EBB 424E Semiconductor Devices and Optoelectronics

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Nov 1, 2013 (3 years and 5 months ago)

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EBB 424E Semiconductor
Devices and Optoelectronics

Part II
-

Optoelectronics

Dr 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% Exam

30% Coursework

Contents of the Course

Light sources

LED

Photodetector

Photoconductor

Photovoltaic

LASERS

Light Detectors

Optoelectronics

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

IR
-

Photodetector

Head Mounted Display


Application猺 N數t
g敮敲etion h敡d mount敤 di獰lay and vi牴ual
牥rlity t牡ining

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

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,










E
ph
= 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
:


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

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

10
8

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 properties

Particles: 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 Spectrum

Shorter 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

Efficiency, 100%

400nm

600nm

700nm

500nm

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

4

1

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


E
gap
= 1.2eV



Need to know visible light photon energy


E
vis

~ 1.8


3.1eV


E
vis

is larger than Silicon E
gap


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 E
gap

of GaP


E
gap

= 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

E
vis
= 1.8eV





3.1eV


If Photon Energy, E
vis

> E
gap



偨otons will b攠
absorbed


If Photon Energy, E
vis

< E
gap



偨otons will

transmitt敤


If Photon Energy is in the range of E
gap

;


Those with higher energy than E
gap

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 (E
gap

>> 5eV)


E
gap

>> 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 = I
o

exp (
-





I = incident beam


I
o

= 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

Absorption coefficient (

),cm
-
1

Photon energy (eV)

Absorption spectrum of a semiconductor.


Vis

E
g

~

vis

Wavelength (



IR

UV

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 (> 10
4

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, E
C

Valance band, E
V

E
gap

E
photon

Process requires the
lowest E of photon to
initiate electron jumping
(excitation)



E
C
-
E
V

= h




E
C
-
E
V

= E
gap



If h


>
gap

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


=
C(min)

-

E
v (max)
=
E
gap

Conservation of
wavevector

K
vmax

+

photon

=
k
c

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

n
1

= refractive index of material 1

n
2

= refractive index of material 2

Total Internal Reflection

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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