EBB 424ELecture 3– LED 2Dr Zainovia Lockman

LIGHT EMITTING DIODE – Materials Issues and Selection

At the end of this lecture you would be able to…

• Cite semiconductor materials suitable for LED of different colours (red, yellow, green, blue, white)

• Describe the GaAsP system as an example of ternary compounds

• Use the knowledge of band gap engineering to design LED material to emit suitable coloured lights

• Discuss the current phenomenon in LED research activities

GaAs(1+x) Px

Ga

PAs

What is GaAs(1+x) Px?

• GaAs(1+x) Px is a ternary compound based on GaAs and GaP

• GaAs is a direct gap semiconductor and GaP is indirect semiconductor

• When alloyed, there is cross over point where GaAs(1+x) Px will transformed from being direct gap material to indirect gap material

• Red, yellow and orange coloured LED can be made with GaAs(1+x) Px

Band DiagramDirect band gap 100%GaAs

Indirect band gap 100% GaP

Red photon

Green photon

Indirect to Direct transition 50% GaP

Doped with nitrogen efficiency increases

Composition of GaP %

GaN+GaP = GaAs (1+x)Px

Spectral response of human eye

eV

GaP = 2.26eV GaAs = 1.42eV

Indirect ----------- > DirectGaP= indirect but when alloyed with

GaAs, the band gap will become direct at x

= 0.45

At the transition, the band gap correspond to

from near IR to the orange-red part of the

vis-spectrum

1.997eV

GaAs (1+x) Px

GaAs(1+x) Px system

x = 0.45 indirect to direct transition

GaAs(1+x) Px system doped with N

• Indirect no radiative transition

• Indirect GaAs(1+x) Px can have radiative transition.

• HOW?• By adding nitrogen to the system

• When N added to GaAs(1+x) Px:– The quantum efficiency increases ~ 100x– The emission wavelength increases

• Quantum efficiencies = rate of emission of photons

Rate of electron supply

How efficient the e-h pair can recombine

Isoelectronic Doping and Heisenberg Uncertainty Principles (N +GaAsP)

N has the same valancy as that of P and As N can enter the As or P site in the GaAsP crystal structure. N and P has similar number of valance electrons but different core shell

structure N produces a perturbance in the electronic confinement Electronics confinement changes and acts as a ‘trap’ Electron trapped at a level just below a conduction band. Hole can be captured to produce electron-hole pair (exciton) The carriers are localised, the momentum and the wavenumber are diffuse

due to Heisenberg uncertainties principle

Typical Exam Question!!!!

The figure below shows the quantum efficiencies of GaAsP based LED as a function of alloy composition with and without nitrogen doping. Explain why the additional of nitrogen leads to such dramatic changes in the quantum efficiencies of the device. Why is this phenomenon important from a practical point of view? (100 marks +5 bonus)

The figure: Quantum efficiencies

N substitution to GaAsP

e

e

No N

N produces perturbances

e falls inside the ‘trap’ producing excitons

VB

CB

VB

CBN doping can dramatically increases the

radiative efficiency of GaP (indirect), the doping changes the

emission wavelength to

longer wavelength because the energy of the transition is

now reduced to Eg-Ed

ED

VB

CB

Heisenberg Uncertainty Principle – the uncertainties of the doped electrons position and momentum

K

h

Px = hP = hk/2P = momentumPx = hx = 2/kSet x = 2/a (a= lattice parameter)

The position of electron is uncertain, when electron is at k=0 then recombination occurs, if not then no recombination. The position and momentum of a particle cannot be simultaneously measured with arbitrarily high precision.

E

Question 2.

• GaP and GaAs can be mixed to produce a direct gap semiconductor that produce red-light, explain this statement.

Question 3. List down al of the possible application of IR LED

Band Gap Engineering

A process of varying the elemental components of the semiconductor alloy in a controlled way to achieve a desired band gap that can emit a desired wavelength of radiation.

2 critical considerations

1. The wavelength of the radiation emitted

2. The lattice parameters of the compounds

The wavelength visible, UV or IR

The lattice parameter for epitaxial growth

Why?

How?A good device requires a defect free semiconductor

films. Defect free good crystallographic orientation of

the grains of the semiconductor materials, low defect

To achieve defect free semiconductor thin film, adopt a so-call epitaxial growth of the film on a substrate growth process where the deposited

films will ‘follow’ the surface structure of a substrate.

Substrate

Thin film

Epitaxial growthP-dopant

Semiconductor materials need to be

deposited onto a textured substrate

(thin film technology)

P-n junction

Substrate must have similar lattice parameter to that of

the semiconductor thin film to avoid lattice mismatch (strain

at the interface will induce crack) and to allow epitaxial

growth

The semiconductor then need to be

doped to achieve both p and n type

require p-n junction

Substrate

np

IR & Red LED

GaAs direct band gap, p-n junctions are readily formed with high radiative efficiency. High radiative efficiency can be induced by doping GaAs with Zn or Si. Si doped GaAs is now the industry standard near IR LEDs.

GaAsP direct – in direct transition

GaInAsP Grown on InP substrate and band gap can be varied to get wavelength from 919nm to

1600nm. A true story of band gap engineering.

Band Gap and Lattice Constant

Substrates must have similar lattice parameter to the

semiconductor films, GaAs, GaN and InP are often used as

substrates.

The band gap energy can be tailored to get

desired visible light radiation

LED + band gap engineering

“LEDs are specialized semiconductor devices that can potentially convert electricity to light, without the wasteful creation of heat. The color emitted is controlled in large part by the energy gap of the semiconductor and in advanced structures by the “photonic band gap,” a range of wavelengths that cannot travel through that particular substance. By suppressing certain wavelengths and enhancing others, the band gap determines the color.”

One of the pioneers in the field of LED; Fred Schubert

Examples of Substrate/semiconductor p-n diode/visible light produces

GaAsP / GaAs 655nm / Red

GaP 568nm / Yellow Green

GaP 700nm / Bright Red

GaAsP / Gap 610nm / Amber

GaP 555nm / Pure Green

GaAsP / GaP 655nm / Hi-Eff.Red

GaP 568nm / Yellow Green 

GaA1As / GaAs 660nm / Red

InGaA1P 574nm / Ultra Green

InGaA1P 574nm/Ultra Green

InGaA1P 620nm / Ultra Orange  

InGaA1P 595nm / Ultra Yellow

Cross section of a typical epitaxial layers

Calculation. InGaAs on InP substrate (Kasap)

The nitrides and blue LED

• Difficulties:– to find suitable substrates for the nitrides. – to get p-type nitrides

• But with constant R&D works, better materials are produced

• GaN, InGaN, AlGaN high efficiency LEDs emitting blue/green part of the spectrum.

• First blue LED 1994 Shuji & Nakamura (10 000 hours lifetime)

• SiC can also be used as blue LED- SiC on GaN substrate

The device

Applications:

Flat panel displays (display requires, R,G,B now B is found, all LED displays can be made.

High resolution printers

Light source for communications

Microwave transistors (electrons have high mobility)

UV-LED

Apart blue LED, UV LED can also be made using nitrides.

UV-LED can be used as UV calibration devices, UV detector etc.

The Blue-Violet LED + Phosphor and White LED

White LEDs are slightly more efficient than a 100W incandescent bulb and three times more efficient than a 7W night light type bulb. The lifetime of white LED could reach >10 000 hours while incandescent filament (100watt) normally reaches about 750-1500 hours.

Phosphor

Another typical exam questionDraw a table to list down some examples of possible materials for visible LEDs. In your table state also the visible wavelength your LED will emit as well as some applications of a given visible LED. Explain why group III-V materials have been selected as an LED emitter for use in an optical fiber network.

(100 marks)

The Selenide

• Group II-V is also important (ZnSe especially even though ZnO has being a contender as well)

• ZnSe can be made into LED, emitting blue and green lights.

• Problem with finding suitable template (substrate) for growth.

• GaAs and GaN can be used as the substrate for selenide. The lattice parameter for GaAs = 5.6Å and ZnSe = 5.5Å

• ZnSe has been used as blue/green laser (study later). • The selenide degrade more rapidly hence shorter

working life-time

ZnSe can be made ternary allow with ZnTe to produce ZnSeTe blue-

green

The selenides - E gap vs lattice parameter

Homework question

A diagram given to you shows the energy gap versus lattice constant of some group III-V semiconductors. Explain the importance of band gap engineering in designing an LED and expand your answer to include some examples of materials used in an IR-LED.