SMMRE,USM

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

Light sources Optoelectronics Light Detectors LED LASERS Photodetector Photoconductor Photovoltaic

Scope of the Course

  By the end of the course you will be able to describe various optoelectronics devices.

1.

In particular you need to be able to describe:

The device configuration 2.

3.

4.

Materials requirements Materials selection 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 White LED LED for displays 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 IR Photodetector

Head Mounted Display generation head mounted display and virtual reality training



Applications: Next

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 properties of the material on the optical To familiarise with the basic photodetector and photovoltaic).

principles of optoelectronic devices (light emitting diode, laser, 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) Frequency (wave side)

e

   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,

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:  2  ,  1 c 2    2  t 2     Changes in the fields propagate through space with speed c.

Speed of Light, c

   Frequency of oscillation, wavelength, 

c =



o

 o  of the fields and their in vacuum are related by; In any other medium the speed, v is given by; 

v= c/n =

   n = refractive index of the medium  = wavelength in the medium And,   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 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

 Particles: photon energy Wave-like properties

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 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 400nm 500nm 600nm 700nm

Interaction Between Light and Bulk Material Incident light 3c

Semi-transparent material

4 3a 1 3b 2

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

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 opaque  metallic luster and 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

E vis = 1.8eV

I B G Y

3.1eV

O R •If Photon Energy, E

vis absorbed > E gap



Photons will be

•If Photon Energy, E

vis < E gap



Photons will transmitted

•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 (-



l)

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 UV

Important region:

Wavelength (



m) Vis IR 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 (> 10 4 gamma rays.

eV) are produced by x-rays and 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)

• E

C -E V = h

 • E

C -E V = E gap

• If h 

> E gap then transition happens

•Electrons in the

conduction band and excited.

E gap

Conduction band, E C

h  E photon

Valance band, E V

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

Direct vertical transition Momentum of photon is negligible Conservation of E h



= E C(min) - E v (max) = E gap K (wave number) Conservation of wavevector K vmax +



photon = kc E

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

Small n High n

n 1 = refractive index of material 1 n 2 = refractive index of material 2

Total Internal Reflection

k

i



i



t



i

Transmitted (refracted) light

k

t

n

2

n

1

>

n

2 k

r



c



c

Evanescent wave 

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 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?

End

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