vg in Figure 4.6, the p-type surface is depleted, and a wider

depletion region will be formed under the gate by increasing Vg up to ~ 2 V, which

decreases the TAT current at the surface [72, 92], as illustrated in Figure 4.5 (c).

Recombination at the surface will be increased due to the increased volume of

depletion in the p-type, resulting in a current increase. Similar to the gated diode

results presented by Westerhout et al. [237], there was no obvious hysteresis observed

in the I-V or RdA curves when Vg was swept from accumulation to depletion and swept

back.

Shown in Figure 4.7 is the measured dynamic resistance-area product, RdA, for the

two photodiodes at 77 K in a cryostat with a cold shield in the absence of

photocurrent for gated photodiodes. The gate voltage Vg is referenced to the p-type

HgCdTe substrate. By increasing Vg from 0 V to 1.5V, RdA improved by

approximately an order of magnitude, whereas RdA dropped by an order of magnitude

when decreasing Vg to -1.5 V. As Vg is increased from -1.5 V to 1.5 V, the depletion

region at the p-type surface is continually widened within this gate bias regime, which

reduces the TAT current, and results in a monotonic increase in RdA. The relatively

low values of dynamic resistance of the diodes may be due to negative charges within

the passivation layer that can accumulate the p-type HgCdTe surface. The

performance of the photodiodes was observed to be improved by varying the gate

bias, also indicating a non-ideal passivated surface which has increased the surface

current component at zero bias [237].

106


4.3 Summary

In this chapter, surface and interface effects were studied in CdTe/HgCdTe devices,

including photoconductive devices and gated photodiode devices. The CdTe

passivation used was MBE low-temperature grown film. In order to determine the


photoconductors were utilised to investigate the passivation effect of MBE grown

CdTe films, by comparing photoresponsivity between devices with and without

sidewall CdTe passivation. Surface recombination simulations of the photodetectors

were performed to evaluate the effectiveness of the sidewall passivation and estimate

the surface recombination velocity of CdTe passivated surfaces. The gated photodiode

was used as a tool to investigate device performance, and the band bending at the

surface can be influenced by varying the gate bias. This allows the magnitude of dark

current and dynamic resistance to be manipulated at the surface of the photodiode,

which also changes the dominant surface GR mechanisms. Gated diode results indicate

that positive charge may be trapped in CdTe/ZnS. This is a way to evaluate the

CdTe/ZnS passivation.

107

Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures

5 Interface Effects in Metal/SiNx/HgCdTe Structures

The capabilities of low-temperature processing, good surface insulation and

hydrogenated films make SiNx a suitable choice for passivating HgCdTe. The MIS

structure has long been utilised as a tool in studying the interface between the

insulator and semiconductor [164, 238], and the interface state density Dit has been

considered as the key parameter in evaluating surface passivation performance and in

correlating passivation quality with other film properties [174, 175, 179, 182, 183]. In

this chapter, SiNx films were deposited under different conditions, and then the

characteristics of SiNx/n-Hg0.68Cd0.32Te MIS structures were determined, with the

interface trap density Dit examined by both quasi-static and conductance methods.

Also, work in this chapter has also been carried out on the correlation of film

passivation performance with film bond densities. The concentration of silicon-

hydrogen bonds, [Si–H], in silicon nitride based passivation has been regularly

considered as a measure of surface passivation quality, since a higher [Si–H] implies a

higher probability that hydrogen terminates any dangling bonds at the interface. In

Section 5.3, the [Si-H] and [N-H] bond densities in the SiNx/Si films will be discussed

as indicators for passivation quality [1].

Given the limitations on the current literature on SiNx/HgCdTe interface effects, it is

difficult to formulate a physical model relating the optical and electrical properties of

SiNx to its passivation performance for HgCdTe. However, based on the literature on

SiNx/Si, the work in this chapter hopes to provide some insight into the issue by

demonstrating the interaction of film bond density with the interface trap density Dit

using SiNx/HgCdTe MIS structures, and also the impact of deposition parameters on

the interface.

5.1 Fabrication of the MIS structures

The HgCdTe epilayers employed were grown at UWA in a Riber-32 MBE facility. A

layer of 2 μm-thick Hg0.6Cd0.4Te was grown on CdZnTe substrate followed by a

5 μm-thick Hg0.68Cd0.32Te layer. The HgCdTe wafer was then diced into four pieces,

and on each piece SiNx was deposited under different conditions. Circular gate

contacts of 5 nm Cr and then 200 nm of Au were deposited onto the SiNx films and

defined photolithographically.

108


(a) D1-80C (b) D1-90C

(c) D1-100C (d) D4-100C

Figure 5.1 C-V curves measured at 1 MHz with the three different sweeping voltage ranges for each of the four MIS sample for the range of ± 2 V (circle ‘o’), ± 4 V (plus ‘+’) and ± 6 V (square ‘’). Each C-V measurement was swept from surface inversion to accumulation (dashed lines) and then swept back to inversion (solid lines).

-6 -4 -2 0 2 4 60.7

0.75

0.8

0.85

0.9

0.95

1

Applied voltage (V)

Chf

/ C

ox

-6 -4 -2 0 2 4 60.8

0.85

0.9

0.95

1

Applied voltage (V)

Chf

/ C

ox

-6 -4 -2 0 2 4 60.7

0.75

0.8

0.85

0.9

0.95

1

Applied voltage (V)

Chf

/ C

ox

-6 -4 -2 0 2 4 60.6

0.7

0.8

0.9

1

Applied voltage (V)

Chf

/ C

ox

111


Flat band voltage, fixed charge density and slow interface trap density were extracted

using standard MIS theory [164, 238], summarised in Table 3.3 and Table 3.9. The

discussion on extracted Dit will be presented in the following sections. The polarities

of fixed charge density were found to be negative for all the four samples. A positive

fixed charge in SiNx film on a n-HgCdTe substrate would make the HgCdTe surface

accumulated at zero-bias, which is advantageous for n-type HgCdTe photoconductive

detectors due to the field-effect surface passivation, whereas a low negative charge

could be beneficial for n+-p photo-diodes. A high negative fixed charge will invert the

n-HgCdTe surface, and this inverted region of p-HgCdTe, separated from the n-

HgCdTe by a depletion region, could lead to a decrease in recombination rate, and

thus leads to a field-effect passivation. In Figure 5.1, the high frequency capacitance

was seen to increase slightly in inversion for increasingly negative gate bias, which is

an indication of an inverted semiconductor surface at zero bias.

Figure 5.2 defines the terms of ∆V+, ∆V–, ∆VH and Vfb used in the C-V analysis. ∆V+

is a measure of negative charge trapping, whereas the corresponding ∆V– is a measure

of positive charge trapping. ∆VH is the sum of ∆V+ and ∆V–. The asymmetrical ∆V+

and ∆V– is an indication of the asymmetrical electron and hole trapping characteristics

of the defects [241]. All the four samples indicate higher electron trapping at he

interface compared hole trapping, with the low-temperature deposited sample D1-80C

exhibiting the highest hole trapping characteristics, and the defects at the interface can

trap either electrons or holes.

It can be seen in Figure 5.1 that the Vfb and ∆V– barely change when voltage sweeping

from negative to positive. But when sweeping from positive back to negative voltage,

Vfb shifted to positive voltages by a significant amount, resulting in a change of ∆V+

for different sweep conditions. This phenomenon becomes more obvious with

increased sweep bias extremes from Vmax (solid C-V curve in Figure 5.2) to Vmax'

(dashed C-V curve in Figure 5.2), similar to what have been observed in anodic

oxide/HgCdTe [242] and ZnS/HgCdTe [243] MIS structures.

112


Figure 5.2 Illustration on the definitions of ∆V+, ∆V– and ∆VH used in the C-V analysis.

It is well-known that a shift in Vfb is caused by the charge exchange between the

semiconductor and the slow traps due to the relative movement of the Fermi levels

with the band edges [164]. The experimental C-V curves indicate that the charge

exchange takes place mainly when the n-HgCdTe surfaces were accumulated. Under

positive bias, the slow traps in SiNx are filled with the electrons accumulated at the

HgCdTe surface, thus exhibiting a higher negative charge and, hence, a positive Vfb

shift. With an increase in the voltage extreme Vmax, the number of electrons captured

by the traps increases, resulting in a greater Vfb shift to more positive voltages.

A summary on Vfb, fixed charge density (Qf), ∆VH, slow interface trapped charge

density (Qit) and Dit extracted from the C-V characteristics of the four SiNx/HgCdTe

MIS samples is given in Table 5.2. The relationship between Vfb and substrate

temperature, [N]/[Si] and n632.8nm for the MIS samples of D1-80C, D1-90C, and D1-

100C, with SiNx deposited at the same condition except for substrate temperature, is

plotted in Figure 5.3. It can be seen that the flat band voltages Vfb decrease with the

increase of substrate temperature and film refractive index n632.8nm, and increase with

[N]/[Si]. The values of VH and Qit at various Vmax for the four MIS samples are listed

in Table 5.3, which are illustrated in Figure 5.4.

CFB

∆V_ ∆V+

∆VH'

∆VH

Vmax Vmax' -Vmax' -Vmax 0 VG

C

113


Table 5.2 Summary on flat band voltage, fixed charge density, slow interface trapped charge density and interface trap density extracted for the four

SiNx/HgCdTe MIS samples for a bias sweep range of ± 2 V

D1-80C D1-90C D1-100C D4-100C

[N]/[Si] 1.17 1.07 1.02 0.90

n632.8nm 1.86 1.90 1.93 1.94

Flat band voltage Vfb (V) 0.98 0.86 0.71 0.76

Fixed charge density Qf (×10 10 cm-2) -12.86 -8.53 -9.42 -11.48

Hysteresis width ∆VH (V) 0.19 0.08 0.208 0.207

Slow interface charge density Qit (×10 10 cm-2) 3.17 1.03 3.87 4.33

Interface trap density Dit_min (×10 10 eV-1cm-2) 5.07 17.47 11.78 4.01

Interface trap density Dit (×10 10 eV-1cm-2) at mid-gap 13.65 50.38 17.56 10.57

Table 5.3 Hysteresis widths VH in the high-frequency C-V curves versus bias extremes for the four SiNx/HgCdTe MIS structures

Hysteresis width VH (V) Slow interface trap density (× 1010 cm-2)

Sample name D1-80C

D1-90C

D1-100C

D4-100C

D1-80C

D1-90C

D1-100C

D4-100C

± 2 V 0.19 0.08 0.208 0.207 3.17 1.03 3.87 4.33

± 4 V 0.84 0.21 0.48 0.46 14.23 2.80 9.01 9.70

± 6 V 1.75 0.36 0.80 0.73 29.81 4.82 15.08 15.57

114


(a) (b)

Figure 5.3 Flat band voltage as a function of (a) substrate temperature, and (b) [N]/[Si], for the three MIS samples of D1-80C, D1-90C, and D1-100C.

(a) (b)

Figure 5.4 The change in (a) hysteresis widths VH and (b) slow interface charge densities as a function of bias extremes for the four MIS structures. The absolute values of the negative slow interface trap density are shown in the plot.

2 3 4 5 60

0.5

1

1.5

2

Vmax (V)

Hyst

eres

is wi

dth

(V)

D1-80CD1-90CD1-100CD4-100C

± 2 3 4 5 6

0

5

10

15

20

25

30

Vmax (V)Slo

w in

terfa

ce c

harg

e de

nsity

(x10

10cm

-2)

D1-80CD1-90CD1-100CD4-100C

±

0.7 0.75 0.8 0.85 0.9 0.95 11.02

1.04

1.06

1.08

1.1

1.12

1.14

1.16

1.18

[N]/[

Si]

Vfb (V)0.7 0.75 0.8 0.85 0.9 0.95 1

1.86

1.87

1.88

1.89

1.9

1.91

1.92

1.93

1.94

n (λ

= 6

32.8

nm

)

0.7 0.75 0.8 0.85 0.9 0.95 11.02

1.04

1.06

1.08

1.1

1.12

1.14

1.16

1.18

[N]/[

Si]

Vfb (V)0.7 0.75 0.8 0.85 0.9 0.95 1

1.86

1.87

1.88

1.89

1.9

1.91

1.92

1.93

1.94

n (λ

= 6

32.8

nm

)

80 85 90 95 1000.7

0.8

0.9

1

Substrate temperature (oC)

Vfb

(V)

115


5.2.2 Interface trap density extracted by quasi-static method

The quasi-static C-V method [164, 244, 245] was employed to extract the density of

fast interface trap Dit from the high-frequency and low-frequency C-V data in the bias

sweep range of ±2V. The basic theory of the quasi-static C-V method was developed

by Berglund [245], which can be carried out by comparing a low-frequency (lf) C-V

with a high-frequency (hf) C-V. The high-frequency C-V curve should be measured at

a frequency high enough so that interface traps can be assumed not to respond,

whereas the low-frequency C-V is where interface traps and minority carrier inversion

charges are able to respond to the probe frequency used in the measurement. The low-

frequency capacitance Clf in depletion-inversion can be expressed as 1

11−

+

+=itSox

lf CCCC (5.1)

where

itit DqC 2= (5.2)

and

hfox

hfoxS CC

CCC

−= (5.3)

therefore,

−−

−=

−

−=

oxhf

oxhf

oxlf

oxlfoxS

lfox

lfoxit CC

CCCC

CCqC

CCC

CCq

D/1

//1

/122 (5.4)

where Cox is oxide capacitance and CS is semiconductor capacitance. Replacing CS in

Eq. (5.4) with Eq. (5.3) eliminates the uncertainty in the calculation of CS. A summary

of the flat band voltage, fixed charge density, slow interface charge density and

interface trap density extracted for the four SiNx/HgCdTe MIS samples is shown in

Table 3.9. The Dit characteristics for the four MIS capacitors as a function of surface

potential at the SiNx/HgCdTe interface are shown in Figure 5.5.

116


Figure 5.5 Comparison of the interface trap densities (Dit) of the SiNx/n-Hg0.68Cd0.32Te MIS structures extracted by the quasi-static method as a function of the energy from mid-gap at 77 K.

Figure 5.5 shows on a logarithmic scale the distribution of the interface trap density

over energy relative to mid-gap energy Ei. The Dit characteristics manifest a typical

U-shaped distribution over the bandgap, with sample D4-100C indicating lower Dit

over most of bandgap range. The interface trap densities were found to increase

strongly toward the band edges. All the four Dit curves were found to have similar

mid-gap values Dit-midgap but differ from each other when the surface potential moves

towards either the valance or conduction band edges. When comparing sample D4-

100C with sample D1-100C, in which all deposition conditions were the same except

for the NH3/SiH4 flow ratio, it is evident that D4-100C with a higher NH3/SiH4 flow

ratio and a more Si-rich film shows better passivation quality. This indicates that the

deposition conditions corresponding to sample D4-100C, which was deposited under

Si-rich conditions, gives the best result and can be employed to passivate HgCdTe

based devices without the need for a CdTe capping layer. The SiNx deposition

conditions for sample D4-100C were found to result in a SiNx/HgCdTe MIS structure

characterised by a negative fixed trap density of -1.2 × 1011 cm-2, a slow interface trap

density of 4.3 × 1010 cm-2, and interface trap density Dit of 4.0 × 1010 eV-1cm-2. These

-0.1 -0.05 0 0.05 0.11010

1011

1012

1013

1014

E-Ei (eV)

Dit

(cm

-2eV

-1)

D1-80CD1-90CD1-100CD4-100C

117


results are a significant improvement on the best reported ECR-PCVD deposited SiNx

films on HgCdTe, which indicated a negative fixed charge density of -1.4 × 1011 cm-2

and an interface trap density Dit of 1 × 1011 eV-1cm-2 [70]. Thus ICPECVD SiNx films

deposited at relatively low temperatures (80 °C - 100 °C) have significant potential as

surface passivation films for HgCdTe-based devices.

5.2.3 Conductance-frequency measurements on MIS structures

The measurements in this section aim to extract Dit by the conductance method, which

was proposed by Nicollian and Goetzberger in 1976 [238]. They observed the

capacitance decreases with increasing frequency, which depends on the relaxation

time of the interface states and frequency of the signal. The conductance method is

based on measuring the equivalent parallel conductance GP of the MIS capacitor as a

function of bias and frequency, f. The change in conductance represents the loss

mechanism due to interface trap capture and emission of carriers, from which Dit can

be extracted [164, 238, 246].

The simplified equivalent circuit used in the conductance method consists of the oxide

capacitance Cox, semiconductor capacitance CS and the interface trap capacitance Cit.

The lossy process of capture-emission of carriers by Dit is represented by Rit, and the

MIS-C has an interface trap time constant τit = RitCit. The Cp and Gp are given by [164,

238, 246]

2)(1 it

itSP

CCCωτ+

+= (5.5)

2)(1 it

ititP DqGωτ

ωτω +

= (5.6)

where itit DqC 2= , and fπω 2= . When assuming a continuous distribution of

interface traps within a few kT/q above and below the Fermi level, Gp/ω is expressed

as

])(1ln[2

2it

it

itP qDG ωτωτω

+= (5.7)

118


Assuming negligible series resistance, the relationship between Gp/ω and measured

capacitance Cm, oxide capacitance Cox and conductance Gm is

222

2

)( moxm

oxmP

CCGCGG−+

=ωω

ω (5.8)

The value of 5.2=itωτ is found by solving 0)(/)/( =∂∂ nPG ωτω [238]. The Gp/ω

curves pass through a maximum at 5.2=itωτ with the values of

max4029.01

≈ω

Pit

Gq

D (5.9)

In order to apply the conductance method to the analysis of the MIS structures,

capacitance- and conductance - frequency measurements were implemented at 77 K

with the frequency ramping from 1 kHz to 1 MHz. The measured capacitance - log ω

characteristics at various biases for the four MIS structures at 77 K are shown in

Figure 5.6. At lower frequency, all the interface traps are able to respond to the

applied signal, so the interface trap capacitance is in parallel with the depletion

capacitance, resulting in a higher value of measured capacitance as shown in

Figure 5.6. As the frequency is increased to an intermediate level, only part of the

interface traps are able to respond to the applied signal, resulting in a decrease of the

measured capacitance. When the sweeping frequency is high enough, none of the

interface traps contribute to the measured capacitance, since the interface trap time

constant is too long to permit charge move in and out of the interface traps in response

to an applied signal [164, 247].

The measured and fitted Gp/ω – log ω characteristics for various biases for the four

MIS structures at 77 K are shown in Figure 5.7. Except for sample D1-90C , the fit to

the Gp/ω - log ω characteristics suggests the presence of a continuous distribution of

interface traps [164].

119


(a) D1-80C (b) D1-90C

(c) D1-100C (d) D4-100C

Figure 5.6 Measured capacitance – log ω characteristics at various biases for the four MIS structures at 77 K.

3.5 4 4.5 5 5.5 6 6.51.5

2

2.5

3

3.5 x 10-8

log ω (Hz)

Cap

acita

nce/

Are

a (F

/cm

2 )

-2.0-1.6-1.2-1.1-0.9

Bias (V)

4 4.5 5 5.5 6 6.51.5

2

2.5

3

3.5x 10-8

log ω (Hz)C

apac

itanc

e/A

rea

(F/c

m2 )

-2.0-1.6-1.2-1.0

Bias (V)

3.5 4 4.5 5 5.5 6 6.51.5

2

2.5

3

3.5x 10-8

log ω (Hz)

Cap

acita

nce/

Are

a (F

/cm

2 )

-2.0-1.6-1.2-1.1

Bias (V)

3.5 4 4.5 5 5.5 6 6.51.5

2

2.5

3

3.5 x 10-8

log ω (Hz)

Cap

acita

nce/

Are

a (F

/cm

2 )

-2.0-1.6-1.2-1.0-0.9-0.8-0.7-0.6-0.5

Bias (V)

120


(a) D1-80C (b) D1-90C

(c) D1-100C (d) D4-100C

Figure 5.7 Measured and fitted Gp/ω versus log ω characteristics at various gate biases for the four MIS structures at 77 K. (a)-(d) have the same scale in the plots. Dots: measured data points, lines: fitted curves.

3.5 4 4.5 5 5.5 6 6.50

1

2

3

4

5

x 10-9

log ω (Hz)

GP/ω

(F/c

m2 )

-2.0-1.6-1.2-1.0-0.9-0.8-0.7-0.6-0.5

Bias

3.5 4 4.5 5 5.5 6 6.50

1

2

3

4

x 10-9

log ω (Hz)

GP/ω

(F/c

m2 )

-2.0-1.6-1.2-1.0

Bias

3.5 4.0 4.5 5.0 5.5 6.0 6.50

0.5

1

1.5

x 10-9

log ω (Hz)G

P/ω

(F/c

m2 )

-2.0-1.6-1.2-1.1

Bias

3.5 4.0 4.5 5.0 5.5 6.0 6.50

0.5

1

1.5

2

2.5

3

3.5 x 10-9

log ω (Hz)

GP/ω

(F/c

m2 )

-2.0-1.6-1.2-1.1-0.9

Bias

121


5.2.4 Interface trap density extracted by conductance method

Based on the conductance method [164, 238, 246], interface trap densities, Dit, for

each of the four MIS structures were extracted, as plotted in Figure 5.8. It can be seen

that Dit decreases with interface energy towards mid-gap. Except for sample D1-90C

that exhibits an anomalous non-continuous distribution of interface trap energy levels

[164], samples D4-100C and D1-80C were found to have lower levels of interface

traps than sample D1-100C. There can be seen differences between the Dit levels

shown in Figures 5.5 and Figures 5.8. The disagreement in the extracted interface trap

densities between the two methods is likely to be due to the influence of leakage

current or series resistance.

The time constant, τit , of the four SiNx/n-Hg0.68Cd0.32Te MIS structures extracted by

the conductance method at 77 K are shown in Figure 5.9. As seen from Figure 5.9, τit

increased with the interface energy from the valance band towards mid-gap.

The majority carrier capture cross section nσ of the interface trap can be calculated

from itτ by the equation [238, 248]

−=

kTq

NVs

Ditthn

φτ

σ exp1 (5.10)

where the thermal velocity thV (cm/s) of electrons and holes can be expressed using

××

××=

∗

∗

2/1

2/1

)/3(

)/3(

h

eth mTk

mTkV (5.11)

where ∗em and ∗

hm are effective mass of electrons and holes, respectively,

corresponding to 101084.7 ×=thV cm/s and 91018.3 × cm/s for electrons and holes at

77 K.

The electron capture cross section, nσ , as a function of energy for the MIS structures

are shown in Figure 5.10. The four curves of nσ were found to decrease with

interface energy towards mid-gap. Further experiments on SiNx/HgCdTe MIS

samples with PECVD SiNx deposited under different conditions combined with

theoretical modelling are required in order to better understand the nature of the

interface defects.

for electrons

for holes

122


Figure 5.8 Comparison of the interface trap densities, Dit, of all the SiNx/n-Hg0.68Cd0.32Te MIS structures extracted by the conductance method as a function of the energy from mid-gap at 77 K.

Figure 5.9 Comparison of the time constant (τit) of all the SiNx/n-Hg0.68Cd0.32Te MIS structures extracted by the conductance method as a function of the energy from mid-gap at 77 K.

-0.1 -0.05 0 0.05 0.1

1010

1011

E-Ei (eV)

Dit

(eV

-1cm

-2)

D1-80CD1-90CD1-100CD4-100C

-0.1 -0.05 0 0.0510-6

10-5

10-4

10-3

10-2

E-Ei (eV)

τ it (s

)

D1-80CD1-90CD1-100CD4-100C

123


Figure 5.10 Electron capture cross section as a function of energy for the SiNx/n-Hg0.68Cd0.32Te MIS structures at 77 K.

5.3 Relationship between SiNx passivation performance and thin film bond concentrations

The IR absorption coefficient curves of the four SiNx /Si reference wafer are shown in

Figure 5.11, with the three inserts zooming in on a specific absorption band. It can be

observed that samples D4-100C and D1-80C with lower levels of Dit, exhibit higher

Si-H (s) peaks at the wavelength of ~ 2200 cm-1 and lower N-H (r) peaks at

~ 1180 cm-1 compared to samples D1-90C and D1-100C.

Bond density calculations were carried out on the SiNx/Si reference wafers, where

more detailed literature is available for silicon substrates, in an attempt to correlate the

bond density with the level of interface traps. The [Si-H] bond density has been

considered as a measure of passivation quality, and good passivation could be

achieved if the SiNx has a higher density of [Si-H] [183]. The sources of hydrogen

incorporation used in the ICPECVD are the SiH4 and NH3 process gases.

A summary on bond and atom concentrations calculated for SiNx/Si reference wafers

of the SiNx/HgCdTe MIS structures were given in Table 3.9, and results from C-V

and IR absorbance analysis on the four SiNx/HgCdTe MIS structures were listed in

Table 3.8. Figure 5.12 plots the relationship between Dit and [Si-H] and [N-H] bond

concentrations, which have been identified as a measure of the quality of interface

-0.14 -0.12 -0.1 -0.08 -0.0610-20

10-18

10-16

10-14

10-12

E-Ei (eV)

σ n (c

m2 )

D1-80CD1-90CD1-100CD4-100C

124


passivation for the SiNx/HgCdTe MIS samples. The variations of [H], [N-H], [Si-H]

and [Si-H]/[N-H] with Dit at mid-gap are shown in Figure 5.13. An approximately

linear relationship is observed between the Dit taken at mid-gap and [Si-H] bond

density of the SiNx film. Sample D4-100C was found to have the highest [Si-H] bond

density among the four wafers, with D1-80C the second, which can be directly

correlated with the observed lower densities of interface traps.

Following the Robertson and Powell model [249], the origin of the interface traps is

assumed to be associated with dangling bonds at the insulator/semiconductor

interface. A higher percentage of [H] bonded with Si can effectively terminate the

dangling bonds, resulting in a decease in Dit. Mäckel and Lüdemann have investigated

the reaction pathway of the passivation of dangling bonds, and shown that hydrogen

plays a fundamental role in the formation of Si-H and =Si-H2 bonds, and found that

the addition of H2 gas to the plasma enhances the passivation of dangling bonds and,

hence, the quality of surface passivation [183].

Generally, within a certain range of [Si-H], Dit is expected to decrease with any

increase in [Si-H] and decrease in [N-H]. This observation could be useful for

optimising the passivation quality of SiNx films. The decrease of Dit with increasing

[Si-H] and the increase of Dit with increasing [N-H] indicates that the formation of

hydrogen bonds at the interface plays an important role in surface passivation [183,

239]. In addition, a correlation has been reported between an increased fixed charge

density Qf and an enhanced [Si-H] bond concentration [250], hence sample D1-80C

and D4-100C with higher level of negative charge can possibly be explained by their

higher level of [Si-H].

Dit at mid-gap has shown to decrease linearly with the increasing [Si-H] bond density

in Figure 5.13, however, Si dangling bonds were reported to produce a near mid-gap

trap state in SiNx with increasing of [Si-H] [249, 251]. Also, SiNx films with non-

detectable Si-H absorption band were found to have improved electrical properties

[172]. Therefore, to some extent, a high [Si-H] could become disadvantageous. More

work needs to be done in investigating the upper limit of [Si-H], where the film

quality and its surface passivation performance starts to decrease or stops increasing

with further increases in [Si-H] bond concentration.

125


Figure 5.11 The IR absorbance spectra of the reference silicon nitride films on Si substrate under four deposition conditions for the MIS structures.

5001000150020002500300035000

0.5

1

1.5

2

2.5 x 104

Wavenumber (cm-1)

Abs

orpt

ion

Coe

ffici

ent (

cm-1

)

D1-80CD1-90CD1-100CD4-100C

Si-N (sym.s)Si-H (w-r)

Si-N (asym. s)


11001150120012500

500

1000

1500

2000

2500

3000

3500

Wavenumber (cm-1)

Abs

orpt

ion

Coe

ffici

ent (

cm-1

)

200021002200230024000

500

1000

1500

2000

Wavenumber (cm-1)

Abs

orpt

ion

Coe

ffici

ent (

cm-1

)

320032503300335034003450400

600

800

1000

1200

1400

1600

1800

Wavenumber (cm-1)

Abs

orpt

ion

Coe

ffici

ent (

cm-1

)

126

Chapter 5 Interface Effects in M

etal/SiNx /H

gCdTe Structures

126


Table 5.4 Summary on bond and atomic concentrations calculated for SiNx/Si reference wafers of the SiNx/HgCdTe MIS structures

Sample NH3/ SiH4

SiH4/ NH3

n632.8nm [N]/[Si] [Si-N] (1022 cm-3)

[N-H] (1022 cm-3)

[Si-H] (1022 cm-3)

[Si-Si] (1022 cm-3)

[Si] (1022 cm-3)

[N] (1022 cm-3)

[H] (1022 cm-3)

Total bond

density (1022 cm-3)

Film density (g/cm3)

D1-80°C 1.49 0.67 1.86 1.17 8.40 3.24 1.69 1.59 3.32 3.88 4.93 14.92 2.53

D1-90°C 1.49 0.67 1.90 1.07 8.95 3.36 0.93 2.73 3.84 4.10 4.29 15.97 2.81

D1-100°C 1.49 0.67 1.93 1.02 9.11 3.32 1.59 2.78 4.06 4.14 4.91 16.80 2.94

D4-100°C 1.29 0.77 1.94 0.90 9.03 2.87 1.75 3.43 4.41 3.97 4.63 17.09 3.06

127

Chapter 5 Interface Effects in M

etal/SiNx /H

gCdTe Structures

127


Table 5.5 Summary of results from C-V and IR absorbance analysis on the four SiNx/HgCdTe MIS structures

D1-80C D1-90C D1-100C D4-100C

[N]/[Si] 1.17 1.07 1.02 0.90

n632.8nm 1.86 1.90 1.93 1.94

Flat band voltage Vfb (V) 0.98 0.86 0.71 0.76

Fixed charge density Qf (×10 10 cm-2) -12.86 -8.53 -9.42 -11.48

Hysteresis width VH (V) 0.19 0.08 0.208 0.207

Slow interface charge density Qit (×10 10 cm-2) 3.17 1.03 3.87 4.33

Interface state density Dit_min (×10 10 eV-1cm-2) 5.07 17.47 11.78 4.01

Interface state density Dit (×10 10 eV-1cm-2) at mid-gap 13.65 50.38 17.56 10.57

[H] (1022 cm -3) 0.41 0.35 0.37 0.36

[Si-H] (1022 cm -3) 1.69 0.93 1.59 1.75

[N-H] (1022 cm -3) 3.24 3.36 3.32 2.87

[Si-H]/[N-H] 0.52 0.28 0.48 0.61

Figure 5.12 Relationship between Dit and [Si-H] and [N-H] bond concentrations.

0 10 20 30 40 50 600.5

1

1.5

2

2.5

3

3.5

Dit ( x 1010 eV-1cm-2)

Bon

d de

nsity

( x

1022

cm

-3)

Dit minimum vs [N-H]

Dit at mid-gap vs [N-H]

Dit minimum vs [Si-H]

Dit at mid-gap vs [Si-H]

128


(a)

(b)

Figure 5.13 Dit at mid-gapwith the variations of (a) [H], [N-H], [Si-H] (a) and (b) [Si-H]/[N-H].

0 1 2 3 4 50

10

20

30

40

50

60

Bond density ( x 1022 cm-3)

Dit-

mid

-gap

( x

1010

cm

-2eV

- 1)

[H][N-H][Si-H]

0.2 0.3 0.4 0.5 0.60

10

20

30

40

50

60

Dit-

mid

-gap

( x

1010

cm

-2eV

-1)

[Si-H]/[N-H] ratio

129


5.4 Summary

In this chapter, work is reported on SiNx thin films for surface passivation of HgCdTe

epitaxial layers without the need for a CdTe capping layer, with SiNx/HgCdTe MIS

structures being utilised as a tool during the study.

The interface trap density, Dit, was extracted and examined by analysing high-

frequency and low-frequency C-V data, as well as by the conductance method. Dit

was considered as the dominant measure in evaluating surface passivation

performance and in correlating passivation quality with other film properties.

Analysis of the SiNx/n-Hg0.68Cd0.32Te MIS structures indicated that Si-rich SiNx films

deposited at 100 °C exhibit electrical characteristics suitable for surface passivation of

HgCdTe-based devices, with interface trap densities in the range of mid-1010 cm-2eV-1,

and fixed negative interface charge densities of ~ 1011 cm-2.

The relationship between different bond concentrations in the SiNx film and surface

passivation performance was also presented using the method presented in

Section 3.4. The Si-H and N-H bond concentrations were found to be directly

correlated with passivation performance, such that SiNx films with a combination of

high [Si-H] and low [N-H] bond concentrations found to be suitable as electrical

passivation layers on HgCdTe.

130

Chapter 6 Conclusions and Future Work

6 Conclusions and Future Work

6.1 Summary and Conclusions

The principal objectives of this thesis have been to study interface effects in HgCdTe

materials and devices. In the process of achieving the objectives outlined in Chapter

1, the main results achieved and conclusions drawn are as follows.

A series of in-situ monitoring characterisations and ex-situ characterisations on

HgCdTe and its passivation materials have been conducted in order to examine the

properties of HgCdTe, its passivants of CdTe and SiNx, and their interfaces of

CdTe/HgCdTe and SiNx/HgCdTe. In particular, some of the characterisations were

able to be conducted and analysed either before or after passivation, such as RHEED,

HR-MSA and XRD, to investigate the influence of passivation.

The effectiveness of MBE low-temperature grown CdTe passivating film was studied

using photoconductive devices. As expected, the HgCdTe photoconductors with their

surface all surfaces passivated with CdTe show significantly higher photoresponsivity

than those without sidewall passivation, which indicates the effectiveness of low-

temperature MBE grown CdTe as a passivation layer in reducing surface

recombination velocity. Characterisation of the responsivity differences between the

photoconductors with and without the sidewall CdTe passivation offers a potential

method to measure the interface/surface recombination velocity. This has been

demonstrated by extracting the value of surface recombination velocity using a

commercial device modelling package (Synopsis Sentaurus) to fit responsivity data

for all surfaces passivated and partially passivated devices.

Low-temperature MBE grown CdTe passivated HgCdTe gated photodiodes were

employed as a tool to investigate device performance, with the band bending at the

surface being influenced by gate bias. By increasing the gate bias from 0 to 1.5 V, the

RdA improved by an order of magnitude, whereas RdA was reduced by an order of

magnitude when decreasing the gate bias to -1.5 V. As the gate bias was increased

from -1.5 V to 1.5 V, the p-type surface depletion region widened within this gate bias

regime and TAT current was reduced, resulting in a monotonic increase in RdA. This

131


was due to the field induced junction formed under the gate, which widened the

depletion area and increased the barrier to tunnelling at the surface.

In this thesis work has been carried out to investigate SiNx thin films for surface

passivation of HgCdTe epitaxial layers without the need for a CdTe capping layer.

Low-temperature (80 °C - 130 °C) deposited SiNx films in this thesis were deposited

employing the Sentech SI500D ICPECVD system with a high-density and low ion

energy plasma source. The low ion energy of the plasma source enables the SiNx film

to be deposited on the HgCdTe without significant surface damage.

In order to investigate suitable SiNx film deposition conditions for HgCdTe surface

passivation, the influence of ICP power on the quality of the deposited SiNx films was

assessed through the IR absorbance of the films. The absorbance spectrum of each

film was measured on the day of the deposition and was regularly monitored over a

six month period, with the films exposed to a standard laboratory atmosphere. The

results indicate that the SiNx/CdTe/GaAs sample C5-SiNx deposited using a relatively

high ICP power of 600 W appeared to be porous and more susceptible to oxidation

under conventional ambient conditions, with the presence of the Si-O stretching peak

appearing near 1080 cm-1. It is noted that deposition conditions C2, C3, C4, D1 and

D4 have demonstrated excellent long-term stability in terms of the IR absorbance

peaks associated with exposure to O2 and H2O in a laboratory atmosphere, with no

evidence of the Si-O oxidation peak in the IR spectra.

The influence of NH3/SiH4 flow ratio on SiNx film properties, such as refractive

index, film composition, deposition rate, IR absorbance and bonding configuration,

were investigated on Si substrates before being applied on HgCdTe substrates. A

series of films were deposited with a fixed SiH4 flow rate of 6.9 sccm and various

NH3 flow rates of 12.4, 10.3, 8.2 and 6.1 sccm at low substrate temperatures (80 °C-

100 °C). Within the investigated range of NH3/SiH4 flow ratio from 0.88 to 1.80, the

[N]/[Si] ratio was found to decrease and n632.8nm to increase with the NH3/SiH4 flow

ratio. The dependence of the SiNx film composition on the NH3/SiH4 flow ratio is as

expected, and can be readily explained by the kinetics of dissociation processes and

the free radical sticking coefficient values. The film density ρ was seen to increase

with elevated deposition temperature, and the defect density is expected to decrease

with increasing film density. The trends of deposition rate and film density with

NH3/SiH4 flow ratio were found to be non-linear. The general trends in bonding

132


configuration are that [N-H], [N] and [H] all increase with NH3/SiH4 flow ratio

whereas [Si-H] and [Si] decrease with NH3/SiH4 flow ratio.

Film composition, deposition rate and IR absorbance can be correlated with the

polyamine concentration in the plasma. For a NH3/SiH4 flow ratio in the range of 0.88

to 1.19, a dramatic increase in deposition rate and [N]/[Si] ratio was observed,

suggesting a change in the dominant radical from disilane to polyaminosilanes. In

terms of film IR absorbance, when the NH3/SiH4 flow ratio increased from 0.88 to

1.19, a significant drop in the area of the Si-H (stretching) peak (~ 2180 cm-1) was

observed, also suggesting a change in the dominant radicals from disilane to

polyaminosilanes. Films with a high NH3/SiH4 flow ratio of 1.80 contained little or no

Si-H bonding and enhanced N-H bonding in the IR absorption spectra, suggesting that

Si[NH2]3 was the principal film precursor, with suppressed Si2H6 in the plasma during

film deposition.

SiNx/n-Hg0.68Cd0.32Te MIS structures were utilised as a tool to study the interface

between SiNx and HgCdTe, and the interface trap density Dit was considered as the

primary parameter to evaluate surface passivation performance and in correlating

passivation quality with other film properties. SiNx films were deposited on HgCdTe

wafers under different conditions, and characterisation of the SiNx/n-Hg0.68Cd0.32Te

MIS structures was carried out to extract Dit by analysing high-frequency and low-

frequency C-V data and by the conductance method. The observed Dit characteristics

manifest a U-shaped distribution over the HgCdTe bandgap, with sample D4-100C

indicating the lowest Dit over most of the bandgap range. This indicates that the

deposition conditions corresponding to sample D4-100C, which was deposited under

Si-rich conditions with a lower NH3/SiH4 flow ratio, provides the best results and can

be employed to passivate HgCdTe based devices without the need for a CdTe capping

layer. The SiNx deposition conditions for sample D4-100C were found to result in a

SiNx/HgCdTe MIS structure characterised by a negative fixed charge density of -1.2 ×

1011 cm-2, a slow interface trap density of 1.6 × 1011 cm-2, and a minimum fast

interface trap density Dit of 4 × 1010 cm-2eV-1. These results represent a significant

improvement on the best reported ECR-PCVD deposited SiNx films on HgCdTe,

which indicated a negative fixed charge density of -1.4 × 1011 cm-2 and an interface

trap density Dit of 1 × 1011 cm-2eV-1. Thus, the ICPECVD SiNx films deposited at

133


relatively low temperatures (80 °C - 100 °C) in the thesis have been shown to have

significant potential as surface passivation films for HgCdTe-based devices.

The correlation between bond concentration and surface passivation performance has

been studied. [Si-H], [N-H] and [Si-H]/[N-H] were identified as potential measures of

surface passivation performance at the SiNx/HgCdTe interface. An approximately

linear relationship is observed between the Dit taken at mid-gap and [Si-H] bond

density of the SiNx film. SiNx films with high [Si-H] and low [N-H] bond

concentrations have been identified as suitable electrical passivation layers. This

could be a useful criteria for optimising the passivation quality of SiNx films for

HgCdTe-based devices. The decrease of Dit with increasing [Si-H] and the increase of

Dit with increasing [N-H] indicates that the formation of hydrogen bonds at the

interface plays an important role in surface passivation.

6.2 Recommendations for future work

In relation to the issues with HgCdTe passivation and the study of interface effects in

this thesis, the following suggestions are noted as requiring further investigation:

1. The nearly lattice-matched CdTe surface passivation for HgCdTe and the grading

at the CdTe/HgCdTe interface can be beneficial for device performance;

however, the additional layer of CdTe may hinder the diffusion of hydrogen

during the PECVD SiNx deposition in a dual-layer passivation process. Thus,

additional work needs to be undertaken on a comparison of the passivation

performance of dual-layer passivation of SiNx/CdTe/HgCdTe with single-layer

passivation of SiNx/HgCdTe and CdTe/HgCdTe.

The HR-MSA technique has shown a decrease of bulk electron mobility after

MBE CdTe passivation deposited at ~ 100 °C compared to prior to

passivation, suggesting that the low-temperature MBE CdTe growth and post

in-situ annealing conditions can be improved.

More work needs to be carried out on passivating nBn detectors and other

photodetector devices to compare the passivation performance of low-

temperature MBE grown CdTe ( ~ 100 °C) and low-temperature SiNx

( < 100 °C).

134


2. In addition to the bond density analysis from IR absorbance, it would be

worthwhile to correlate the SiNx passivation quality with the analysis via the

following:

transport measurements - in particular, the HR-MSA can provide information

not only on the surface passivation but also on bulk passivation, which could

be a powerful tool in examining hydrogenation related processes;

Photoluminescence measurements;

transient photoconductive decay measurements;

surface recombination velocity - the effective surface recombination velocity

was found to depend strongly on Dit, irrespective of the varied process

parameters, and it depends primarily on Dit rather than the insulator

charge [240];

nano-indentation - the SiNx film densities have been calculated by IR

absorbance analysis in the thesis, which need to be correlated with the results

from nano-indentation;

XPS measurements

3. The mechanisms of SiNx hydrogenated from PECVD have been extensively

studied on silicon substrates, yet there are issues that still remain unclear. There

has been little published work on the mechanisms of hydrogenated SiNx

deposited on HgCdTe and their interface effects. This area is worthy of further

investigation.

4. As to the passivation performance and its correlation with interface traps, more

work could be carried out on the aspects listed below, based on the results from

the thesis:

Dit at midgap has been observed to decrease approximately linearly with

increasing [Si-H] bond density, however, Si dangling bonds were reported to

produce a near midgap trap states in SiNx with increasing [Si-H] [249, 251].

Also, SiNx films with non-detectable Si-H absorption band were found to

have improved electrical properties [172]. Therefore, to some extend, a high

[Si-H] may eventually become a disadvantage. More work could be done in

investigating the upper limit of [Si-H], where the quality of the thin film and

135


its surface passivation performance start to decrease or stop increasing with

an increase of the [Si-H] bond concentration.

With a series of SiNx films with different [N]/[Si] ratio deposited on HgCdTe

wafers, work could be carried out to reveal whether there is any correlation

between film composition and passivation performance, since the literature

has indicated different views on this issue for silicon substrates, and no

relevant work has been published on HgCdTe substrates as yet.

It is possible to attribute the dominant type of interface traps for SiNx

samples deposited at different substrate temperatures by their capture cross

section characteristics, by using small-pulse DLTS [252]. Further

measurements and analysis on SiNx/HgCdTe MIS structures with PECVD

SiNx deposited under various conditions, combined with theoretical

modelling, would be useful in order to explore the nature of the interface

traps at the interface.

5. In addition to the conductance method, quasi-static C-V under triangular voltage

sweep with varied sweep rates could be useful in studying the interface traps.

Under varied sweep rates, interface states will show different characteristics when

being filled and emptied. Modelling of quasi-static C-V on the MIS capacitors

can be performed by using the simulation tool of Synopsis Sentaurus.

6. The deposition parameters for SiNx passivation on HgCdTe could be optimised

through a statistical experimental design [199] and/or a central composition

experiments (CCE) [240]. With the aid of genetic algorithm (GA) optimised

generalized regression neural network (GRNN) [199], the effect of deposition

parameters on the quality of surface and/or bulk HgCdTe passivation could be

characterized systematically, and hence be optimised.

Defect passivation of dangling bonds can be enhanced by the addition of a

further hydrogen source to the plasma gas [183]. Additional H2 could be

added to increase the hydrogen content in the SiNx film. The composition of

the film would be affected by the additional H2, as well as n632.8nm and

[N]/[Si].

As to the modulation of fixed charge in the passivant, gamma irradiation on

the SiNx film has been found to have an impact on the fixed charge

density [253], which could be a possible method in manipulating the fixed

136


charge density in the film and, hence, improving passivation quality. Work

on dual-layer passivation of SiNx/SiO2/HgCdTe has reported that different

signs and values of the fixed charge are possible [254], which could be

another possible direction.

The effect of thickness of SiNx passivant could also be examined.

7. The work presented in thesis has focused on SiNx surface passivation for MWIR

n-HgCdTe, and more work could be carried out on CdTe/p-HgCdTe, SiNx/p-

HgCdTe, and on HgCdTe layers with different mole fractions.

137

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[268] J. Schmidt and M. Kerr, "Highest-quality surface passivation of low-resistivity

p-type silicon using stoichiometric PECVD silicon nitride," Solar Energy

Materials and Solar Cells, vol. 65, pp. 585-591, 2001.

[269] A. W. Weeber, H. C. Rieffe, M. Goris, J. Hong, W. M. M. Kessels, M. C. M.

van de Sanden, and W. J. Soppe, "Improved thermally stable surface and bulk

passivation of PECVD SiN/sub x: H using N/sub 2/and SiH/sub 4," in

Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference

on, 2003, pp. 1131-1134.

163

Appendix A: HgCdTe Properties


A.1 Bandgap

CdTe is a semiconductor with a relatively wide energy bandgap of 1490 meV at room

temperature, and HgTe is a semimetal with an inverted bandgap of -144 meV.

Hg1−xCdxTe, as a compound semiconductor, has an energy bandgap, Eg, that lies

between these two extremes of bandgap. The conduction band minimum and valence

band maximum are located at the Г-point of the Brillouin zone [255], making

HgCdTe a direct bandgap semiconductor, as shown in Figure A1.1. This inherent

property leads to a large photon absorption co-efficient and a high quantum efficiency

of HgCdTe.

One of the most widely used bandgap energy, Eg (eV), expressions for HgCdTe is a

function of the mole fraction, x, and temperature T (K), derived empirically by

Hansen et al., which is expressed as [256]

( ) 324 832081021103559313020 x. x. x) - T( . x . . x,TEg +−×++−= − (A1.1

)

where Eq.(A1.1) is plotted in Figure A1.2. It can be seen that at 77K Eg varies from

approximately -0.3 eV to 1.6 eV when adjusting x from 0 to 1. The relationship

between Eg and the cut-off wavelength of the material, λc, is given by:

g g c E

.Ehc λ 2451

== (A1.2)

Figure A1.3 shows the relationship between HgCdTe cut-off wavelength and x using

Eq.(A1.1 ) and Eq.(A1.2), which illustrates one of the advantages of HgCdTe - its cut-

off wavelength being adjustable by varying x from the region of shortwave infrared

(SWIR) (0.75 to 3 μm) to mid-wave infrared (MWIR) (3 to 5 μm) and long-wave

infrared (LWIR) (8 to 14 μm).

164


Figure A1.1 Schematics of energy bandgap of (a) HgTe. (b) HgTe-CdTe transition (zero bandgap). (c) CdTe. The Г6 and Г8 point refer to the electron band and light/heavy hole band, respectively.

Figure A1.2 Bandgap of Hg1−xCdxTe as a function of cadmium composition, x.

0 0.2 0.4 0.6 0.8 1-0.5

0

0.5

1

1.5

2

Cadmium composition (x)

Ener

gy g

ap (e

V)

77 K200K300K

EnergyΓ8

e

Γ6 lh

Γ8 hh

EnergyΓ6

e

Γ8 hh

Γ8 lh

Γ8 hh

Γ6 e

Γ8 lh

Energy

(a) (b) (c)

165


Figure A1.3 Cut-off wavelength of Hg1−xCdxTe as a function of cadmium composition, x.

A.2 Lattice constant The lattice mismatch between CdTe and HgTe is very small, and the lattice constant

across the entire composition range changes by only 0.3%, making multilayer

crystalline growth possible. The relationship between lattice constant of Hg1−xCdxTe,

a (Å), and mole fraction, x, given by Higgins et al., is expressed as [257]

x - x x a 32 0057.00168.00084.04614.6 ++= (A1.3)

which is plotted in Figure A1.4.

Figure A1.4 Lattice constant of Hg1−xCdxTe as a function of cadmium composition, x.

0.2 0.3 0.4 0.5 0.6 0.7 0.80

2

4

6

8

10

12

14

16


Cut

-off

wav

elen

gth

(µm

) 77 K200K300K

LWIR

MWIR

0 0.2 0.4 0.6 0.8 16.44

6.45

6.46

6.47

6.48

6.49

6.5


Latti

ce c

onst

ant (

A)

o

166


A.3 Intrinsic carrier concentration The most widely used expression for intrinsic carrier concentration, ni, of HgCdTe is

an empirical expression given by Hansen and Schmit as [258]:

( ) ( ) )2

exp10001364.0001753.082.3585.5 234314

kTE

(TExTTx x, Tn g//gi −××××−+−=

(A1.4)

which is valid for 0.16 < x < 0.7 and 50 K < T < 359 K. k is the Boltzmann constant

(eV K−1). The change of ni as a function of x for varying temperatures is illustrated in

Figure 3.5. At a given temperature, ni decreases with increasing x (that is, with

increasing bandgap).

Figure A1.5 Intrinsic carrier concentration of Hg1-xCdxTe as a function of x for T = 77 K, 150 K, 200 K and 300 K.

A.4 Mobility The mobility of electrons and holes in HgCdTe vary with mole fraction and

temperature. The empirical expression for the electron mobility in HgCdTe with

0.2 < x < 0.6 is given by Rosbeck et al. as [259]:

ae Zb

2

8109×=µ (A1.5a)

where 6.02.0

=

xa (A1.5b)

0.1 0.2 0.3 0.4 0.5 0.61010

1012

1014

1016

1018


Intri

nsic

car

rier c

once

ntra

tion

(cm

-3)

77 K 150 K200 K

300 K

167


75.02.0

=

xb (A1.5c)

and

>

≤−−

×=

KTT

KTTZ

50

50352600

1018.107.2

5

(A1.5d)

The electron mobility as a function of temperature (T > 50 K) for varying x is plotted

in Figure A1.6 using Eq.(A1.5).

Figure A1.6 Electron mobility in Hg1-xCdxTe as a function of temperature for varying mole fraction, x.

The mobility of holes in HgCdTe is approximately two orders of magnitude lower

than that of the electrons, due to the material’s small electron effective mass, being

expressed as:

eh µµ 01.0= (A1.6)

A more detailed description of the hole mobility is presented in the work by Yadava

et al. [260].

50 100 150 200 250 300103

104

105

106

Temperature (T)

Ele

ctro

n m

obilit

y (c

m-2

V-1

s-1

)

x = 0.20

0.25

0.300.350.40

168

Appendix B: Deposition Parameters Concerning High-temperature Deposited SiNx Films

Appendix B: Deposition Parameters Concerning High-

temperature Deposited SiNx Films

The properties of SiNx and its passivation quality can be affected by the gas reactants

used [261, 262], the mode of PECVD reactor [84, 167, 263], and specific details of

the deposition conditions [174, 175, 179].

There are several chemistries available for plasma deposition of SiNx, the gas

reactants utilised in deposition have a significant effect on the quality of surface

passivation and film properties [261, 262]. For plasma enhanced deposited silicon

nitride, films are typically deposited using SiH4 and other reactant gases, such as NH3

and/or N2. Different plasma chemistries result in different radicals responsible for the

film deposition and hence film properties [172, 212, 264]. The hydrogen content

incorporated in silicon nitride films decreases significantly if N2 is employed rather

than NH3 [262, 265].

Secondly, the quality of surface passivation and film properties were found to be

strongly affected by the mode of PECVD [84, 167, 263]. SiNx films grown using

direct high-frequency or remote PECVD have lower surface recombination velocity

than those using low-frequency direct PECVD. The low-frequency systems (in the 10

- 500 kHz range) tend to produce comparatively poorer and unstable passivation [266].

The SiNx films discussed in this chapter were deposited in a high-frequency

(13.56 MHz) direct ICPECVD system.

In addition, deposition conditions have an influence over film properties and the

quality of surface passivation; hence deposition conditions need to be tuned to achieve

specific film characteristics such as film composition, film refractive index and

hydrogen concentration.

Deposition temperature is a crucial parameter, and may be independent of other

factors [190, 263, 266, 267]. For SiNx films deposited on Si, Lauinger reported a

substrate temperature of around 350 °C as the optimum surface passivation, which is

independent of the deposition technique. Cuevas et al. reported the deposition

temperature to be the dominant parameter in achieving low surface recombination

velocity for silicon-based solar cells, with SiNx films deposited at around 400 °C

169


providing a high quality surface passivation [263]. The difference in the reported

optimum temperature is possibly due to the difference between the hot-plate

temperature and wafer temperature. For example, a hot-plate temperature of 400 °C

leads to a wafer temperature of around 350 °C if the wafer is only bottom

heated [190].

The ratio of NH3/SiH4 gas flow rate has been found to be another crucial parameter in

determining film properties and passivation performance. Varying the NH3/SiH4 flow

ratio will result in changes in film stoichiometry and passivation performance. In

general, Si-rich SiNx films can be obtained by reducing the NH3/SiH4 flow ratio,

whereas N-rich films will result from increasing the ratio [183]. The influence of SiNx

stoichiometry on the interface quality with the semiconductor has been the subject of

extensive studies, although different trends can be seen in the literature when using

different modes of PECVD, gas reactants, deposition temperatures, surface treatment

and post-annealing steps [174, 175, 179, 180, 189, 190].

Generally speaking, in terms of effective surface recombination velocities at the

SiNx/Si interface, as-deposited Si-rich SiNx films provide a superior surface passivant

in comparison to as-deposited N-rich films if there is no high temperature processing

involved, whereas N-rich films tend to give the best properties after high temperature

annealing step(s) resulting in denser and thermally more stable films than Si-rich SiNx

films [175, 207]. After the high-temperature treatment, N-rich hydrogenated films are

considered to be denser and thermally more stable than the Si-rich ones.

In the literature, Si-rich SiNx with n > 2.3 was reported by Lauinger et al. [167, 239,

266] to have the best surface passivation using either a remote or high-frequency

(13.56 MHz) PECVD system. The gas reactants used in the PECVD were pure SiH4

and NH3 at a temperature of around 375 °C. Soppe et al. obtained their optimum

surface passivation with Si-rich films with refractive indices between 2.3 to 2.4

deposited at temperatures between 350 °C and 400 °C, using gas ratios of NH3/SiH4 ≈

1.1 and N2/SiH4 ≈ 1.3, respectively. Silicon nitride deposited with NH3 and SiH4 gas

reactants with a refractive index of about 2.1 showed further improvement after

thermal annealing, while films with refractive index of 2.3 appear to be very stable

under thermal annealing [93]. The silicon-rich SiNx films (n > 3) reported by Mäckel

170


and Lüdemann show higher [Si-H] bond density, and hence better passivation of the

dangling bonds at the silicon surface [183].

Stoichiometric Si3N4 films (n ≈ 1.95) were reported by Schmidt and Kerr to give the

best surface passivation for silicon using 4.5 % SiH4 in N2 and NH3 in parallel-plate

reactor (Oxford Plasma Technology, Plasmalab 80+) [268]. Sanjoh et al. [174] also

found near stoichiometric films to give the best surface passivation for silicon. The

films were deposited at 300 °C, using a conventional, capacitively-coupled plasma

CVD system with a 13.56 MHz RF generator using SiH4+N2+NH3 mixture. A rapid

drop in Dit with increasing x up to 1.33 (stoichiometric film) was observed. For the

case of N-rich films (x > 1.33), they observed a slight increase in the minimum Dit.

The decrease in excess silicon or nitrogen atoms from a non-stoichiometric SiNx film

was found to be effective for the reduction of Dit. In addition, there are other

publications from different research groups that have shown that SiNx films with

n633nm between 1.95 (stoichiometric film) and 2.4 (Si-rich film) deposited at around

400 °C can give high quality surface passivation for solar cells [93, 167, 190, 269].

Ghosh et al. observed that N-rich nitride films (x = 1.63) gave the best surface

passivation with the lowest Dit [179]. The defect density caused by silicon dangling

bonds was noted to increase with increasing silicon content in the films. As nitrogen

atoms can terminate silicon dangling bonds, the minimum Dit was found to decrease

when the nitrogen content increased, and increased with increasing silicon content.

Basa et al. reported that the minimum interface state density decreased with an

increase in [N]/[Si] by analysing C-V characteristics. The films were deposited at

250 °C using SiH4+N2+NH3 mixture in a capacitively coupled parallel-plate

commercial PECVD system [182].

171

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