51
THE LIGHT EMITTING THE LIGHT EMITTING DIODE DIODE Chapter 6 Chapter 6

THE LIGHT EMITTING DIODE

  • Upload
    enya

  • View
    57

  • Download
    1

Embed Size (px)

DESCRIPTION

THE LIGHT EMITTING DIODE. Chapter 6. When the electron falls down from conduction band and fills in a hole in valence band, there is an obvious loss of energy. CB. VB. The question is; where does that energy go?. - PowerPoint PPT Presentation

Citation preview

Page 1: THE LIGHT EMITTING DIODE

THE LIGHT EMITTING THE LIGHT EMITTING DIODEDIODE

Chapter 6Chapter 6

Page 2: THE LIGHT EMITTING DIODE

CB

VB

When the electron When the electron falls down from falls down from conduction band and conduction band and fills in a hole in fills in a hole in valence band, there is valence band, there is an obvious loss of an obvious loss of energy.energy.

The question is; The question is; where does that energy go?where does that energy go?

Page 3: THE LIGHT EMITTING DIODE

In order to achieve a In order to achieve a reasonable efficiency reasonable efficiency for photon emission, for photon emission, the semiconductor the semiconductor must have a direct must have a direct band gap.band gap.

CB

VB

The question is; The question is; what is the mechanism what is the mechanism

behind photon emission in LEDs?behind photon emission in LEDs?

Page 4: THE LIGHT EMITTING DIODE

For example;For example;SiliconSilicon is known as an is known as an indirect band-gap indirect band-gap

material.material.

as an electron goes from the bottom of as an electron goes from the bottom of the conduction band to the top of the the conduction band to the top of the valence band;valence band;

it must also undergo a it must also undergo a significant significant change in change in momentum.momentum.

CB

VB

What this means is thatWhat this means is that

E

k

Page 5: THE LIGHT EMITTING DIODE

As we all know, whenever something changesAs we all know, whenever something changes

state, one must conserve not only energy, but also state, one must conserve not only energy, but also momentum.momentum.

In the case of an electron going from conduction In the case of an electron going from conduction band to the valence band in silicon, both of these band to the valence band in silicon, both of these things can only be conserved:things can only be conserved:

The transition also creates a quantized set of lattice vibrations,

called phonons, or "heat“ .

Page 6: THE LIGHT EMITTING DIODE

Phonons possess both energy and momentum.Phonons possess both energy and momentum. Their creation upon the recombination of an Their creation upon the recombination of an

electron and hole allows for complete electron and hole allows for complete conservation of both energy and momentum. conservation of both energy and momentum.

All of the energy which the electron gives up in All of the energy which the electron gives up in going from the conduction band to the valence going from the conduction band to the valence band (1.1 eV) ends up in phonons, which is band (1.1 eV) ends up in phonons, which is another way of saying that the electron heats up another way of saying that the electron heats up the crystal.the crystal.

Page 7: THE LIGHT EMITTING DIODE

In a class of materials called In a class of materials called direct band-gap direct band-gap semiconductorssemiconductors; ;

the transition from conduction band the transition from conduction band to valence band involves essentially to valence band involves essentially no change in momentumno change in momentum..

Photons, it turns out, possess a fair Photons, it turns out, possess a fair amount of energy ( several eV/photon amount of energy ( several eV/photon in some cases ) but they have very in some cases ) but they have very little momentum associated with little momentum associated with them.them.

Page 8: THE LIGHT EMITTING DIODE

Thus, for a direct band gap material, the excess Thus, for a direct band gap material, the excess energy of the electron-hole recombination can energy of the electron-hole recombination can either be taken away as heat, or more likely, as either be taken away as heat, or more likely, as a photon of light.a photon of light.

This radiative transition then This radiative transition then

conserves energy and momentum conserves energy and momentum

by giving off light whenever an by giving off light whenever an

electron and hole recombine.electron and hole recombine. CB

VB

This gives rise to This gives rise to (for us) a new type (for us) a new type of device;of device;

the light emitting diode (LED).the light emitting diode (LED).

Page 9: THE LIGHT EMITTING DIODE

Mechanism is “injection Mechanism is “injection Electroluminescence”. Electroluminescence”. Luminescence Luminescence part tells us that we are producing photons.part tells us that we are producing photons.

Electro part tells us that Electro part tells us that the photons are being produced the photons are being produced by an electric current.by an electric current.

e-

Injection tells us that Injection tells us that photon production is by photon production is by the injection of current carriers.the injection of current carriers.

Mechanism behind photon Mechanism behind photon emission in LEDs?emission in LEDs?

e-

Page 10: THE LIGHT EMITTING DIODE

Producing photonProducing photon

Electrons recombine with holes.Electrons recombine with holes.

Energy of photon is the energy of Energy of photon is the energy of band gap.band gap.

CB

VB

e-

h

Page 11: THE LIGHT EMITTING DIODE

Method of injectionMethod of injection We need putting a lot of eWe need putting a lot of e--’s where there are lots ’s where there are lots

of holes.of holes. So electron-hole recombination can occur.So electron-hole recombination can occur. Forward biasing a p-n junction will inject lots of eForward biasing a p-n junction will inject lots of e --’s ’s

from n-side, across the depletion region into the p-from n-side, across the depletion region into the p-side where they will be combine with the high side where they will be combine with the high density of majority carriers.density of majority carriers.

n-side

p-side

-

+

I

Page 12: THE LIGHT EMITTING DIODE

Notice that:Notice that: Photon emission occurs whenever we have Photon emission occurs whenever we have

injected minority carriers recombining with the injected minority carriers recombining with the majority carriers.majority carriers.

If the eIf the e- - diffusion length is greater than the hole diffusion length is greater than the hole diffusion length, the photon emitting region will diffusion length, the photon emitting region will be bigger on the p-side of the junction than that be bigger on the p-side of the junction than that of the n-side.of the n-side.

Constructing a real LED may be best to consider Constructing a real LED may be best to consider a na n++++p structure.p structure.

It is usual to find the photon emitting volume It is usual to find the photon emitting volume occurs mostly on one side of the junction region.occurs mostly on one side of the junction region.

This applies to LASER devices as well as LEDs.This applies to LASER devices as well as LEDs.

Page 13: THE LIGHT EMITTING DIODE

MATERIALS FOR LEDSMATERIALS FOR LEDS The semiconductor bandgap The semiconductor bandgap

energy defines the energy of the energy defines the energy of the emitted photons in a LED.emitted photons in a LED.

To fabricate LEDs that can emit To fabricate LEDs that can emit photons from the infrared to the photons from the infrared to the ultraviolet parts of the e.m. ultraviolet parts of the e.m. spectrum, then we must consider spectrum, then we must consider several different material several different material systems.systems.

No single system can span this No single system can span this energy band at present, although energy band at present, although the 3-5 nitrides come close.the 3-5 nitrides come close.

CB

VB

Page 14: THE LIGHT EMITTING DIODE

Unfortunately, many of potentiallly useful 2-6 Unfortunately, many of potentiallly useful 2-6 group of direct band-gap semiconductors group of direct band-gap semiconductors (ZnSe,ZnTe,etc.) (ZnSe,ZnTe,etc.) come naturally doped come naturally doped either p-either p-type, or n-type, but they don’t like to be type-type, or n-type, but they don’t like to be type-converted by overdoping.converted by overdoping.

The material reasons behind this are The material reasons behind this are complicated and not entirely well-known.complicated and not entirely well-known.

The same problem is encountered in the 3-5 The same problem is encountered in the 3-5 nitrides and their alloys InN, GaN, AlN, InGaN, nitrides and their alloys InN, GaN, AlN, InGaN, AlGaN, and InAlGaN. The amazing thing about AlGaN, and InAlGaN. The amazing thing about 3-5 nitride alloy 3-5 nitride alloy systems is that appear to be systems is that appear to be direct gap direct gap throughout.throughout.

Page 15: THE LIGHT EMITTING DIODE

When we talk about light ,it is conventional to When we talk about light ,it is conventional to specify its wavelength, specify its wavelength, λλ, instead of its , instead of its frequency. frequency.

Visible light has a wavelength on the order of Visible light has a wavelength on the order of nanometers. nanometers.

Thus, a semiconductor with a 2 eV band-gap should give a light at about 620 nm (in the red). A 3 eV band-gap material would emit at 414 nm, in the violet.

The human eye, of course, is not equally responsive to all colors.

( )( )

hcnm

E eV

1242( )

( )nm

E eV

Page 16: THE LIGHT EMITTING DIODE

Relative response of the human eye to various colors

350 400 450 500 550 600 650 700 750

100

10-1

10-2

10-3

10-4

Relative eye responseRelative eye response

Wavelength in nanometers

The materials which are used for important light emitting diodes (LEDs) for each of the different spectral regions.

GaN

GaN

Zn

Se

Zn

Se

violet blue

GaP

:NG

aP:Ngreen yellow

GaA

sG

aAs .1

4.1

4pp868

6

GaA

sG

aAs .3

5.3

5pp656

5

redorange

GaA

sG

aAs .6.6

pp44

Page 17: THE LIGHT EMITTING DIODE

Properties of InGaNProperties of InGaN

InGaN alloy has one composition at a time only.InGaN alloy has one composition at a time only. This material This material will emit one wavelengthwill emit one wavelength only only

corresponding to this particular composition.corresponding to this particular composition. An InGaN LED would An InGaN LED would not emit white light not emit white light (the (the

whole of the visible spectrum at once) since its whole of the visible spectrum at once) since its specific compositionspecific composition. .

For a For a white light sourcewhite light source we have to form a we have to form a complicated multilayercomplicated multilayer device device emitting lots of emitting lots of different wavelengths. different wavelengths.

Page 18: THE LIGHT EMITTING DIODE

Properties of InGaNProperties of InGaN

A LED fabricated A LED fabricated in a in a graded materialgraded material where on either side of the junction region where on either side of the junction region the the material changes slowly from InN to material changes slowly from InN to GaNGaN via InGaN alloys. via InGaN alloys.

Minority carriers need to get through the Minority carriers need to get through the whole of this alloy region if efficient photon whole of this alloy region if efficient photon production at all visible wavelengths was production at all visible wavelengths was to occur. to occur.

Page 19: THE LIGHT EMITTING DIODE

GaNGaN InNInN

Concentration:Concentration:

The highly The highly gallium rich gallium rich

alloyalloy

The highly The highly indium rich indium rich

alloyalloy

Band gap:Band gap: 3.3eV3.3eV 2 eV2 eV

Wavelength of Wavelength of photons:photons:

376 nm376 nm 620 nm620 nm

Part of the Part of the electromagnetic electromagnetic spectrum:spectrum:

In the ultravioletIn the ultraviolet In the visible In the visible (orange)(orange)

Page 20: THE LIGHT EMITTING DIODE

GaNInN

3.3 eV(376 nm)ultraviolet

Page 21: THE LIGHT EMITTING DIODE

GaNInN

3 eV (414 nm)violet

Page 22: THE LIGHT EMITTING DIODE

GaNInN

2.7 eV(460 nm)

Page 23: THE LIGHT EMITTING DIODE

GaNInN

2.4 eV(517 nm)

Page 24: THE LIGHT EMITTING DIODE

GaNInN

2.1 eV(591 nm)

Page 25: THE LIGHT EMITTING DIODE

GaN

InN

2.00 eV

2 eV(620 nm)

Page 26: THE LIGHT EMITTING DIODE

A number of the important LEDs are based on the GaAsP system.

GaAsGaAs is a direct band-gap S/C with a band gap of 1.42 eV1.42 eV (in the infrared).

GaPGaP is an indirect band-gap material with a band gap of 2.26 eV2.26 eV (550nm, or green).

GaAsGaP

1.42 eV

Page 27: THE LIGHT EMITTING DIODE

GaAsGaP

1.52 eV

Page 28: THE LIGHT EMITTING DIODE

GaAsGaP

1.62 eV

Page 29: THE LIGHT EMITTING DIODE

GaAsGaP

1.72 eV

Page 30: THE LIGHT EMITTING DIODE

GaAsGaP

1.80 eV

Page 31: THE LIGHT EMITTING DIODE

GaAsGaP

1.90 eV

Page 32: THE LIGHT EMITTING DIODE

GaAsGaP

2.00 eV

Page 33: THE LIGHT EMITTING DIODE

GaAsGaP

2.26 eV

Page 34: THE LIGHT EMITTING DIODE

_

h

+

En

erg

y

Momentum

• Addition of a nitrogen recombination center to indirect GaAsP .

Both As and P are group V elements. (Hence the nomenclature of the materials as III-V compound semiconductors.)

Page 35: THE LIGHT EMITTING DIODE

We can replace some of the As with P in GaAs and make a mixed compound semiconductor GaAs1-xPx.

When the mole fraction of phosphorous is less than about 0.45 the band gap is direct, and so we can "engineer" the desired color of LED that we want by simply growing a crystal with the proper phosphorus concentration!

Page 36: THE LIGHT EMITTING DIODE

X CB

Minimum

Γ VB

Maximum

N Level

Γ CB

Minimum

(a) Direct-gap GaAs

N Level

(b) Crossover GaAs0.50P0.50

N Level

(c) Indirect-gap GaP

Schematic band structure of GaAs, GaAsP, and GaP. Also shown is the nitrogen level. At a P mole fraction of about 45-50 %, the direct-indirect crossover occurs.

Page 37: THE LIGHT EMITTING DIODE

Materials for visible wavelength LEDsMaterials for visible wavelength LEDs

We see them almost everyday, either on calculator We see them almost everyday, either on calculator displays or indicator panels.displays or indicator panels.

Red LED use as “ power on” indicatorRed LED use as “ power on” indicator Yellow, green and amber LEDs are also widely available Yellow, green and amber LEDs are also widely available

but very few of you will have seen a blue LED.but very few of you will have seen a blue LED.

Page 38: THE LIGHT EMITTING DIODE

Red LEDsRed LEDs

can be made in the GaAsP can be made in the GaAsP (gallium arsenide phosphide). (gallium arsenide phosphide).

GaAsGaAs1-x1-xPPxx

for 0<x<0.45 has direct-gapfor 0<x<0.45 has direct-gap for x>0.45 the gap goes for x>0.45 the gap goes

indirect andindirect and for x=0.45 the band gap for x=0.45 the band gap

energy is 1.98 eV.energy is 1.98 eV. Hence it is useful for red Hence it is useful for red

LEDs.LEDs.

N-GaAs substrate

N-GaAsP P = 40 %

p-GaAsP region

Ohmic Contacts

Dielectric(oxide or nitride)

Fig. GaAsP RED LED on a GaAs sub.

Page 39: THE LIGHT EMITTING DIODE

Isoelectronic CentreIsoelectronic Centre

IsoelectronicIsoelectronic means that t means that the centre being introduced he centre being introduced has the has the same number of valance electrons same number of valance electrons as the element it is as the element it is replacing.replacing.

For example, nitrogen can replace some of the phosphorus For example, nitrogen can replace some of the phosphorus in GaP. It is isoelectronic with phosphorus, but in GaP. It is isoelectronic with phosphorus, but behaves behaves quite differentlyquite differently allowing reasonably efficient green allowing reasonably efficient green emission.emission.

Page 40: THE LIGHT EMITTING DIODE

How isoelectronic centres work?How isoelectronic centres work?

For our isoelectronic centre For our isoelectronic centre the the position is very well-position is very well-defined,defined, hence there is a hence there is a considerable considerable spread in its spread in its momentum momentum state.state.

Isoelectronic centre has the Isoelectronic centre has the same valance configuration same valance configuration as the phosphorus it is as the phosphorus it is replacing.replacing.

It doesn't act as a dopantIt doesn't act as a dopant..

E

dE

Isoelectroniccentre

CB edge

electrons

Electron-hole recombination

Holes

VB edge

k = 0

Page 41: THE LIGHT EMITTING DIODE

Isoelectronic centres provide a ‘Isoelectronic centres provide a ‘stepping stonestepping stone’ for ’ for electrons in E-k space so that transitions can occur electrons in E-k space so that transitions can occur that are radiatively efficient.that are radiatively efficient.

The recombination event The recombination event shown has no change in shown has no change in momentum, so it behaves momentum, so it behaves like a direct transition. like a direct transition.

E

dE

Isoelectroniccentre

CB edge

electrons

Electron-hole recombination

Holes

VB edge

k = 0

Because the effective transition is occurring between Because the effective transition is occurring between the isoelectronic centre and VB edge, the photon that the isoelectronic centre and VB edge, the photon that is emitted has a lower energy than the band-gap is emitted has a lower energy than the band-gap energy.energy.

Page 42: THE LIGHT EMITTING DIODE

GaP : NGaP : N

(dE = 50 meV) Photon energy (dE = 50 meV) Photon energy is less than the semiconductor is less than the semiconductor band-gap energy it means that band-gap energy it means that the photon is not absorbed by the photon is not absorbed by the semiconductor, and so the the semiconductor, and so the photon is easily emitted from photon is easily emitted from the material.the material.

This lack of absorption pushes This lack of absorption pushes up the efficiency of the diode up the efficiency of the diode as a photon source.as a photon source.

E

dE

Isoelectroniccentre

CB edge

electrons

Electron-hole recombination

Holes

VB edge

k = 0

50 meV

Page 43: THE LIGHT EMITTING DIODE

For emission in the red part of the spectrum using GaP For emission in the red part of the spectrum using GaP the isoelectronic centre introduced contathe isoelectronic centre introduced contaiins zinc (Zn) and ns zinc (Zn) and oxygen (O). These red LEDs are usually designated oxygen (O). These red LEDs are usually designated GaP:ZnO and they are quite efficient. GaP:ZnO and they are quite efficient.

Their main drawback is that their emission at 690 nm is in Their main drawback is that their emission at 690 nm is in a region where the eye sensitivity is rather low, which a region where the eye sensitivity is rather low, which means that commercially, the AlGaAs/GaAs diodes are means that commercially, the AlGaAs/GaAs diodes are more successful devices.more successful devices.

Page 44: THE LIGHT EMITTING DIODE

Orange (620 nm) and yellow (590 nm) LEDs are Orange (620 nm) and yellow (590 nm) LEDs are commercially made using the GaAsP system. However, commercially made using the GaAsP system. However, as we have just seen above, the required band-gap as we have just seen above, the required band-gap energy for emission at these wavelengths means the energy for emission at these wavelengths means the GaAsP system will have an indirect gap. GaAsP system will have an indirect gap.

The isoelectronic centre used in this instance is nitrogen, The isoelectronic centre used in this instance is nitrogen, and the different wavelengths are achieved in these and the different wavelengths are achieved in these diodes by altering the phosphorus concentration.diodes by altering the phosphorus concentration.

The green LEDs (560 nm) are manufactured using the The green LEDs (560 nm) are manufactured using the GaP system with nitrogen as the isoelectronic centre.GaP system with nitrogen as the isoelectronic centre.

Orange-Yellow & Green LEDsOrange-Yellow & Green LEDs

Page 45: THE LIGHT EMITTING DIODE

Blue LEDsBlue LEDs

Blue LEDs are commercially available and are fabricated Blue LEDs are commercially available and are fabricated using silicon carbide (SiC). Devices are also made using silicon carbide (SiC). Devices are also made based on gallium nitride (GaN).based on gallium nitride (GaN).

Unfortunately both of these materials systems have Unfortunately both of these materials systems have major drawbacks which render these devices inefficient.major drawbacks which render these devices inefficient.

The reason silicon carbide has a low efficiency as an The reason silicon carbide has a low efficiency as an LED material is that it has an indirect gap, and no ‘magic’ LED material is that it has an indirect gap, and no ‘magic’ isoelectronic centre has been found to date.isoelectronic centre has been found to date.

Page 46: THE LIGHT EMITTING DIODE

Blue LEDsBlue LEDs

The transitions that give rise to blue photon emission in The transitions that give rise to blue photon emission in SiC are between the bands and doping centres in the SiC are between the bands and doping centres in the SiC. The dopants used in manufacturing SiC LEDs are SiC. The dopants used in manufacturing SiC LEDs are nitrogen for n-type doping, and aluminium for p-type nitrogen for n-type doping, and aluminium for p-type doping. doping.

The extreme hardness of SiC also requires extremely The extreme hardness of SiC also requires extremely high processing temperatures.high processing temperatures.

Page 47: THE LIGHT EMITTING DIODE

Gallium Nitride (GaN)Gallium Nitride (GaN)

Gallium nitride has the advantage of being a direct-gap Gallium nitride has the advantage of being a direct-gap semiconductor, but has the major disadvantage that bulk semiconductor, but has the major disadvantage that bulk material cannot be made p-type.material cannot be made p-type.

GaN as grown, is naturally nGaN as grown, is naturally n++ . ++ .

Light emitting structures are made by producing an Light emitting structures are made by producing an intrinsic GaN layer using heavy zinc doping. Light intrinsic GaN layer using heavy zinc doping. Light emission occurs when electrons are injected from an nemission occurs when electrons are injected from an n+ +

GaN layer into the intrinsic Zn-doped region.GaN layer into the intrinsic Zn-doped region.

Page 48: THE LIGHT EMITTING DIODE

A possible device structure is A possible device structure is shown in fig.shown in fig.

Unfortunately, the recombination Unfortunately, the recombination process that leads to photon process that leads to photon production involves the Zn production involves the Zn impurity centres, and photon impurity centres, and photon emission processes involving emission processes involving impurity centres are much less impurity centres are much less efficient than band-to-band efficient than band-to-band processes.processes.

SapphireSubstrate

(transparent)

n + GaN

i-GaN

Ohmic Contacts

Dielectric(oxide or nitride)

Fig. Blue LED

Blue photons

Page 49: THE LIGHT EMITTING DIODE

It is generally true to say that if we order the photon It is generally true to say that if we order the photon producing processes (in semiconductors) in terms of producing processes (in semiconductors) in terms of efficiency, we would get a list like the one below.efficiency, we would get a list like the one below. band-to-band recombination in direct gap material,band-to-band recombination in direct gap material, recombination via isoelectronic centres,recombination via isoelectronic centres, rreecombination via impurity (not isoelectronic) centres,combination via impurity (not isoelectronic) centres, band-to-band recombination in indirect-gap materials.band-to-band recombination in indirect-gap materials.

So, the current situation is that we do have low-efficiency So, the current situation is that we do have low-efficiency blue LEDs commercially available. We are now awaiting blue LEDs commercially available. We are now awaiting a new materials system, or a breakthrough in GaN or a new materials system, or a breakthrough in GaN or SiC technology, for blue LEDs of higher brightness and SiC technology, for blue LEDs of higher brightness and higher efficiency to be produced.higher efficiency to be produced.

Page 50: THE LIGHT EMITTING DIODE

Color NameColor Name WavelengthWavelength(Nanometers)(Nanometers)

SemiconductorSemiconductorCompositionComposition

InfraredInfrared 880880 GaAlAs/GaAsGaAlAs/GaAs

Ultra RedUltra Red 660660 GaAlAs/GaAlAsGaAlAs/GaAlAs

Super RedSuper Red 633633 AlGaInPAlGaInP

Super OrangeSuper Orange 612612 AlGaInPAlGaInP

OrangeOrange 605605 GaAsP/GaPGaAsP/GaP

YellowYellow 585585 GaAsP/GaPGaAsP/GaP

IncandescentIncandescentWhiteWhite 4500K (CT)4500K (CT) InGaN/SiCInGaN/SiC

Pale WhitePale White 6500K (CT)6500K (CT) InGaN/SiCInGaN/SiC

Cool WhiteCool White 8000K (CT)8000K (CT) InGaN/SiCInGaN/SiC

Pure GreenPure Green 555555 GaP/GaPGaP/GaP

Super BlueSuper Blue 470470 GaN/SiCGaN/SiC

Blue VioletBlue Violet 430430 GaN/SiCGaN/SiC

UltravioletUltraviolet 395395 InGaN/SiCInGaN/SiC

Page 51: THE LIGHT EMITTING DIODE

MaterialMaterial Wavelength Wavelength (µm)(µm) MaterialMaterial Wavelength Wavelength

(µm)(µm)

ZnS ZnS ZnOZnOGanGanZnSeZnSeCdSCdSZnTeZnTeGaSeGaSeCdSeCdSeCdTeCdTe

0.33 0.33 0.370.370.400.400.460.460.490.490.530.530.590.59

0.6750.6750.7850.785

GaAsGaAsInPInP

GaSbGaSbInAsInAsTeTe

PbSPbSInSbInSbPbTePbTePbSePbSe

0.84-0.950.84-0.950.910.911.551.553.13.1

3.723.724.34.35.25.26.56.58.58.5