Dislocations in SiGe planar waveguides - …fisica.unipv.it/CILSOMAF/OldSite/files/activities/student seminars...Spintronics) – Politecnico di ... Dislocations in SiGe planar waveguides

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Dislocations in SiGe planar waveguides

Andrea Trita

Laboratory of Quantum Electronics and Nonlinear OpticsElectronics Department - University of Pavia

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OUTLINEIntroduction

Dislocations in SiGe planar waveguides:Consequences on the guided optical field

Silicon photonicsThe SiGe/Si approach to Silicon PhotonicsDislocations formation at SiGe/Si interfaces

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OUTLINEIntroduction



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Opticalinterconnects

Silicon Photonics

Cheap photonic chips foroptical communications industry

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Silicon PhotonicsSilicon as optical material

Indirect bandgap Poor efficiency as light emitter

Centrosimmetric crystallinestructure

Does not exhibit electro-optic effect

Disadvantages

Bandgap energy = 1.12eV High transparency at telecom wavelength(1.3-1.5μm)

At 1.3-1.5μm nsi~3.5 Tight optical confinement in combination withSiO2

Crystalline structure High nonlinear optical coefficients(gR,Si~20cm/MW – gR,Silica=0.93X10-5cm/MW)

Advantages

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Silicon PhotonicsThe optical waveguide.

Cladding (n1)

Core (n2)

n2>n1

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Silicon PhotonicsThe optical waveguide.

n2>n1

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Silicon PhotonicsTechnological approaches

SOI approachSi

SiO2

SiO. Boyraz and B. Jalali, "Demonstration of a silicon Raman laser," Opt. Express 12, 5269-5273 (2004)

SiGe/Si approachSi

SiGe

Si

SiGeSi

Possibility to tune the optical properties of the material acting on the Gefraction

At low Ge fraction the small index contrast between Si and SiGe allows formonomodal structures with micrometric dimensions

Elevated nonlinear optical coefficients

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J.W. Matthews and A.E. Blakeslee, J. Crystal Growth 32 (1974), p. 265.

Ge has 4,2% larger lattice constant than Si

Si

SiGe

Si

SiGe

StrainedRelaxed withdislocations

Silicon PhotonicsSiGe approach to optical waveguides

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OUTLINEIntroduction



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PRIN Project 2005: “SiGe optical waveguides: design, fabrication, characterisation and their application to Raman amplication”

Laboratory of Quantum Electronics and Nonlinear Optics- University of Pavia

L-NESS (Laboratory for Epitaxial Nanostructures on Silicon and Spintronics) – Politecnico di Milano

Material Science Department - University of Milano BicoccaPhysics Department -University of Insubria


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Si capSi1-xGex layer

Si Substrate

d


6102#75425102#7544

4102#7546

2102#7382

2101.5#76892100.8#76932100.4#7646

2100.2#7645

Nominalx

SiGe cap

[μm]SiGe layer

d [μm]Samplenumber

Fixedd

varyingx

Fixedx

varyingd

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Dislocations in SiGe planar waveguidesConsequences on the guided optical field

1.55 μm LASER

SOURCE


Si Substrate

VIDICON CAMERA

Cylindricallens

40X objective

Collimator

xz

y

6102#75425102#7544

4102#7546

2102#7382

2101.5#76892100.8#76932100.4#7646

2100.2#7645

Nominalx

SiGe cap

[μm]SiGe layer

d [μm]Samplenumber

Fixedd

varyingx

Fixedx

varyingd

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1.55 μm LASER

SOURCE


Si Substrate

VIDICON CAMERA

Cylindricallens

40X objective

Collimator

xz

y

x

y

Increasing Ge fraction(x)

d=0.2 μm x=2%

Inc

rea

sin

gS

iGe

co

re t

hic

kn

ess(d

)

d=0.4 μm x=2%

d=0.8 μm x=2%

d=1.5 μm x=2%

d=2 μm x=2% d=2 μm x=4% d=2 μm x=5% d=2 μm x=6%

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Si cap

Si substrate

PLANAR WAVEGUIDE Si/SiGe/Si

SiGe core

?


Experimental results indicate that the refractive index inside the waveguides presents sharp localized variations

The origin of these sharp refractive index variations can be ascribed tothe presence of dislocations

Dislocations are organized in bunches so that refractive index variationsoccur over a spatial scale comparable with λ

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Transmission Electron Microscope (TEM) at EMEZ (Electron Microscopycentre of the ETH Zurich)


Si cap

Si substrate


SiGe core

?

λ

Si Cap

Si Sub

SiGecore

d=2 μm x=2%

SiGecore

Si Cap

Si Sub

d=0.4 μm x=2%

Si Cap

Si Sub

SiGecore

d=2 μm x=6%

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Raman micro-spectroscopy at the University of Milano Bicocca


Si cap

Si substrate


SiGe core

?

|ε|=10-4

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Finite Element Method (FEM) opto-structural model of the waveguide. (University of Pavia-University of Milano Bicocca)


Si cap

Si substrate


SiGe core

?

The strain field due to the bunches of dislocations is evaluated analiticallyand inserted in the FEM model

Strain field

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Finite Element Method (FEM) opto-structural model of the waveguide.(University of Pavia-University of Milano Bicocca)

Dislocations in SiGe planar waveguidesconsequences on the guided optical field

The strain field is converted into a refractive index distribution through the elasto-optic tensor and optical modes supported by the structure are evaluated

Optical modesIntensity

Optical modesElectric Field

Refractive indexdistribution(nzz)

Refractive indexdistribution(nzz)

Refractive indexdistribution(nxx)

Optical modesIntensity(section)

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Dislocations in SiGe/Si planar waveguidesconsequences on the guided optical field

Bunches of dislocations

Experimentally (optical characterization, TEM, Micro-raman) and theoretically (FEM model), it has been shown that:

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Sharp localized perturbations to the strain fields


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Sharp localized perturbation to the refractive index


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Sharp localized perturbation to the refractive index

Detectable output intensity perturbation


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Dislocations in SiGe planar waveguides - …fisica.unipv.it/CILSOMAF/OldSite/files/activities/student seminars...Spintronics) – Politecnico di ... Dislocations in SiGe planar waveguides