3.46 optical and optoelectronic materials · extended response of ge-on-si strained epitaxial...

3.46 OPTICAL AND OPTOELECTRONIC MATERIALS Spring 2003 M,W 2:30-4:00pm

Band Gap Engineering: Strain, Composition, and Temperature March 15, 2004

Badgap Engineering: Precise Control of Emission Wavelength

Wavelength Division Multiplexing

Fiber Transmission Window

Optical Amplification Spectrum Design and Fabrication of emitters and detectors

Composition Binary, Ternay, Quaternary (alloy) semiconductors

Quantum size effect Superlattices, Quantum wells, Quantum wires, Quantum dots

Strain effect Lattice mismatch and thermal mismatch

Case studies 1. Emitters 2. EDFA pump light sources 3. Detectors

1



Figure 1 Absorption spectrum of optical fibers Light Sources

0.7eV< Eg < 1eV for networks 1eV< Eg < 2eV for interconnects

2



Figure 2 Bandgap engineering 1 – Alloy composition

1. Bowing parameter Alloy Eg does not follow Vegard’s law (linear)

2. Substrates Alloys lattice matched to GaAs and InP cover the desired Eg

range

3



Figure 4 Lattice-matched InGaAsP on InP for 1.55 and 1.31 m laser

diode.

4



Figure 5 Bandgap engineering 2: Quantum Confinement Dimensions < Bohr orbit in dielectric medium Quantum size effect: En=n2h2/8mL2

Bandgap discontinuity : C= e1- e2 (Anderson’s rule) AlGaAs/GaAs C = 0.65 G (0.85 0.57 0.70 0.65)

5



Figure 6 Erbium Doped Fiber Amplifier (EDFA) Pump Light Sources 1. Pump light source at 980nm

Shorter wavelengths: GaAs Longer wavelengths: InP

2. Ternary alloy requires strain tuning of Eg

Strained In0.2GaAs/GaAs

6



Figure 7 Strain and strain relaxation (dislocations) 1. Critical layer thickness

InxGaAs/GaAs 2. Strained layers

GaAs/GaAs MQW LDs on GaAs substrate

7



Figure 8 Biaxial-strained semiconductor bandgap 1. Deformation potentials

Band extrema 2. Quantum confined states

LH (light hole) HH (heavy hole)

8



Figure 9 Absorption coefficient of Ge and Si 1. Choice of detector material

Monolithic integration with receiver electronics

9



Figure 10 Annealing effect of Ge epi on Si 1. Lattice mismatched epitaxy

Misfit dislocations Threading dislocations

2. Defect reduction Morphology: low T growth Defect density: strain anneal

Direct Growth Ge-on-Si

Deposit flat Ge epilayer directly on Si by a two-step CVD process

550C

300C

10µm

10 cycles

10µmGe SiO2

1 cycle

Cyclic annealing allows for dislocation free mesas

• Substrate: silicon as the universal platform• ‘Glue’ layers• Low T, high flux, post growth heat treatment

10



Silicon Microphotonics, Massachusetts Institute of Technology

Mechanism for Defect Reduction

After Dislocation Annihilation AnnealThreading Dislocation Density = 8×106cm-2

600 650 700 750 800 850 9000.0

0.5

1.0

Nor

mal

ized

Disl

ocat

ion

Vel

ocity

(a.u

)

Temperature, TL (C)

⎟⎟⎠

⎞⎜⎜⎝

⎛ −×∆×∆∝

kTaE

expV TCTE

As Grown Ge on SiThreading Dislocation Density ~109cm-2

11



Effect of lattice strain in Ge layer grown on Si Bandgap change absorption property

Possible strain Compressive: lattice mismatch

Lattice constant: Ge 5.66 Å > Si 5.43 Å Tensile: thermal mismatch

Expansion coefficient: Ge 5.9x10-6 K-1 > Si 2.6x10-6 K-1

Thermal expansion mismatch - Bi-metal effectRoom temp. Equilibrium at high temp.

Tensile strain in Gei
Thermal expansion coefficient: Ge > S Figure 11 Annealing effect of Ge on Si
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XRD: 0.20%

0.15 - 0.23%

Figure 12 Theoretical estimation of lattice strain in Ge and Ge band structure.

13



14

Figure 13 Deformation potential calculation for enhanced long wavelength absorption coefficient

30 meV bandgap shrinkage and L-band optical wavelength detection

15

Wavelength (nm)

Abs

orpt

ion

Coe

ffici

ent (

cm-1

)

10 2

10 3

10 4

1400 1450 1500 1550 1600 1650

Ge/Si (MIT)

Bulk Ge

C-band L-band

0.0 0.1 0.2 0.30.75

0.76

0.77

0.78

0.79

0.80

1640

1620

1600

1580

1560

Ge/Si/C54-TiSi2 Ge/Si Bulk Ge

EgΓ (lh

) (e

V)

In plane strain (%)

Wavelength(nm

)

Extended Response of Ge-on-Si Strained Epitaxial Layers

Strained Ge layers show absorption spectrum ‘red’ shift of ~30 nm.

Cannon, Jongthammanurak, Liu, MIT

Ge Band Structure

Tensile strain shifts light hole band up in energy with respect to heavy hole band, reducing direct band gap

3.46 optical and optoelectronic materials · extended response of ge-on-si strained epitaxial...

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