EBB 424E Semiconductor Devices and Optoelectronics

Part II - OptoelectronicsDr Zainovia Lockman

SMMRE,USM

EBB 424:Semiconductor Devices and Optoelectronics

Part 1:Semiconductor DevicesDr. Sabar D. Hutagalung

Part 2:Optoelectronics

DevicesDr Zainovia Lockman

70% Exam30% 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 configuration2. Materials requirements3. Materials selection4. 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 ReceiverIR - Lasers

Transmitter ChannelReceiver

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 DUALITYE = 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

WavelengthAssociated with colours

EnergyEach 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 ~ 3.17eVB ~ 2.73eVG ~ 2.52eV Y ~ 2.15eVO ~ 2.08eVR ~ 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

Effic

ienc

y, 1

00%

400nm 600nm 700nm500nm

Interaction Between Light and Bulk Material

1- Refraction2- Transmission3a – Specular reflection3b – Total internal reflection3c – Diffused reflection4 – ScatteringThere 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 SemiconductorsAb

sorp

tion

coe

ffici

ent

(),

cm

-1

Photon energy (eV)Absorption spectrum of a semiconductor.

Vis

E g ~

vi

s

Wavelength (m)

IRUVImportant 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 Eh = 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

n 2

i

n 1 > n2i

Incidentlight

t

Transmitted(refracted) light

Reflectedlight

k t

i>cc

TIRc

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 TIROptical fibre for communication

What sort of materials do you think are suitable for fibre optics cables?

EndRead EBB424 notes