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 (

E g

or

E

 ). Some are indirect bandgap materials with indirect bandgap energy (

E X

or

E L

)

Solid line: direct bandgap materials Dotted line: indirect bandgap materials Matched system to reduce the strain effect and epitaxial growth defects!

III-V Material systems with important optoelectronic applications

Materials substrate Lattice matched Important strained Main optoelectronic system members members applications AlGaAs GaAs GaAs Ga 1-

x

In

x

As Emitter and modulators Al

x

Ga 1-

x

As 0 

x

 0.25 0.75  m    1.1  m AlAs Detectors: 0.4  m    1.1  m GaInAsP InP Ga 0.47

In 0.53

As Ga 1-

x

In

x

As Optoelectronic devices /InP Ga

x

In 1-

x

As

y

P 1-

y

0.4 

x x

  0.6 at 0.2

  = 1.3  m and = 1.55  m AlGaInAs InP Ga 0.47

In 0.53

As Ga 1-

x

In

x

As Optoelectronic devices /InP (Al

x

Ga 1-

x

) 0.47

In 0.53

As 0.4  0 

x



x

 0.6 at 1   = 1.3  m and = 1.55  m Al 0.48

In 0.52

As AlGaInP GaAs GaAs Ga 1-

x

In

x

As Red emitter Ga 0.5

In 0.5

P 0 

x

 0.25

(Al

x

Ga 1-

x

) 0.5

In 0.5

P Ga 1-

x

In

x

P 0 

x

 1 0.4 

x

 0.6

III-V Material systems with important optoelectronic applications

Materials substrate Lattice matched Important strained Main optoelectronic system members members applications AlGaAsSb GaSb GaSb Emitter and Detectors /GaInAsSb Al

x

Ga 1-

x

As

y

Sb 1-

y

/GaSb

x

= 12

y

; 0 

x

 1  ~ 2-3  m Al

x

In 1-

x

As

y

Sb 1-

y x

= 1.1

y

; 0 

x

 1 GaAsP GaAs GaAs GaAsP Visible LED’s InP GaP  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: t ~ 1ms (for Si,Ge), t ~ 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 Epitaxy Material Characterization Device Processing Device Testing Device Packaging Reliability Screening Key Technology areas Final Packaged Device

 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: h n p n +V -V Holes Electrons Active Layer (e.g. GaAs) Cladding Layers (e.g. GaAlAs) Conduction band E fn h n E fn Valence band Double heterostructure p-n junction.

Energy E = h n or E(eV) = 1.24/  (  m) Refractive Index

 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 Light output n p type epitaxial layer Dome LED ohmic contacts diffused p-type n type substrate Planar LED ohmic contacts  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.

Burrus LED Metal tab Multimode optical fiber Epoxy resin ~ 250  m 250  m 50  m Etched well 50  m SiO 2 n-AlGaAs p-GaAs p-AlGaAs p + -GaAs Gold stud Metal contact ~ 50  m Primary light emitting region n-GaAs substrate Metal contact SiO 2 n-AlGaAs Edge-emitting p + -AlGaAs LED p-AlGaAs AlGaAs (Active layer) n-GaAs Carrier confinement layers : p-AlGaAs and n-AlGaAs

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

60 T Output power is typically linear with the drive current.

Advantages of these devices: ease of modulation, long lifetime, low cost, and high yield 0 0 Current (mA) 100 * Semiconductor Lasers: Laser has a much narrower spectral range and a much more intense light output (at least 100 times more intense than LED) Diode Laser LED < 3 nm ~ 75 nm Wavelength (nm)

 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.

7 Slope gives external efficiency  T  Lasing emission 0 0 I th Current (mA) 40 Spontaneous emission  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: Classical buried heterostructure (BH) Laser Double channel planar burried heterostructure (DCPBH) Laser Distributed feedback (DFB)laser Simple oxide stripe DH (double Heterostructure)

 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.

h n -V p n +V Applying a suitable reverse bias voltage to a simple p-n junction => create an electric field profile => separation the photo-generated electron-hole pairs (absorption of light within the semiconductor).

Distance (x) Speed of response is determined by the device capacitance => governed by the thickness of depletion region => small area device and low doped active 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 materials AlAs: indirect bandgap materials

E

 : direct bandgap;

E

X : indirect bandgap

E

 ( eV )  1 .

423  1 .

36

x

 0 .

22

x

2

E

X ( eV )  1 .

906  0 .

207

x

 0 .

55

x

2 For effective light emission the

x

< 0.4in Ga

x

Al 1-

x

As Refractive index of Ga

x

Al 1-

x

As

First Brillouin zone of diamond structure k z k x X X  L X k y

 Ga

x

In 1-

x

As

y

P 1-

y

Materials system: RT

E

g and refractive index of Ga

x

In 1-

x

As

y

P 1-

y

for

x

= 0.47

y

, 0 

y

 1; (lattice matched to InP!)

RT absorption spectra of Ga 0.47

In 0.53

As, GaInAsP/InP ( 1.3

 m), and InP. Solid lines: experimental data; dashed lines: a fit to  = constant  (

E

-

E g

) 1/2 .

 1/2 

g

= indirect h 

 AlGaInAs/InP Materials system: RT

E

g of Al

x

Ga 1

-x

In 0.53

As

E g

( eV )  0 .

76  0 .

49

x

 0 .

2

x

2 RT refractive index of Al

x

Ga 1

-x

In 0.53

As

 AlGaInP Materials system: main application is red diode lasere

E

X : indirect

E

 : direct RT

E

g , refractive index, and absorption coefficient of (Al

x

Ga 1

-x

) 0.52

In 0.48

As (match GaAs)

E

 ( eV ) 

E

X ( eV )  1 .

89 2 .

25   0 .

64 0 .

09

x x

2.5

2.0

Solid lines: Al x Ga 1-x As y Sb 1-y ;

y

= 0.083

x

Dashed lines: Ga 1-x In x As y Sb 1-y ;

y

= 0.911

x

Al x Ga 1-x As y Sb 1-y :

E

 1.5

1.0

0.5

0.0

0.0

 GaSb Materials system: two quaternaries, AlGaAsSb, GaInAsSb lattice matched to GaSb substrate  Al

x

Ga 1-

x

As

y

Sb 1-

y

= (GaSb)

x

(AlAs 0.083

Sb 0.917

) 1-

x

 Ga 1-

x

In

x

As

y

Sb 1-

y

= (GaSb) 1-

x

(InAs 0.911

Sb 0.089

)

x

Al x Ga 1-x As y Sb 1-y :

E

X Al x Ga 1-x As y Sb 1-y :

E

L Ga 1-x In x As y Sb 1-y :

E

X 0.2

Ga 1-x In x As y Sb 1-y :

E

L Ga 1-x In x As y Sb 1-y :

E

 0.4

0.6

composition, x 0.8

1.0

RT

E

g of Al

x

Ga 1-

x

As

y

Sb 1-

y

and Ga 1-

x

In

x

As

y

Sb 1-

y

; at RT and

x

(indirect) >  = 0,

E

L

E

 => GaSb is barely a direct bandgap materials!

Al

x

Ga 1-

x

As

y

Sb 1-

y

is nearly indirect in the whole range!

RT refractive index of Al

x

Ga 1-

x

As

y

Sb 1-

y

/GaSb RT refractive index of Ga 1-

x

In

x

As

y

Sb 1-

y

/GaSb

 GaAsP Materials system: RT

E

g of GaAs

x

P 1-

x

; Crossover of direct indirect is ~

x

= 0.5

E g

( eV )  1 .

428  1 .

125

x

 1 .

952

x

2 Absorption coefficient of GaAs

x

P 1-

x

 II-VI semiconductor materials:  Wide spectrum of energy gaps (

E

g ) => wide range of optoelectronic properties; ranging from the far infrared to the UV  Large

E

g 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 B semiconductor heterostructures; e.g.

A AB graded ZnSe/Zn 0.9

Fe 0.1

Se, magnetic field => conduction band and valence band are different for carriers with different angular momentum!

DMS ZnSe DMS -1/2 +1/2 -3/2 hh Type II semiconductor +3/2 Type I semiconductor  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 SiC and 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) SiC 6H SiC 2H A B C A B C A A B C A B  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 Seed SiC 1800 o C

dT

/

dx

~ 20 K/cm pressure gradients to form high Poly SiC 2000 o C 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.

 SiC LEDs: Spectrum of SiC LED.

 SiC Photodiodes: Temperature dependent responsivity of a SiC photodiode.

SiC efficiency as a function of bias.

 GaN:  Grown by CVD, MOCVD, ECR-CDV, etc.

 The best substrate is sapphire (Al 2 O 3 ). 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 Nichia InGaN/GaN LED Output power and quantum efficiency of Nichia GaN LED

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

Important semiconductor materials for optoelectronics Materials Type Substrate Devices Wavelength range(  m) Si SiC Ge GaAs AlGaAs IV IV IV III-V III-V GaInP GaAlInP GaP GaAsP InP InGaAs InGaAsP InAlAs InAlGaAs III-V III-V III-V III-V III-V III-V III-V III-V III-V GaSb/GaAlSb CdHgTe ZnSe II-VI II-VI] II-VI ZnS II-VI Si SiC Ge GaAS GaAS GaAs GaAS GaP GaP InP InP InP InP InP GaSb CdTe ZnSe ZnS Detectors, Solar cells Blue LEDs Detectors LEDs, Lasers, Detectors, Solar Cells, Imagers, Intensifiers LEDs, Lasers, Solar Cells, Imagers Visible Lasers, LEDs Visible Lasers, LEDs Visible LEDs Visible LEDs Solar Cells Detectors Lasers, LEDS Lasers, Detectors Lasers, Detectors Lasers, Detectors Long wavelength Detectors Short wavelength LEDs Short wavelength LEDs 0.5-1 0.4

1-1.8

0.85

0.67-0.98

0.5-0.7

0.5-0.7

0.5-0.7

0.5-0.7

0.9

1-1.67

1-1.6

1-2.5

1-2.5

2-3.5

3-5 and 8-12 0.4-0.6

0.4-0.6

Commercial Applications of Optoelectronic Devices Materials Devices Applications GaAs/AlGaAs InP/InP InP/InGaP InP/InGaAs InGaAlAs/InGaAs GaAs/GaInP/ GaInAlP Detectors, Infrared LEDs and Lasers Solar cell Infrared LEDs, Lasers (1-1.6

 m) 1-1.67

 m Detectors 1.67-2.4

 m Detectors 0.5-0.7

 m LEDs and Lasers Si Ge SiC GaSb/GaAlSb/InSb ZnSe/ZnS Detectors and Solar Cells Detectors Blue LEDs Long wavelength detectors/smitters Visible LEDs Remote control TV, etc., video disk players, range-finding, solar energy conversion, optical fiber communication systems (local networks), image intensifiers Space solar cell Optical fiber communications (long-haul and local loop) Optical fiber communications, instrumentation Military applications, medicine, sensor Displays, control, compact disk players, laser printers/scanners, optical disk memories, laser medicine equipment Solar energy conversions, e.g. watches, calculators, cooling, heating, detectors Detectors Displays, optical disk memories, etc.

Infrared imaging, night vision sights, missile seekers, other military applications Commercial applications (R&D stages only)