Transcript LIGHT EMITTING DIODE – Materials Issues and Selection

LIGHT EMITTING
DIODE – Materials
Issues and Selection
EBB 424E
Lecture 3– LED 2
Dr Zainovia
Lockman
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
Ga
As
P
GaAs(1+x) Px
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 Diagram
Direct band gap
100%GaAs
Indirect to Direct
transition 50% GaP
Red
photon
Indirect band gap
100% GaP
Composition of GaP %
Green
photon
Doped with nitrogen
efficiency increases
GaN+GaP = GaAs
Spectral response
of human eye
(1+x)Px
At the transition, the band
gap correspond to  from
near IR to the orange-red
part of the vis-spectrum
eV

GaP = 2.26eV
GaP= indirect but when
alloyed with GaAs, the
band gap will become
direct at x = 0.45
1.997eV
GaAs = 1.42eV
Indirect ----------- > Direct
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
CB
No N
VB
e
N produces
perturbances
CB
VB
e falls inside
the ‘trap’
producing
excitons
CB
ED
VB
e
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
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 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
h
K
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
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?
A good device requires a defect free semiconductor films.
Defect free  good crystallographic orientation of the
How?
grains of the semiconductor materials, low defect
1.
Thin film
Substrate
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.
Epitaxial growth
P-dopant
p
n
P-n junction
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
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
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)
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
The selenides - E gap vs lattice
parameter
ZnSe can be made
ternary allow with ZnTe
to produce ZnSeTe 
blue-green
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.