Download EBB 424E Semiconductor Devices and Optoelectronics

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EBB 424E Semiconductor
Devices and Optoelectronics
Part II - Optoelectronics
Dr Zainovia Lockman
EBB 424:
Semiconductor Devices and Optoelectronics
Part 1:
Semiconductor Devices
Dr. Sabar D. Hutagalung
Part 2:
Optoelectronics
Devices
Dr Zainovia Lockman
70% Exam
30% Coursework
Contents of the Course
Optoelectronics
Light sources
Light Detectors
LED
Photodetector
LASERS
Photoconductor
Photovoltaic
Scope of the Course


By the end of the course you will be able to
describe various optoelectronics devices.
In particular you need to be able to describe:
1. The device configuration
2. Materials requirements
3. Materials selection
4. Materials issues
What is Optoelectronics?
"Optoelectronics, the alliance of optics and electronics, [is] one of the most
exciting and dynamic industries of the information age. As a strategic
enabling technology, the applications of optoelectronics extend throughout
our everyday lives, including the fields of computing, communication,
entertainment, education, electronic commerce, health care and
transportation. Defense applications include military command and control
functions, imaging, radar, aviation sensors, and optically guided weapons.
Optoelectronics businesses manufacture components such as lasers,
optical discs, image sensors, or optical fibers, and all sorts of equipment
and systems that are critically dependent on optoelectronics components.
Optoelectronics technology is a key enabler of the USD$1.5 Trillion global
information industry."
Light- Emitting
Diodes
LEDs
Red LED
LED for displays
White LED
Blue LED
LED for traffic light
DIODE LASERS
Diode lasers have been used for cutting,
surgery, communication (optical fibre),
CD writing and reading etc
Producing Laser in the Lab
Optoelectronic devices for
Photovoltaic Applications
Solar Cells
Fibre optics Communication
Transmitter  Channel  Receiver
IR - Lasers
IRPhotodetector
Head Mounted Display  Applications: Next
generation head mounted display and virtual
reality training
What is expected of you?
Objectives of the Part II EBB424E




To describe the fundamentals of photon-electron
interaction in solid and to relate such
understanding with the optoelectronics devices
To develop an appreciation of intrinsic properties
of semiconductors focusing on the optical
properties of the material
To familiarise with the basic principles of
optoelectronic devices (light emitting diode, laser,
photodetector and photovoltaic).
To state the materials issues, requirements and
selection for a given optoelectronic devices
Introduction to
Optoelectronics
- Lights
Lecture 1
Lights- Newton and Huygens
 Lights as
wave?
 Lights as particles?
Huygens
They did not agree
with each other!
Newton
Lights – Einstein and Planck

1905 Einstein –related wave and
particle properties of light

Planck - WAVE-PARTICLES DUALITY
E = h
Total E of the Photon
(particle side)
e



Frequency (wave side)
Light is emitted in multiples of a certain minimum
energy unit. The size of the unit – photon.
Explain the photoelectric effect - electron can be
emitted if light is shone on a piece of metal
Energy of the light beam is not spread but propagate
like particles
Photons

When dealing with events at an atomic scale it is
often best to regard light as composed of particles
– photon. Forget it being wave.
 A quanta of light
 Electromagnetic radiation quantized and occurs
in finite "bundles" of energy = photons
 The energy of a single photon is given, in terms
of its frequency, f, or wavelength, , as,
Eph = hf = hc/
Maxwell – Electromagnetic wave
Light as Electromagnetic Wave


Light as an electromagnetic wave is characterised
by a combinations of time-varying electric field ()
and magnetic field (H) propagating through space.
Maxwell showed both  and H satisfy the same
partial differential equation:
1 
 , H   2  2 , H 
c  t 
2
2
Changes in the fields propagate
through space with speed c.
Speed of Light, c

Frequency of oscillation, of the fields and their
wavelength, o in vacuum are related by;


c = o
In any other medium the speed, v is given by;

v= c/n = 



And,


n = refractive index of the medium
 = wavelength in the medium
n  r r
r = relative magnetic permeability of the medium
r = relative electric permittivity of the medium
The speed of light in a medium is related to the
electric and magnetic properties of the medium, and
the speed of light can be expressed
Question 1
Relate Planck’s Equation (E = h) with the
Speed of Light in a medium (c = )
h =
Planck’s constant = eV
c =
Speed of light =
2.998 x 108 ms-1
 Why do you think this equation is important
when designing a light transmission devices
based on semiconductor diodes?
 Relate this with Photon Energy.

Answer 1
E = hc

Particles: photon energy
Wave-like properties
Answer 1
= 1.24x 10-6 /E
Wavelength
Energy
Associated
with colours
Each colour has energy
associated with it
Question 2

Based on the equation you have produced in
question 1, calculate the photon energy of
violet, blue, green, orange and red lights.

Electromagnetic Spectrum
Shorter wavelength
Larger Photon
Energy (eV)
Answer 2:
V ~ 3.17eV
B ~ 2.73eV
G ~ 2.52eV
Y ~ 2.15eV
O ~ 2.08eV
R ~ 1.62eV
Longer wavelength
Visible Lights


Lights of wavelength detected by human eyes ~ 450nm to
650nm is called visible light:
3.1eV
1.8eV
Human eyes can detect lights with different colours
Each colour is detected with different efficiency.
Spectral Response of Human
Eyes
Efficiency, 100%

400nm 500nm 600nm 700nm
Interaction Between Light and Bulk
Material
3c
Semi-transparent
material
Incident light
4
1- Refraction
1
2- Transmission
3a – Specular reflection
3a
3b – Total internal
reflection
3b
3c – Diffused reflection
2
4 – Scattering
There is also dispersion –
where different colours
bend differently
Appearance of insulator, metal and
semiconductor



Appearance in term of colour depends on the interaction
between the light with the electronics configuration of the
material.
Normally,
 High resistiviy material: insulator  transparent
 High conductivity material: metals  metallic luster and
opaque
 Semiconductors  coloured, opaque or transparent, colour
depending on the band gap of the material
For semiconductors the energy band diagram can explain the
appearance of the material in terms of lustre and colouration
Question 3. Why is Silicon Black
and Shiny?
Answer 3.

Need to know, the energy gap of Si


Need to know visible light photon energy



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
Egap = 1.2eV
Evis ~ 1.8 – 3.1eV
Evis is larger than Silicon Egap
All visible light will be absorbed
Silicon appears black
Why is Si shiny?
A lot of photons absorption occurs in silicon, there are
significant amount of electrons on the conduction
band. These electrons are delocalized which induce
the lustre and shines.
Question 4. Why is GaP yellow?
Answer 4
Need to know the Egap of GaP
 Egap = 2.26eV
 Equivalent to  = 549nm.
 E photons with h > 2.26ev absorb light (i.e.
green, blue and violet)
 E photons with h < 2.26eV transmit light
(i.e. yellow, orange and red).
 Sensitivity of human eye is greater for yellow
than red therefore GaP appears
yellow/orange.

Colours of Semiconductors
Evis= 1.8eV
I
3.1eV
B
G
Y
O
R
•If Photon Energy, Evis > Egap  Photons will be
absorbed
•If Photon Energy, Evis < Egap  Photons will transmitted
•If Photon Energy is in the range of Egap ;
•Those with higher energy than Egap will be absorbed.
•We see the colour of the light being transmitted
•If all colours are transmitted = White
Why do you think glass is
transparent?




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Glass is insulator (huge band gap)
The electrons find it hard to jump across a big energy gap (Egap >> 5eV)
Egap >> E visible spectrum ~2.7- 1.6eV
All colored photon are transmitted, no absorption hence light transmit –
transparent.
Defined transmission and absorption by Lambert’s law:
 I = Io exp (- l)





I = incident beam
Io = transmitted beam
 = total linear absorption coefficient (m-1)
 = takes into account the loss of intensity from both scattering centers and
absorption centers.
 = approaching zero for pure insulator.
What happens during
photon absorption process?
Photon interacts with the lattice
Photon interacts with defects
Photon interacts with valance electrons
Absorption Process of Semiconductors
Vis
UV
IR
Important region:
Eg ~ vis
Absorption coefficient (), cm-1
Wavelength (m)
Photon energy (eV)
Absorption spectrum of a semiconductor.
Absorption – an important phenomena
in describing optical properties of
semiconductors



Light, being a form of electromagnetic radiation, interacts
with the electronic structure of atoms of a material.
The initial interaction is one of absorption; that is, the
electrons of atoms on the surface of a material will absorb the
energy of the colliding photons of light and move to the
higher-energy states.
The degree of absorption depends, among other things, on
the number of free electrons capable of receiving this photon
energy.
Absorption Process of Semiconductors





The interaction process is a characteristic of a photon and
depends on the energy of the photon (see the pervious slide
– the x-axis).
Low-energy photons interact principally by ionization or
excitation of the outer orbitals in solids’ atoms.
Light is composed of low-energy photons (< 10 eV)
represented by infrared (IR), visible light, and ultraviolet
(UV) in the electromagnetic spectrum.
High-energy protons (> 104 eV) are produced by x-rays and
gamma rays.
The minimum photon energy required to excite and/or ionize
the component atoms of a solid is called the absorption
edge or threshold.
Valance-Conduction-Absorption
Process requires the
lowest E of photon to
initiate electron jumping
(excitation)
Conduction band, EC
• EC-EV = h
• EC-EV = Egap
• If h > Egap then
transition happens
•Electrons in the
conduction band and
excited.
Egap
h
Ephoton
Valance band, EV
After the absorption then what?


Types  Direct and Indirect photon absorption
For all absorption process there must be:
 Conservation of energy
 Conservation of momentum or the wavevector
The production of e-h pairs is very important
for various electronics devices especially the
photovoltaic and photodetectors devices.
 The absorbed light can be transformed to
current in these devices

Direct Band Gap
E
Direct
vertical
transition
Conservation of E
h = EC(min) - Ev (max) =
Egap
K (wave number)
Momentum
of photon is
negligible
Conservation of
wavevector
Kvmax + photon = kc
h
Indirect Band Gap
E
K (wave number)
h
Question 5.
 For
indirect band gap transition,
how do the energy and
momentum or the wavevector
are being conserved?
Answer Question 5 yourself
Refraction, Reflection and Dispersion
Light when it travels
in a medium can be
absorbed and
reemitted by every
atom in its path.
Defines by
refractive index; n
High n
Small n
n1 = refractive index of material 1
n2 = refractive index of material 2
Total Internal Reflection
t
ki
i
i
Transmitted
(refracted) light
kt
n2
n 1 > n2
kr
Evanescent wave
c c
i >c
TIR
Incident
light
Reflected
light
(a)
(b)
(c)
Light wave travelling in a more dense medium strikes a less dense medium. Depending on
the incidence angle with respect to  c, which is determined by the ratio of the refractive
indices, the wave may be trans mitted (refracted) or reflected. (a) i < c (b)  i = c (c) i
>  c and total internal reflection (TIR).
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Mechanism and Application of TIR
Optical fibre for
communication
What sort of materials do
you think are suitable for
fibre optics cables?
End
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