slide1 l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Introduction to Optical Properties BW, Chs 10 & 11; YC, Chs 6-8; S, Chs 11-13 PowerPoint Presentation
Download Presentation
Introduction to Optical Properties BW, Chs 10 & 11; YC, Chs 6-8; S, Chs 11-13

Loading in 2 Seconds...

play fullscreen
1 / 38

Introduction to Optical Properties BW, Chs 10 & 11; YC, Chs 6-8; S, Chs 11-13 - PowerPoint PPT Presentation


  • 141 Views
  • Uploaded on

Introduction to Optical Properties BW, Chs 10 & 11; YC, Chs 6-8; S, Chs 11-13. Recall : Semiconductor Bandgaps E g are usually in the range: 0 < E g < 3 eV (up to 6 eV if diamond is included) Also, at equilibrium, at temperature T = 0 , the valence band is full & the

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Introduction to Optical Properties BW, Chs 10 & 11; YC, Chs 6-8; S, Chs 11-13' - mika


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

Introduction to Optical Properties

BW, Chs 10 & 11; YC, Chs 6-8; S, Chs 11-13

slide2
Recall:Semiconductor BandgapsEgare usually in the range: 0 < Eg < 3 eV

(up to 6 eVif diamond is included)

  • Also, at equilibrium, at temperature T = 0,

the valence band is full & the

conduction band is empty.

  • Now, consider what happens if electromagnetic radiation (“light”) is shined on the material.
  • In the photon representation of this radiation

If hν  Eg, some electrons can be promoted to the conduction band leaving some holes in the valence band.

slide3
Now, consider some of the various possible types of spectra associated with this process:

Absorption

Looks at the number of absorbed photons (intensity) vs.

photon frequency ω

Reflection

Looks at the number of reflected photons (intensity) vs.

photon frequency ω

Transmission

Looks at the number of transmitted photons (intensity)

vs. photon frequency ω

Emission

Looks at the number of emitted photons (intensity) vs.

photon frequency ω

slide4
A (non-comprehensive) list of

Various Spectra Types:

Absorption, Reflection,

Transmission, Emission

  • Each of these types of spectra is

very rich, complicated, & varied!

  • Understanding such spectra gives

huge amounts of information about:

electronic energy bands, vibrational properties, defects, …

slide5

Interaction Between Light & Bulk Material Many differentpossible processes can occur!

1. Refraction

2. Transmission

3. Reflection

a. Specular

b. Total internal

c. Diffused

4. Scattering

There is also

Dispersion

where different colors bend differently

3c

“Semi-

transparent”

material

Incident

light

4

1

3a

3b

2

a quick review of light photons history newton huygens on light
A Quick Review of “Light” & PhotonsHistory: Newton & Huygens on Light
  • Light as waves
  • Light as particles

Christiaan Huygens

They strongly

disagreed with

each other!

Isaac Newton

slide8

Light – Einstein & Planck

  • 1905 Einstein– Related the wave & particle properties of light when he looked at the
  • Photoelectric Effect.
  • Planck – Solved the “black body” radiation problem by making the (first ever!) quantum hypothesis: Light is quantized into quanta (photons) of energy
  • E = h. Wave-Particle duality.

(waves)

(particles)

  • Light is emitted in multiples of a certain minimum energy unit. The size of the unit – the photon.
  • Explains how anelectron can be emitted if light is shined on a metal
  • The energy of the light is not spread but propagates like particles .
photons
Photons
  • When dealing with events on the atomic scale, it is often best to regard light as composed of quasi- particles:

PHOTONS

Photonsare Quanta of light

Electromagnetic radiation is quantized

& occurs in finite "bundles" of energy

Photons

  • The energy of a single photon in terms of its frequency , or wavelength  is,

Eph = h = (hc)/

slide11

Light as an Electromagnetic Wave

  • Light as an electromagnetic wave is characterized by a combinationof atime-varying electric field (E)& a time-varying magnetic field (H) propagating through space.
  • Maxwell’s Equations give the result that E & H satisfy the same wave equation:

2

(E, H)

(E, H)

Changes in the fields

propagate through free space with speed c.

speed of light c
Speed of Light, c
  • The frequency of oscillation, of the fields & their wavelength, oin vacuum are related by:c = o
  • In any other medium the speed, v is given by: v = c/n = 

n  refractive indexof the medium

wavelengthin the medium

rrelative magnetic permeabilityof the medium

r relative electric permittivityof the medium

The speed of light in a medium is related to the electric & magnetic properties of the medium. The speed of light c, in vacuum, can be expressed as

slide13

The Electromagnetic Spectrum

Shorter

Wavelengths

Increasing Photon

Energy (eV)

Color & Energy

Violet ~ 3.17eV

Blue ~ 2.73eV

Green ~ 2.52eV

Yellow ~ 2.15eV

Orange ~ 2.08eV

Red ~ 1.62eV

Longer

Wavelengths

slide14

Visible Light

  • Light that can be detected by the human eye has wavelengths in the rangeλ ~ 450nm to 650nm & is called visible light:

1.8eV

3.1eV

  • The human eye can detect light of many different colors.
  • Each color is detected with different efficiency.

Spectral Response of Human Eyes

Efficiency, 100%

400nm

500nm

600nm

700nm

slide15

Visual Appearance of

Insulators, Metals, & Semiconductors

  • A material’s appearance & color depend on the interaction between light with the electron configuration of the material.
slide16

Visual Appearance of

Insulators, Metals, & Semiconductors

  • A material’s appearance & color depend on the interaction between light with the electron configuration of the material.
  • Normally

High resistivity materials(Insulators) are Transparent

High conductivity materials(Metals) have a “Metallic

Luster” & are Opaque

Semiconductorscan beopaque or transparent

This & their color depend on the material band gap

  • For semiconductors the energy band diagram can explain the appearance of the material in terms of both luster & color.
to answer this
To Answer This:
  • We need to know that the energy gap of Si is:

Egap = 1.2eV

  • We also need to know that, for visible light, the photon energy is in the range:

Evis ~ 1.8 – 3.1eV

So, for Silicon, Evisis larger than Egap

  • So, all visible light will be absorbed & Silicon appears black

So, why is Si shiny?

  • The answer is somewhat subtle: Significant photon absorption occurs in silicon, because there are a significant number of electrons in the conduction band. These electrons are delocalized. They scatter photons.
why is gap yellow
Why is GaP Yellow?

To Answer This:

  • We need to know that the energy gap of GaP is:
  • Egap= 2.26 eV
  • This is equivalent to a
  • Photon of Wavelength  = 549 nm.
  • So photons with E = h > 2.26 eV(i.e. green, blue, violet)are absorbed.
  • Also photons with E = h < 2.26eV (i.e. yellow, orange, red) are transmitted.
  • Also, the sensitivity of the human eye is greater for yellow than for red, so
  • GaP Appears Yellow/Orange.
colors of semiconductors
Colors of Semiconductors

Evis= 1.8eV 3.1eV

I B G Y O R

If the Photon Energy is Evis > Egap

Photons will be absorbed

If the Photon Energy is Evis < Egap

Photons will transmitted

If the Photon Energy is in the range of Egap

those with higher energy thanEgapwill be absorbed.

We see the color of the light being transmitted.

If all colors are transmitted the light is White

slide21

Why is Glass Transparent?

  • Glass is an insulator (with a huge band gap). Its is difficult for electrons to jump across a big energy gap:Egap >> 5eV
  • Egap >> E(visible light) ~ 2.7- 1.6eV
  • All colored photons are transmitted, with no absorption, hence the light is transmitted & the material is transparent.
  • Define transmission & absorption by
  • Lambert’s Law: I = Ioexp(-x)
  • Io = incident beam intensity, I = transmitted beam intensity
  • x = distance of light penetration into material from a surface
  • total linear absorption coefficient(m-1)
  • takes into account the loss of intensity from scattering centers & absorption centers.approaches zero for a pure insulator.
slide22

What happens during the photon absorption process?

Photons interact with the lattice

Photons interact with defects

Photons interact with

valence electrons

Photons interact with …..

slide23

Wavelength (m)

Vis

UV

IR

Absorption coefficient (, cm-1)

Eg ~ Evis

Photon Energy (eV)

Absorption spectrum of a semiconductor.

Absorption Processes in Semiconductors

Important region:

Lllllllllllllllllllllllllllllllllllllllllllllllllllllllllll lllllllllllllllllll

slide24

Absorption

An Important Phenomena in the Description of the Optical Properties of Semiconductors

  • Light (electromagnetic radiation) interacts with the electronic structure of the material.
  • The Initial Interaction is Absorption
  • This occurs because valence electrons on the surface of a material absorb the photon energy & move to higher-energy states.
  • The degree of absorption depends,among many other things,on the number of valence electronscapable of receiving the photon energy.
slide25
The photon-electron interactionprocess obviously depends strongly on the photon energy.
  • Lower Energy Photonsinteract principally by ionization or excitation of the solid’s valence electrons.
  • Low Energy Photons (< 10 eV)are in the infrared (IR), visible&ultraviolet (UV)in the EM spectrum.
  • High Energy Photons (> 104 eV) are in the X-Ray&Gamma Ray region of the EM spectrum.
  • The minimum photon energy to excite and/or ionize a solid’s valence electrons is called the

Absorption Edgeor

Absorption Threshold.

slide26

Valence Band – Conduction Band Absorption

(Band to Band Absorption)

Conduction Band, EC

h = Ephoton

Egap

Valence Band, EV

slide27

Valence Band – Conduction Band Absorption

(Band to Band Absorption)

This process obviously requires that the minimum energy of a photon to initiate an electron transition must satisfy

EC - EV = h = Egap

Conduction Band, EC

h = Ephoton

Egap

Valence Band, EV

slide28

Valence Band – Conduction Band Absorption

(Band to Band Absorption)

This process obviously requires that the minimum energy of a photon to initiate an electron transition must satisfy

EC - EV = h = Egap

Conduction Band, EC

If h > Egapthen obviously a transition can happen. Electrons are then excited to the conduction band.

h = Ephoton

Egap

Valence Band, EV

after the absorption then what
After the Absorption Then What?

2 Primary Absorption Types

Direct Absorption & Indirect Absorption

  • Allabsorption processes must satisfy:

Conservation of Total Energy

Conservation of Momentum or Wavevector

  • The production of electron-hole pairs is very important for electronics devices especially photovoltaic & photodetector devices.
  • The conduction electrons produced by the absorbed light can be converted into a current in these devices.
slide30

A Direct Vertical Transition!

E

Conservation of Energy

h = EC(min) - Ev (max) = Egap

K (wave number)

h

The Photon Momentum is Negligible

Conservation of Momentum

Kvmax + qphoton = kc

Direct Band Gap Absorption

slide31

K (wave number)

h

Indirect Band Gap Absorption

E

another viewpoint
Another Viewpoint
  • If a semiconductor or insulator does not have many impurity levels in the band gap, photons with energies smaller than the band gap energy can’t be absorbed
    • There are no quantum states with energies in the band gap
  • This explains why many insulators or wide band gap semiconductors are transparent to visible light, whereas narrow band semiconductors (Si, GaAs) are not
slide33
Some of the many applications
  • Emission:

light emitting diodes (LED) & Laser Diodes (LD)

  • Absorption:
  • Filtering: Sunglasses, ..

Si filters (transmission of infra red light with simultaneous blocking of visible light)

slide34
If there are many impurity levels the photons with energies smaller than the band gap energy can be absorbed, by exciting electrons or holes from these energy levels into the conduction or valence band, respectively
    • Example: Colored Diamonds
photoconductivity
Charge carriers (electrons or holes or both) created in the corresponding bands by absorbed light can also participate in current flow, and thus should increase the current for a given applied voltage, i.e., the conductivity increases

This effect is calledPhotoconductivity

Want conductivity to be controlled by light. So want few carriers in dark → A semiconductor

But want light to be absorbed, creating photoelectrons

→ Band gap of intrinsic photoconductors should be smaller than the energy of the photons that are absorbed

Photoconductivity
slide36

Refraction, Reflection &Dispersion

Light, when it travels in a medium can be absorbed and reemitted by every atom in its path.

High n

Small n

n1 = refractive index of material 1

n2 = refractive index of material 2

Defined by refractive index; n

slide38

Mechanism & Applications of TIR

Optical fiber for communication

What kinds of materials do you think are suitable for fiber optics cables?