EBB 424E

1
LIGHT EMITTING DIODE Materials Issues and
Selection

• EBB 424E
• Lecture 3 LED 2
• Dr Zainovia Lockman

2
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

3
Ga
P
As
GaAs(1x) Px
4
What is GaAs(1x) Px?

• GaAs(1x) 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(1x) Px will transformed from being direct
  gap material to indirect gap material
• Red, yellow and orange coloured LED can be made
  with GaAs(1x) Px

5
Band Diagram
Direct band gap 100GaAs
Indirect band gap 100 GaP
Indirect to Direct transition 50 GaP
Composition of GaP
Red photon
Green photon
Doped with nitrogen efficiency increases
6
GaNGaP GaAs (1x)Px
At the transition, the band gap correspond to ?
from near IR to the orange-red part of the
vis-spectrum
Spectral response of human eye
eV
?
GaP 2.26eV
GaAs 1.42eV
1.997eV
Indirect ----------- gt Direct
GaP indirect but when alloyed with GaAs, the
band gap will become direct at x 0.45
GaAs (1x) Px
7
GaAs(1x) Px system

• x 0.45 ? indirect to direct transition

8
GaAs(1x) Px system doped with N

• Indirect ? no radiative transition
• Indirect GaAs(1x) Px can have radiative
  transition.
• HOW?
• By adding nitrogen to the system
• When N added to GaAs(1x) 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
9
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

10
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)
11
The figure Quantum efficiencies
12
N substitution to GaAsP
CB
No N
VB
e
N produces perturbances
CB
N 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
VB
e falls inside the trap producing excitons
CB
e
ED
VB
13
Heisenberg Uncertainty Principle the
uncertainties of the doped electrons position and
momentum
?P?x h P hk/2? P momentum ?P?x h ?x
2?/k Set ?x 2?/a (a lattice parameter) The
position of electron is uncertain, when electron
is at k0 then recombination occurs, if not then
no recombination. The position and momentum of a
particle cannot be simultaneously measured with
arbitrarily high precision.
E
h?
K
14
Question 2.

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

15
Question 3. List down al of the possible
application of IR LED
16
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.
17
2 critical considerations

• The wavelength of the radiation emitted
• 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.
Thin film
Substrate
18
Epitaxial growth
p
P-n junction
n
Substrate
Semiconductor materials need to be deposited onto
a textured substrate (thin film technology)
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
19
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.

20
Band Gap and Lattice Constant
The band gap energy can be tailored to get
desired visible light radiation
Substrates must have similar lattice parameter to
the semiconductor films, GaAs, GaN and InP are
often used as substrates.
21
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
22
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
23
Cross section of a typical epitaxial layers
24
Calculation. InGaAs on InP substrate (Kasap)
25
The nitrides and blue LED

• Difficulties
• to find suitable substrates for the nitrides.
• to get p-type nitrides
• But with constant RD 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

26
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)

27
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.
28
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 gt10 000 hours
while incandescent filament (100watt) normally
reaches about 750-1500 hours.
29
Phosphor
30
Another typical exam question
Draw 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)
31
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

32
The selenides - E gap vs lattice parameter
ZnSe can be made ternary allow with ZnTe to
produce ZnSeTe ? blue-green
33
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.