Advanced Analytical Chemistry

Lecture 7

Chem 4631

Semiconductor LED vs LASER

Light Emitting Diode

•Light is mostly monochromatic (narrow energy spread comparable to the distribution of electrons/hole populations in the band edges)

•Light is from spontaneous emission (random events in time and thus phase)•Light diverges significantly

LASER

•Light is essentially single wavelength (highly monochromatic)•Light is from “stimulated emission” (timed to be in phase with other photons•Light has significantly lower divergence (Semiconductor versions have more

than gas lasers though)

Chem 5570

Semiconductor LED vs LASER

Chem 5570

Advantages of Light Emitting Diodes (LEDs)

Longevity: The light emitting element in a diode is a small conductor chip rather than a filament which greatly extends the diode’s life in comparison to an incandescent bulb (10 000 hours life time compared to ~1000 hours for incandescence light bulb)

Efficiency: (Presently ~75 Lumens/Watt) Diodes emit almost no heat and run at very low amperes

Lower energy consumption

Smaller size

Red 10x Better that (filtered) incandescent (15 Lm/W)

White 2x better than incandescent

Potential efficiency 150+ Lumens/Watt (2x better than fluorescent)

Greater Light Intensity: Since each diode emits its own light

Chem 5570

Advantages of Light Emitting Diodes (LEDs)

Cost:

Coming down

Robustness:

Solid state component, not as fragile as incandescence light bulb

No catastrophic failures

Much greater design freedom:Color Flexibility including many “whites” without filters

Size and shape flexibility for styling and fixture design

Instant on and fully dimmable with no color change

Environmentally friendly:Minimal disposal required

No mercury

Potential savings of $17B in annual energy costs (30 large power plants)

Potential reduction on CO2 emissions of 155 million tons

Chem 5570

Light Emitting Diodes

Problems:

LEDs are powerful enough for room lighting are relatively expensive and require more precise current and heat management than compact fluorescent lamp sources of comparable output.

Chem 5570

Requirements:

•Cost reduction (100 x needed)

•Improvements in efficiency (5x needed)

Chem 5570

Chem 5570

Light output

pEpitaxial layers

Substrate

n+

n+

Fig. 6.44: A schematic illustration of one possible LED devicestructure. First n+ is epitaxially grown on a substrate. A thin p layeris then epitaxially grown on the first layer.

From Principles of Electronic Materials and Devices, Second Edition, S.O. Kasap (© McGraw-Hill, 2002)

http://Materials.Usask.Ca

General Structure

A simple LED is a pn junction on a suitable substrate.

Chem 5570

Construction of Typical LED

Chem 5570

n

Al

SiO2

Electrical contacts

p

Light output

Substrate

Luminescence is the process behind light emission

Luminescence is a term used to describe the emission of radiation from a solid when the solid is supplied with some form of energy.

Electroluminescence - excitation results from the application of an electric field.

In a p-n junction diode injection electroluminescence occurs resulting in light emission when the junction is forward biased.

Chem 5570

LEDs are semiconductor p-n junctions that under forward bias conditions can emit radiation by electroluminescence in the UV, visible or infrared regions of the electromagnetic spectrum.

When pn junction is forward biased, large number of carriers are injected across the junctions. These carriers recombine and emit light.

The quanta of light energy released is approximately proportional to the band gap of the semiconductor.

The emitted photons must escape without being reabsorbed, so the p-side has to be narrow.

Chem 5570

Chem 5570

P-n junction

Electrical Contacts

A typical LED needs a p-n junction

Junction is biased to produce even more

e-h and to inject electrons from n to p for

recombination to happen

There are a lot of electrons and holes at

the junction due to excitations

Electrons from n need to be injected to p

to promote recombination

Recombination

produces light!!

Excitation

Chem 5570

Electron (excited by the

biased forward voltage) is

in the conduction band

Hole is in valance band

Normally the recombination takes place between

transition of electrons between the bottom of the

conduction band and the top of the valance band.

The emission of light is;

hc/ = Ec-Ev = Eg(only direct band gap allows radiative transition)

E

Emission wavelength, g

◘ The number of radiative recombination is proportional to the carrier injection rate

◘ Carrier injection rate is related to the current flowing in the junction

◘ If the transition takes place between states (conduction and valance bands) the emission wavelength,

g = hc/(EC-EV)

◘ EC-EV = Eg

◘ g = hc/Eg

Chem 5570

Injection Luminescence in LED

Under forward bias – majority carriers from both sides of the junction can cross the depletion region and entering the material at the other side.

Upon entering, the majority carriers become minority carriers. For example, electrons in n-type (majority carriers) enter the p-type

to become minority carriers. The minority carriers will be larger minority carrier injection. Minority carriers will diffuse and recombine with the majority

carrier. For example, the electrons as minority carriers in the p-region will

recombine with the holes. Holes are the majority carrier in the p-region.

The recombination causes light to be emitted. Such process is termed radiative recombination.

Chem 5570

Recombination and Efficiency

◘ Ideal LED will have all injection electrons to take part in the recombination process.

◘ In real device not all electron will recombine with holes to radiate light.

◘ Sometimes recombination occurs but no light is emitted (non-radiative).

◘ Efficiency of the device is not 100%.

◘ Efficiency is the rate of photon emission over the rate of supply electrons.

Chem 5570

eVo

Eg

p n+

h =Eg

Eg

p n+(a) (b)

Electrons in CB

Holes in VB

Efficient LED

Need a p-n junction (preferably the same semiconductor materialonly different dopants).

Recombination must occur Radiative transmission to give outthe correct color.

Color of LED hc/ = Ec-Ev = Eg so choose material with the right Eg

Direct band gap semiconductors to allow efficient recombination.

All photons created must be able to leave the semiconductor.

Little or no reabsorption of photons.

Chem 5570

LED Characteristic

The energy of an emitted photon = to the size of theband gap.

BUT this is a simplified statement.

The energy of an emitted photon from LED isdistributed appropriately according to theenergy distribution of electrons on the conductionband and holes in the valance band.

You need to know the distribution of electrons andholes in the CB and VB respectively.

Chem 5570

LED Construction

Efficient light emitter is also an efficient absorbers of radiationtherefore, a shallow p-n junction required.

Active materials (n and p) will be grown on a lattice matchedsubstrate.

The p-n junction will be forward biased with contacts made bymetallization to the upper and lower surfaces.

Carrier from n must be injected into the p-side efficiently.

Consider the fraction of the total diode current that is carried byelectrons being injected into the p-side of the junction (e)

Leave the upper part ‘clear’ so photon can escape.

The silica provides passivation/device isolation and carrierconfinement .

Chem 5570

LED Construction - Aim – 100% light emitting efficiency

Chem 5570

Correct band gap Direct band gap

Material can be made p and n-type

Efficient radiativepathways must exist

Band Gap Engineering

A process of varying the elemental components of the semiconductor alloy in a controlled way to achieve a desired band gap that can emit a desired wavelength of radiation.

Chem 5570

2 critical considerations

1. The wavelength of the radiation emitted

2. The lattice parameters of the compounds

The wavelength visible, UV or IR

The lattice parameter for epitaxial growth

Why?

How?

Chem 5570

A good device requires a defect free semiconductor films. Defect free good crystallographic orientation of the

grains of the semiconductor materials, low defects.

To achieve defect free semiconductor thin film, adopt a so-call epitaxial growth of the film on a substrate growth process

where the deposited films will ‘follow’ the surface structure of a substrate. Substrate

Thin film

Epitaxial growth

Chem 5570

P-dopant

Semiconductor materials need to be deposited

onto a textured substrate(thin film technology)

P-n junction

Substrate must have similar lattice parameter to that of the

semiconductor thin film to avoid lattice mismatch (strain at the

interface will induce crack) and to allow epitaxial growth

The semiconductor then need to be doped to achieve both p and n type require p-n

junction

Substrate

n

p

Band Gap and Lattice Constant

Chem 5570

Substrates must have similar lattice parameter to the semiconductor films, GaAs, GaN and InP are often used as

substrates.

The band gap energy can be tailored to get desired visible light

radiation

Cross section of a typical epitaxial layers

Chem 5570

Chem 5570

Direct band gap materials

e.g. GaAs not Si

UV-LED ~0.5-400nm Eg > 3.25eV

vis-LED - ~450-650nm Eg = 3.1eV to 1.6eV

IR-LED- ~750nm- 1mmEg = 1.65eV

Readily doped n or p-typesMaterials with refractive index that could allow light to ‘get out’

Visible LED

Definition:

LED which could emit visible light, the band gap of the materialsthat we use must be in the region of visible wavelength = 390-770nm. This coincides with the energy value of 3.18eV- 1.61eVwhich corresponds to colours as shown below:

Chem 5570

Violet ~ 3.17eVBlue ~ 2.73eVGreen ~ 2.52eV Yellow ~ 2.15eVOrange ~ 2.08eVRed ~ 1.62eV

Colour an LED should

emit

The band gap, Egthat the

semiconductor must posses to emit each light

IR & Red LED Candidates

GaAs direct band gap, p-n junctions are readily formed with high radiative efficiency.

High radiative efficiency can be induced by doping GaAs with Zn or Si. Si doped GaAs is now the industry standard near IR LEDs.

GaAsP direct – in direct transition

GaInAsP Grown on InP substrate and band gap can be varied to get wavelength from 919nm to 1600nm.

Chem 5570

Candidate Materials for LED’s

Chem 5570

Candidate Materials Group III-V & Group II-IV

Chem 5570

iviii v

ii

Group II Group III Group IV Group V

Al

Ga

In

N

P

As

Group III-V (1950)

The era of III–V compound semiconductors started in the early 1950swhen this class of materials was postulated and demonstrated byWelker (1952, 1953).

The class of III–V compounds had been an unknown substance prior tothe 1950s that does not occur naturally.

The novel man-made III–V compounds proved to be optically veryactive and thus instrumental to modern LED technology.

Chem 5570

Group III-V LED materials

Chem 5570

Al

Ga

In

N

P

As

AlN, AlP,AlAs

GaN, GaP, GaAs

InN, InP, InAs

GaAsGaP

GaAl

GaAsP

GaAsAl

Questions to ask when choosing the right material:1. Can it be doped?

2. What wavelength can it emit?3. Would the material allow radiative recombination?

4. Direct or indirect semiconductor?

Ternary

compounds

Binary

compounds

What is GaAs(1+x) Px?

Chem 5570

GaAs(1+x)

Px

Ga

PAs

LEDs

Chem 5570

What is GaAs(1+x) Px?

• GaAs(1+x) Px is a ternary compound based on GaAs and GaP

• GaAs is a direct gap semiconductor and GaP is indirect semiconductor

• When alloyed, there is cross over point where GaAs(1+x)Px will transformed from being direct gap material to indirect gap material

• Red, yellow and orange coloured LEDs can be made with GaAs(1+x) Px

Chem 5570

Band Diagram

Chem 5570

Direct band gap 100%GaAs

Indirect band gap 100% GaP

Indirect to Direct transition 50% GaP

Doped with nitrogen - efficiency increases

Composition of GaP %

Red photon

Green photon

GaN+GaP = GaAs (1+x)Px

Chem 5570

Spectral response of human eye

eV

GaP = 2.26eV GaAs = 1.42eV

Indirect ----------- > DirectGaP= indirect but when alloyed with GaAs, the band gap will become

direct at x = 0.45

At the transition, the band gap correspond to from near IR to the orange-red part of the vis-spectrum

1.997eV

GaAs (1+x) Px

http://images.google.co.uk/imgres?imgurl=http://animaha.com/media/eye.jpg&imgrefurl=http://www.animaha.com/archives/2004_12.html&h=180&w=267&sz=15&tbnid=WZsjGJ0DIHEJ:&tbnh=72&tbnw=108&hl=en&start=1&prev=/images?q=eye&svnum=10&hl=en&lr=http://images.google.co.uk/imgres?imgurl=http://animaha.com/media/eye.jpg&imgrefurl=http://www.animaha.com/archives/2004_12.html&h=180&w=267&sz=15&tbnid=WZsjGJ0DIHEJ:&tbnh=72&tbnw=108&hl=en&start=1&prev=/images?q=eye&svnum=10&hl=en&lr=

GaAs(1+x) Px system

Chem 5570

x = 0.45 indirect to direct transition

GaAs(1+x) Px system doped with N

• Indirect no radiative transition

• Indirect GaAs(1+x) Px can have radiative transition.

• HOW?

• By adding nitrogen to the system

• When N added to GaAs(1+x) Px:

– The quantum efficiency increases ~ 100x

– The emission wavelength increases

• Quantum efficiencies = rate of emission of photons

Rate of electron supply

Chem 5570

How efficient the e-h pair can recombine

Isoelectronic Doping and Heisenberg Uncertainty Principles (N +GaAsP)

N has the same valancy as that of P and As.

N can enter the As or P site in the GaAsP crystal structure.

N and P has similar number of valance electrons but different coreshell structure.

N produces a perturbance in the electronic confinement .

Electronics confinement changes and acts as a ‘trap’.

Electron trapped at a level just below a conduction band.

Hole can be captured to produce electron-hole pair (exciton).

The carriers are localized, the momentum and the wavenumber arediffuse due to Heisenberg uncertainties principle.

Chem 5570

N substitution to GaAsP

Chem 5570

e

No N

N produces perturbances

e falls inside the ‘trap’ producing excitons

VB

CB

VB

CB N doping can dramatically increases the radiative efficiency of GaP (indirect), the doping changes the

emission wavelength to longer wavelength because the energy of the transition is now

reduced to Eg-Ed

ED

VB

CB e

The nitrides and blue LED

• Difficulties:

– to find suitable substrates for the nitrides

– to get p-type nitrides

• But with constant R&D works, better materials are produced

• GaN, InGaN, AlGaN high efficiency LEDs emitting blue/green part of the spectrum.

• First blue LED 1994 Shuji & Nakamura (10 000 hours lifetime)

• SiC can also be used as blue LED- SiC on GaN substrate

Chem 5570

The device

Chem 5570

Applications:

Flat panel displays (display requires, R,G,B now B is found, all LED displays can be made.

High resolution printers

Light source for communications

Microwave transistors (electrons have high mobility)

UV-LED

Chem 5570

Apart from blue LED, UV LED can also be made using nitrides.

UV-LED can be used as UV calibration devices, UV detector etc.

The Blue-Violet LED + Phosphor and White LED

Chem 5570

White LEDs are slightly more efficient than a100W incandescent bulb and three times moreefficient than a 7W night light type bulb. Thelifetime of white LED could reach >10 000 hourswhile incandescent filament (100 watt) normallyreaches about 750-1500 hours.

The Selenide

• Group II-VI is also important (ZnSe especially eventhough ZnO has been a contender as well).

• ZnSe can be made into LED, emitting blue and greenlights.

• Problem with finding suitable template (substrate) forgrowth.

• GaAs and GaN can be used as the substrate for selenide.The lattice parameter for GaAs = 5.6Å and ZnSe = 5.5Å

• ZnSe has been used as blue/green laser (study later).• The selenide degrade more rapidly hence shorter

working life-time

Chem 5570

The selenides - E gap vs lattice parameter

Chem 5570

ZnSe can be made ternary with ZnTe to produce ZnSeTe

blue-green

Important parameter -quantum efficiency (η): a number of photons generated per electron-hole pairs

Factors which determine quantum efficiency

Efficiency of radiative recombination

Internal loses (due to recombination in the depletion region)loses

Chem 5570

The quantum efficiency

Internal quantum efficiency can for some LEDsapproaches 100% but the external efficiencies are muchlower. This is due to reabsorption and TIR (Totalinternal reflectance).

III-V materials have small critical angles therefore theradiation emitted suffers from TIR.

Chem 5570

Total Internal Reflection

Chem 5570

Light Emitting Diodes

Chem 5570

How to solve TIR problem

GaAs-air interface, the C = 16o which means that much

of the light suffers TIR. To solve the problem we could:

1. Shape the surface of the semiconductor into a dome orhemisphere so that light rays strike the surface angles < Ctherefore does not experience TIR. But expensive and notpractical to shape p-n junction with dome-like structure.

2. Encapsulation of the semiconductor junction within a dome-shaped transparent plastic medium (an epoxy) that has higherrefractive index than air.

Chem 5570

Why do we need the dome?

Chem 5570

n+

Electrodes Electrodes

Pn junction

Plastic dome

1. Semiconductor material is shaped like a hemisphere

to reduce TIR…

p

2. Semiconductor material encapsulated in a hemisphere

Homo- and Hetro-Junction

Homojunction = a p-n junction made out of twodifferently doped semiconductors that are of the samematerial (i.e having the same band gap).

Heterojunction = junction formed between twodifferent band gaps semiconductors.

Heterostructure device = semiconductor devicestructure that has junctions between different bandgapmaterials. Higher intensity LEDs can be made by adding a junction

between different bandgap materials (heterostructure device).

Chem 5570

Why Homojunction is bad?

1. Shallow p-region narrow to allow photons to escape without reabsorption.

If the p-region is too shallow, electrons can escape the p-region by diffusion and recombine through crystal defect in the surface of the layer.

This recombination is non-radiative and decreases the efficiency of the LED.

2. Thick p–region then reabsoprtion will be the main problem as the photons will have a long way to go before can be successful emitted.

Create a heterojunction instead since heterojunction solves:Reabsoption problem (photon confinement)Also carrier confinement

Chem 5570

Chem 5570

Band-gap and refractive index engineering.

Heterostructured LED

Avoiding

losses in LED

Carrier confinement

PhotonConfinement

Chem 5570

LED cross section - heterostructure

Double Heterostructure

The double heterostructure is invariably used for optical sourcesfor communication.

Heterostucture can be used to increase:

Efficiency by carrier confinement (band gap engineering)

Efficiency by photon confinement (refractive index)

The double heterostructure enables the source radiation to bemuch better defined, but further, the optical power generated perunit volume is much greater as well, if the central layer of a doubleheterostructure, the narrow band-gap region is made no more than1mm wide.

Chem 5570

Carrier confinement

Double heterostructure allows confinement of carriers and generated photons in the active layer.

Chem 5570

p+-AlGaAsn+-AlGaAs p-GaAsholes

electrons

Simplified band diagram of the ‘sandwich’ top show carrier confinement

Reabsorption Problem

Chem 5570

In order to prevent reabsorption, the upper layer (one that is above the active region) needs to have higher band gap therefore the emitted photons will not be absorbed by the upper layer (photons will be absorbed when Ep < Eg).

2eV1.4eV

n-AlGaAs p-GaAs

Active region – Photons willnot be absorbed by the n-AlGaAs since the band gap ismuch higher than GaAs

p-AlGaAs

n+

GaAs

p Al GaAsp GaAs (active region)

n AlGaAs

Metal contact

Metal contact

Epoxy

n+ GaAs

Photon confinement - Reabsorption problem

Chem 5570

Source of electrons

Source of holes

Active region (micron in thickness)

Active region (thin layer of GaAs) has smaller band gap, energy of photons emitted is smaller then the band gap of the P and N-GaAlAs hence could not be reabsorbed.

Double Heterojunction LED

Chem 5570

Double heterostructure

Burrus type LED

Shown bonded to a fiber with index-matching epoxy.

n+ GaAs

p Al GaAs

p GaAs (active region)

n AlGaAs

Metal contact

Metal contact

Epoxy

Fiber Optics

Burrus-Type LED

Chem 5570

Communication LED

Chem 5570

Heterostructures

Chem 5570

LED 1. Surface emitter

2. Edge emitter

1. Surface Emitter

In surface emitter the emitting area is definedby oxide isolation, with the metal contact area acircle of diameter ~ 10mm-15 mm.

The surface layer is kept as thin as possible (10-15 mm) to minimize reabsorbtion.

Chem 5570

2. Edge Emitter

In edge emitter a double heterostructure band gapengineering is used to achieve carrier confinement andrecombination in an active layer but in addition layersof relatively low refractive index are included toproduce optical guide. A large fraction of the photonsare therefore confined between two ‘plates’ of materialand emerge at the edge of the device as highlydirectional flux compatible with coupling to a fiberoptic cable.

Chem 5570

Edge emitter - using double heterostructure

Chem 5570

Active layer n- GaAlAs

N GaAlAs

N+- GaAlAs

GaAs(n) substrate

Metal contact

Metal contact

P GaAlAs

P+ GaAlAs

n- GaAlAsLight emits

from the edge

The waveguide

Chem 5570

We can use refractive index engineering to create a multilayerstructure in which light can be trapped within the central layers.This layer act as waveguide. (TIR is used in Edge Emitter)

Another Example of Edge Emitter

Chem 5570

OLEDs

Chem 5570

OLED Advantages & Challenges

Advantages• No Backlight => Thin and Low Power

• Power Only Given to ON Pixel => Efficient

• Faster (100x Faster than LCDs)

• Simple, Cheap Processing

Problems

• Current Driven => More complex drive circuitry• Short Lifetime

• Color Drift

• Size

• Price

Chem 5570

OLED

Chem 5570

Chem 5570

OLED Construction

Chem 5570

OLEDs

examples

Chem 5570

OLEDs

examples

Chem 5570

OLED Display (vs LED) Advantages

• Wide Color Gamut

• High Contrast

• High Viewing Angle

• Rapid Response Time

• Low Cost (Fabrication)

Chem 5570

OLED Display (vs LED) Challenges

Product Life Expectancy

– TVs & Monitors 50,000 hrs or more (20+ yrs)

– OLEDS much less

Large Display Size

– Amorphous-Si…Less stable; Low mobility…

– Poly-Si…Great mobility; Vt shifts…

Chem 5570

OLED Lighting Advantages

• Large area diffuse light source

• Thin, flat, lightweight

• Form freedom in design

• Fast switch-on; fully dimmable

• Many colors, including whites

• Robust source (no wires inside)

• Transparent, mirror-like, black or white appearance

• Low voltage technology

• Potentially high efficiency

• "Green" product (energy efficient, recyclable)

• Potentially cheap fabrication

Chem 5570

OLED Lighting Challenges

• High Luminance: Lighting applications needs at least 1,000 cd/m2 brightness

• Long Lifetime: Long operation and shelf lifetime is necessary

• High Efficiency: At least 30 lm/W in WHITE

• Good Homogeneity: Especially on large areas applications

• High CRI at high brightness: > 80 for direct lighting applications

• Very low costs

Chem 5570

Test I (Take Home) (next tuesday)

Outline of research paper Due Oct 10th

Read Chapters 4 – 9

Chem 5570