Part 3. Semiconductor Materials for Optoelectronic Application

The major semiconductor materials used for optoelectronic applications are III-V and II-V group. Group V materials are used in some cases, but mostly in indirect applications. The reason for group III-V to be popular in optoelectronic applications is due to the fact that most III-V materials are direct bandgap semiconductors, which is an necessary condition for efficient conversion of electric energy to light emission. The integration of light source with the photonic devices is desired => photonic devices of III-V system.

III-V semiconductor materials: Column III elements: Al, Ga, and In and column V elements: N, P, As, Sb. In general, nitride is not included in typically claimed III-V compound semiconductor (discussed separately). The main optical process of importance in optoelectronic applications are reflection, waveguiding, diffraction, absorption, emission, and electrooptics and nonlinear optical effect! parameters to express above properties are refractive index (n), absorption coefficient (), and direct bandgap energy (Eg or E). Some are indirect bandgap materials with indirect bandgap energy (EX or EL)

Solid line:direct bandgapmaterials

Dotted line:indirect bandgapmaterials

Matched system toreduce the straineffect and epitaxialgrowth defects!

Materials substrate Lattice matched Important strained Main optoelectronicsystem members members applications

III-V Material systems with important optoelectronic applications

AlGaAs GaAs GaAs Ga1-xInxAs Emitter and modulators AlxGa1-xAs 0 x 0.25 0.75 m 1.1 m AlAs Detectors: 0.4 m 1.1 mGaInAsP InP Ga0.47In0.53As Ga1-xInxAs Optoelectronic devices/InP GaxIn1-xAsyP1-y 0.4 x 0.6 at = 1.3 m and

x=0.47y; 0 y 1 InAsxP1-x = 1.55 m InP 0 x 0.2AlGaInAs InP Ga0.47In0.53As Ga1-xInxAs Optoelectronic devices/InP (AlxGa1-x)0.47In0.53As 0.4 x 0.6 at = 1.3 m and 0 x 1 = 1.55 m Al0.48In0.52AsAlGaInP GaAs GaAs Ga1-xInxAs Red emitter Ga0.5In0.5P 0 x 0.25 (AlxGa1-x)0.5In0.5P Ga1-xInxP 0 x 1 0.4 x 0.6

AlGaAsSb GaSb GaSb Emitter and Detectors/GaInAsSb AlxGa1-xAsySb1-y ~ 2-3 m /GaSb x = 12y; 0 x 1 AlxIn1-xAsySb1-y

x = 1.1y; 0 x 1

GaAsP GaAs GaAs GaAsP Visible LED’s InP GaP

Materials substrate Lattice matched Important strained Main optoelectronicsystem members members applications

III-V Material systems with important optoelectronic applications

Quantum Wells and Strained Materials: The Optical properties of a semiconductor are altered by quantum size effects; at least one of the dimensions of material is on the order of De Broglie’s wavelength of an electron: = h/p; if p ~ eV =>

= ~ a few nm; 1D confinement: quantum wells; structures consisting of a thin well materials sandwiched between two layers of a barrier materials 2D confinement: quantum wires; structures consisting of a thin and narrow well materials surrounded by barrier materials 3D confinement: quantum dots; nano-size particles in a barrier materials. The quantum confinement => allowed electron and hole states are quantized in the well region => energy required to generate e-h pair or radiation emitted from the process of e-h pair recombination is modified => wavelength tuning of the radiation (used in LED or laser applications) A B

As a light source, the efficiency of the source is strongly influenced by defects (line defects, etc) => crystal A and B should be grown as perfect as possible (epitaxial system) Typically, A and B will not have the same lattice constant => strained system => stability issue of the system (relaxation and chemical aspect) and critical thickness for the epitaxial growth become important! Strained epitaxial semiconductors can be used in high speed electronic applications: HEMTs, HBTs! Typically, the resonator have to be constructed with the same kind of material system used for light source (for possible epitaxial relation) => easier to integrate without affecting the quality of the active materials.

Optical efficiency determined by (a) bandgap type: direct or indirect (b) minority carrier lifetime: ~ 1ms (for Si,Ge), ~ 1ns (for GaAs) Introduction of impurities in some indirect bandgap materials=> efficient recombination; e.g. isoelectronic N impurities in GaP Epitaxy: process of depositing thin layers of single- crystal compounds onto a single crystal substrate.

Substrate EpitaxyMaterial

CharacterizationDevice

Processing

Device Testing

DevicePackaging

ReliabilityScreening

Final PackagedDevice

Key Technology areas

Epitaxial growth techniques: liquid phase epitaxy (LPE) (for low end product); metal organic vapor phase epitaxy (MOVPE), and molecular beam epitaxy (MBE).

Light-Emitting Devices:

+V -V

ElectronsHoles

p n

Cladding Layers (e.g. GaAlAs)

Active Layer(e.g. GaAs)

h

Double heterostructure p-njunction.

Energy E = h orE(eV) = 1.24/(m)

Efn

Efn

h

Conductionband

Valenceband

RefractiveIndex

Spontaneous emission: random recombination of electrons and holes => light is emitted at a wavelength corresponding to the band energy, but random in phase => light emitting diodes (LEDs). Stimulated emission: a photon of light traveling through the semiconductor interacts with the electron and hole population to cause the radiative recombination of another electron-hole pair => light is emitted at a wavelength corresponding to the band energy with the same phase => lasers. Light is absorbed within the semiconductor to produce electron-hole pairs => photodetectors. Light with energy smaller than the bandgap of the semiconductor will not be absorbed.

Structures of LED (Important ones):Light output

n

diffusedp-type

ohmiccontacts

n typesubstrate

p typeepitaxial layer

ohmic contacts

Light output

Dome LED

Planar LED

Dome and planar LED are used in most display devices where the interest is in extracting the maximum amount of light from the device. => light is emitted in all directions and using a lens arrangement to focus the light. Burrus and edge-emitting LED are used mainly in optical fiber communication systems.

Epoxyresin

Multimodeoptical fiber

Metal tab

Gold stud

Metal contact~ 50 m Primary light-

emitting region

Etchedwell

50m

SiO2

n-AlGaAsp-GaAsp-AlGaAsp+-GaAs

Burrus LED 250 m~ 250 m

50 m

Metal contact

SiO2

Carrier confinement layers : p-AlGaAs and n-AlGaAs

p+-AlGaAsp-AlGaAsAlGaAs (Active layer)n-AlGaAsn-GaAs

n-GaAs substrate

Edge-emittingLED

Current (mA)0 100Opt

ical

Pow

er O

utpu

t (W

)

0

60

TOutput power is typically linearwith the drive current.

Advantages of these devices: ease ofmodulation, long lifetime, low cost,and high yield

Operation: apply a suitable voltage (5V) => a forward current of between 5 and 100 mA.

* Semiconductor Lasers:

Rel

ativ

e op

tical

pow

er

Wavelength (nm)

< 3 nm

~ 75 nmLED

DiodeLaser

Laser has a much narrowerspectral range and a muchmore intense light output (atleast 100 times more intensethan LED)

Create a lasing cavity that acts as an extremely high Q resonator. The cavity is usually created by the formation of mirrors at each end of the laser device. Mirrors: cleaving along the crystallographic plane [(110)] => abrupt refractive index change at the semiconductor-air interface (refreactivity ~ 0.33). The semiconductor between the two mirrors forms the laser cavity. A high rate of stimulated emission => optical gain g. For lasing to occur a further threshold condition must be met which is that the round trip gain of a photon is greater than unity.

Current (mA)

Lig

ht o

utpu

t (m

W) 7

00 40Ith Spontaneous

emission

Lasingemission

T Slope gives

external efficiency

In general, a reduction in the threshold current, an increase in the total light output, and an increase in the external quantum efficiency all leads to improved lasing devices

Some Laser Structures:

Simple oxide stripe DH (double Heterostructure)

Classical buried heterostructure

(BH) Laser

Double channelplanar burriedheterostructure(DCPBH) Laser

Distributedfeedback

(DFB)laser

Present designs are almost exclusively the double heterostructure type with many variations used to constrain the device to operate in a single lateral (transverse) mode. DFB: single longitudinal mode operation can also be achieved in order to obtain extremely narrow linewidth emission by using diffraction gratings placed adjacent to the active layer of the laser

* Optical detectors: A device that changes its properties by the absorption of light. A great variety of different types of optical detector ranging from thermal and pneumatic detectors to pyroelectric detectors. The most important devices are semiconductor photodiodes.

+V-V

p nh

Ele

ctri

c fi

eld

Distance (x)

Applying a suitable reverse biasvoltage to a simple p-n junction=> create an electric field profile=> separation the photo-generatedelectron-hole pairs (absorption oflight within the semiconductor).

Speed of response is determined bythe device capacitance => governedby the thickness of depletion region=> small area device and low dopedactive regions for low capacitance,i.e. high speed.

Noise: low noise could be obtained by minimizing any leakage current (typically surface leakage current) by having a large band gap materials on the surface.

Integration of different materials in a single chip is challenging. A lot of issues are related to materials science: such as selective area epitaxy (SAE), ion beam etching for the formation of laser cavities and waveguide components, the depositing of insulating materials, and metallization methodology .

AlGaAs Materials system:

GaAs: direct bandgap materialsAlAs: indirect bandgap materials

For effective light emission the x < 0.4in GaxAl1-xAs

222.036.1423.1)eV( xxE

E: direct bandgap; EX: indirect bandgap

2X 55.0207.0906.1)eV( xxE

Refractive index of GaxAl1-xAs

First Brillouin zone of diamond structure

kx

kz

ky

X

XX

L

GaxIn1-xAsyP1-y Materials system:

RT Eg and refractive indexof GaxIn1-xAsyP1-y forx = 0.47y, 0 y 1;(lattice matched to InP!)

RT absorption spectra of Ga0.47In0.53As, GaInAsP/InP (g =1.3m), and InP. Solid lines: experimental data; dashed lines:a fit to = constant (E - Eg)1/2.

1/2

h

indirect

AlGaInAs/InP Materials system:

RT Eg of AlxGa1-xIn0.53As

RT refractive index ofAlxGa1-xIn0.53As

22.049.076.0)eV( xxEg

AlGaInP Materials system: main application is red diode lasere

xE 64.089.1)eV(

xE 09.025.2)eV(X

RT Eg, refractive index, andabsorption coefficient of(AlxGa1-x)0.52In0.48As(match GaAs)E : direct

EX : indirect

GaSb Materials system: two quaternaries, AlGaAsSb, GaInAsSb lattice matched to GaSb substrate AlxGa1-xAsySb1-y = (GaSb)x(AlAs0.083Sb0.917)1-x

Ga1-xInxAsySb1-y = (GaSb)1-x(InAs0.911Sb0.089)x

RT Eg of AlxGa1-xAsySb1-y andGa1-xInxAsySb1-y;at RT and x = 0, EL

(indirect) > E =>GaSb is barely a directbandgap materials!AlxGa1-xAsySb1-y is nearly indirect in thewhole range!

0.0 0.2 0.4 0.6 0.8 1.00.0

0.5

1.0

1.5

2.0

2.5

Dashed lines: Ga1-x

InxAs

ySb

1-y; y = 0.911x

Solid lines: AlxGa

1-xAs

ySb

1-y; y = 0.083x

AlxGa

1-xAs

ySb

1-y: E

X

AlxGa

1-xAs

ySb

1-y: E

L

AlxGa

1-xAs

ySb

1-y: E

Ga1-x

InxAs

ySb

1-y:E

L

Ga1-x

InxAs

ySb

1-y:E

X

Ga1-x

InxAs

ySb

1-y:E

Eg (

eV)

composition, x

RT refractive index of AlxGa1-xAsySb1-y/GaSb

RT refractive index of Ga1-xInxAsySb1-y/GaSb

GaAsP Materials system:

RT Eg of GaAsxP1-x;Crossover of direct-indirect is ~ x = 0.5

Absorption coefficientof GaAsxP1-x

2952.1125.1428.1)eV( xxEg

II-VI semiconductor materials: Wide spectrum of energy gaps (Eg) => wide range of optoelectronic properties; ranging from the far infrared to the UV Large Eg difference => large band offset => adds variety and flexibility to bandgap engineering Compare to III-V, II-VI semiconductors have stronger polarity (bonds have more ionic characteristics and less covalent characteristics). Magnetic ions (Mn++ and Fe++) can be easily incorporated => magnetic semiconductors II-VI semiconductors are mainly prepared using MBE or MOCVD.

Energy gaps and lattice constants for cubic group IV,III-V, and II-VI semiconductors.

ZnSe based blue-green LED; the importance is diminishing, due to the successful development of GaN. Material issue is more complicated than group IV and III-V semiconductor; doping is one important issue in II-VI semiconductor; n-type doing is easier than p-type doping (for ZnSe two promising dopants Ga and Cl); the difficulty in doping any II-VI semiconductors arises intrinsically from the size of the energy gap itself, large gap require high energy to shift the Fermi surface => enough to promote compensation through defect formation Ohmic contacts in another major problem.

ideally one wish to use metals with work functions above the bottom of the conduction and on the n-type semiconductor and below the top of the valence band on the p-type semiconductor; as gap => harder to find proper metals => overcome the problem by (a) heavily dope the semiconductor layer to which the contact is to be made; (b) a graded alloy to move the top of the valence band close to the metal workfunction. Diluted magnetic semiconductor heterostructures; e.g. ZnSe/Zn0.9Fe0.1Se, magnetic field => conduction band and valence band are different for carriers with different angular momentum!

A

BAB

graded

DMS DMSZnSe

hh-3/2

-1/2

+3/2

+1/2

Type Isemiconductor

Type IIsemiconductor

So far the real application of II-VI is not that much, however the system provides rich variety in phenomenon for academic studies;

SiC and GaN as optoelectronic Materials: The need to operate devices at high temperature => studies of wide bandgap semiconductors (SiC, GaN, and diamond) The need for denser optical storage (light with shorter wavelength) => blue laser or even UV laser (AlGaN).

Zinc-blende and wurtzite SiCand GaN lattice constants vs.the energy gap.

SiC 2H and SIC 6H:polymorph

SiC: a family of close-packed materials which exhibit a 1-D polymorphism (called polytypism)

A B A BC AC A B A BC

SiC 6H SiC 2H

3C-SiC: cubic zinc-blende structure (3periodicity; C: cubic)

SiC substrate crystal growth: commercial substrates were grown by sublimation growth technique; SiC is transported in the vapor phase to a SiCseed crystal held at a lower temperature Sublimated SiC must diffuse through porous graphite under carefully controlled thermal and pressure gradients to form high quality single crystal 6H SiC SiC thin film epitaxy: Nishino, et. al. => the clean Si substrate is exposed to a C-containing gas at growth temperature => a thin monocrystalline 3C SiC on Si; template for epitaxial SiC growth => abundant defects: antiphase domain boundary, misfit dislocations, microtwins, stacking faults,etc.

Seed SiC

Poly SiC

1800oC

2000oC

dT/dx ~ 20 K/cm

SiC LEDs:

SiC Photodiodes:

SiC efficiency as a functionof bias.

Spectrum of SiC LED.

Temperature dependent responsivity of a SiC photodiode.

GaN: Grown by CVD, MOCVD, ECR-CDV, etc. The best substrate is sapphire (Al2O3). No nitride substrate available => on sapphire a buffer layer were prepared (typically AlN); the buffer were initially amorphous => converts to single crystal during subsequent growth; recently, low temperature GaN as buffer layer was used, but AlN seems to produce better results! Polytypism in nitride: typical structure for GaN, AlN, and InN are wurtzite (2H), metastable zinc-blende structure can also be formed Doping (p-type dopant: Mg)and ohmic contact (Au/Ni, Ti/Al) are important issues for GaN.

GaN-based LEDs:

EL spectra of NichiaInGaN/GaN LED

Output power and quantum efficiency of Nichia GaN LED

Output power comparison of Nichia InGaN, GaN LED, Sanyo SiC and Cree SiC (solid circle) LED.

Refractive indices of important nitrides

Important semiconductor materials for optoelectronicsMaterials Type Substrate Devices Wavelength range(m)

SiSiCGe

GaAs

AlGaAs

GaInPGaAlInP

GaPGaAsP

InPInGaAs

InGaAsPInAlAs

InAlGaAsGaSb/GaAlSb

CdHgTeZnSeZnS

IVIVIV

III-V

III-V

III-VIII-VIII-VIII-VIII-VIII-VIII-VIII-VIII-VII-VIII-VI]II-VIII-VI

SiSiCGe

GaAS

GaAS

GaAsGaASGaPGaPInPInPInPInPInP

GaSbCdTeZnSeZnS

Detectors, Solar cellsBlue LEDsDetectors

LEDs, Lasers, Detectors, SolarCells, Imagers, Intensifiers

LEDs, Lasers,Solar Cells, ImagersVisible Lasers, LEDsVisible Lasers, LEDs

Visible LEDsVisible LEDs

Solar CellsDetectors

Lasers, LEDSLasers, DetectorsLasers, DetectorsLasers, Detectors

Long wavelength DetectorsShort wavelength LEDsShort wavelength LEDs

0.5-10.4

1-1.80.85

0.67-0.98

0.5-0.70.5-0.70.5-0.70.5-0.7

0.91-1.671-1.61-2.51-2.52-3.5

3-5 and 8-120.4-0.60.4-0.6

Commercial Applications of Optoelectronic DevicesMaterials Devices Applications

Remote control TV, etc., video disk players, range-finding, solar energy conversion, optical fiber communication systems (local networks), image intensifiersSpace solar cellOptical fiber communications (long-haul and local loop)Optical fiber communications, instrumentationMilitary applications, medicine, sensorDisplays, control, compact disk players, laser printers/scanners, optical disk memories, laser medicine equipmentSolar energy conversions, e.g. watches, calculators, cooling, heating, detectorsDetectorsDisplays, optical disk memories, etc.Infrared imaging, night vision sights, missile seekers, other military applicationsCommercial applications (R&D stages only)

Detectors, InfraredLEDs and Lasers

Solar cellInfrared LEDs, Lasers (1-1.6m)1-1.67m Detectors1.67-2.4m Detectors0.5-0.7m LEDs and Lasers

Detectors and Solar CellsDetectorsBlue LEDsLong wavelength detectors/smittersVisible LEDs

GaAs/AlGaAs

InP/InPInP/InGaP

InP/InGaAs InGaAlAs/InGaAsGaAs/GaInP/ GaInAlP

Si

GeSiCGaSb/GaAlSb/InSb

ZnSe/ZnS