EBB 424E Semiconductor Devices and Optoelectronics
Part II - OptoelectronicsDr Zainovia Lockman
SMMRE,USM
EBB 424:Semiconductor Devices and Optoelectronics
Part 1:
Semiconductor Devices
Dr. Sabar D. Hutagalung
Part 2:
Optoelectronics
Devices
Dr Zainovia Lockman
70% Exam30% Coursework
Contents of the Course
Light sourcesLight sources
LED Photodetector
Photoconductor
Photovoltaic
LASERS
Light DetectorsLight Detectors
OptoelectronicsOptoelectronics
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
Red LED White LED
LED for displays Blue LED LED for traffic light
LEDs
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
TransmitterChannel
Receiver
Fibre Optics IR- Photodetector
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?
Newton
They did not agree They did not agree with each other!with each other!
Huygens
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)
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
e
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:
H,tc
1H,
2
2
22
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 =
n = refractive index of the medium = wavelength in the medium
And,
r = relative magnetic permeability of the medium r = relative electric permittivity of the medium
rrn
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
Wave-like propertiesParticles: photon energy
Answer 1
= 1.24x 10-6 /E
Wavelength
Associated with colours
Energy
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 SpectrumShorter wavelength
Longer wavelength
V ~ 3.17eV
B ~ 2.73eV
G ~ 2.52eV
Y ~ 2.15eV
O ~ 2.08eV
R ~ 1.62eV
Larger Photon Energy (eV) Answer 2:
Visible Lights
Lights of wavelength detected by human eyes ~ 450nm to 650nm is called visible light:
Human eyes can detect lights with different colours Each colour is detected with different efficiency.
3.1eV 1.8eV
Spectral Response of Human Eyes
Eff
icie
ncy,
100
%
400nm 600nm 700nm500nm
Interaction Between Light and Bulk Material
1- Refraction
2- Transmission
3a – Specular reflection
3b – Total internal reflection
3c – Diffused reflection
4 – Scattering
There is also dispersion –where different colours bend differently
1- Refraction
2- Transmission
3a – Specular reflection
3b – Total internal reflection
3c – Diffused reflection
4 – Scattering
There is also dispersion –where different colours bend differently
41
3b
2
3a
3c
Incident light
Semi-transparent material
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 Egap = 1.2eV
Need to know visible light photon energy 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
I B G Y O R
EEvisvis= 1.8eV = 1.8eV 3.1eV3.1eV
•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?
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 SemiconductorsA
bsorp
tion
coeffi
cie
nt
(),
cm
-1
Photon energy (eV)
Absorption spectrum of a semiconductor.
Vis
Eg ~
vis
Wavelength (m)
IRUV
Important region:
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
h
Conduction band, EC
Valance band, EV
EgapEphoton
Process requires the lowest E of photon to initiate electron jumping (excitation)
• EC-EV = h
• EC-EV = Egap
• If h > Egap then transition happens
•Electrons in the conduction band and excited.
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
K (wave number)h
Conservation of E
h = EC(min) - Ev (max) = Egap
Conservation of wavevector
Kvmax + photon = kc
E
Direct vertical transition
Momentum of photon is negligible
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
Light when it travels in a medium can be
absorbed and reemitted by every
atom in its path.
Refraction, Reflection and Dispersion
Defines by refractive index;
n
Small n
High n
n1 = refractive index of material 1
n2 = refractive index of material 2
Total Internal Reflection
n2
i
n1 > n
2i
Incidentlight
t
Transmitted(refracted) light
Reflectedlight
k t
i>cc
TIR
c
Evanescent wave
k i k r
(a) (b) (c)
Light wave travelling in a more dense medium strikes a less dense medium. Depending onthe incidence angle with respect to c, which is determined by the ratio of the refractiveindices, the wave may be transmitted (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?
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