integrated si-based photonics

Integrated Si-Based Photonics

Peking University Summer SchoolBeijing, ChinaJuly 19, 2013

James S. HarrisStanford University

Peking University Summer School, July 19, 2013

STANFORD

JSH 2

Demands for Optics at Shorter Distances

Internet, Wide Area Network (WAN)

Local Area Network (LAN)

Rack-to-Rack Card-to-Card

Distance Multi-km 10 - 2000m 30+m 1m

# of Lines 1 1-10 ~100 ~100 - 1000

Use of Optics

Since 1980s to early 1990s

Since late 1990s

Present Present ++

On-card Inter-chip Intra-chip

Distance 0.1 - 0.3m 10 - 100mm <10mm

# of Lines ~1000 ~10,000 ~100,000

Use of Optics

2010 - 2015 Probably after 2015

Sometime in the future

Adapted from IBM ResearchCan evolution of telecom technology address Inter/Intra chip applications?


STANFORD

JSH 3

Architecture change

• Multiple cores on a chip are already available– Trend: increase # of cores NOT speed or complexity

• Parallel architectures increased bandwidth• Nanophotonic communication is a credible solution

D. Fattal & M. Fiorentino HP Labs


STANFORD

JSH 4

Communications ChallengeBroadway, New York City, 1887Intel Microprocessor, 2005


STANFORD

JSH 5

On-chip Interconnects

LOW COST, LOW POWER, INTEGRATED, CMOS COMPATIBLE, OPTICAL TRANSCEIVERS

ITRS Roadmap 2005Globalinterconnects

Local connectsCMOS device

RC limited

Process Technology Node (nm)250 190 130 90 66 45 32

100

10

1

100

Rel

ativ

e D

elay

Gate DelayFO = 4Local

Global WORepeaters

Global WRepeaters

What is required to solve this challenge?


STANFORD

JSH 6

• Cu, CNT: small wire width → Energy per bit decreases as wire pitch is scaling (CV2). Latency increases as wire pitch scales down

• Optics favorable for longer wires

- Electrical interconnects

power dissipated by wire and repeaters latency by wire and repeaters - Optical interconnect (1 Channel)

power dissipated by end devices latency by end devices

Wmin for Cu CNT from ITRSfor optics = 0.6µm

Cdet=Cmod=10fF

Interconnect Performance

Koo, Kapur and Saraswat, IEEE Trans. Electron Dev., Sept. 2009

Energy/bit Latency


STANFORD

JSH 7

Photonic Integrated Circuit-1993

Soref, Proc. IEEE, 1687 (1993)

Waveguide architecture with butt coupled fibers III-V edge emitting lasers, modulators, detectors and high-speed electronics (HBT or HEMT)

All off-chip and Mostly III-V devices


STANFORD

JSH 8

Silicon-Compatible Photonics: A Materials Challenge

Intel

Can a new material be engineered to suit our needs?

Integrate the required photonic devices on silicon

Y.-H. Kuo, et.al., Nature 437 (2005)http://www.bit-tech.net/news/2007/09/18/intel_has_worlds_fastest_si-

ge_photo_detector/1

http://www.research.ibm.com/photonics/images/soi_phwire.jpg


STANFORD

JSH 9

Si

PoorEmission & Absorption

k[100][111]

E

Band Structures of GaAs, Si & Ge

E

k

GaAs

EfficientEmission & Absorption

Global Minimaat zone center

[111] [100]

Silicon Based Germanium

Ge

Local Minimaat zone center

EfficientAbsorption

E

k[100][111]


STANFORD

JSH 10

Unique Multiple Band Ge/SiGe QW

Deep direct band gap, QW

∆EC, direct = 0.4 eVEc,

Direct band gap transition

h+

e- <1ps tunneling>100 GHz modulation

Strain causes valence band splitting

Ec,LLower, shallow indirect band L minima

StrainedSi1-xGexbarrier

RelaxedSi1-yGeybuffer

StrainedGeQW

∆EV = 0.1 eV for heavy hole

Ev,hh

Ev,lh


STANFORD

JSH 11

Quantum-Confined Stark Effect

1. Red shift of absorption edge2. Smaller wave function overlap – lower α3. Change of n through Kramers-Kronig relationship

Electro-absorption and electro-optic modulation by tuning electron-hole coupling in quantum wells

More pronounced for excitons (bound electron-hole pairs)

Ec

EvNo E-field E-field


STANFORD

JSH 12

Graded SiGe buffer is widely used Low defect density Thick buffer layer Large surface roughness-Critical for QWs

Direct growth of SMOOTH, THIN buffer Low surface roughness Post anneal reduces dislocation density Buffer thickness is critical for single

mode waveguide devices on SOI

Two-Tgrowth direct growth

SiGe and GE QW Growth on Si

Si

Graded SiGe

Ge or SiGe

Graded buffer

10µm

Ge or SiGe

SiSingle-Tgrowth direct growth

400n

m

Si

Low-T SiGe

High-TGe or SiGe

400n

m Anneal

High Dislocation Density


STANFORD

JSH 13

SiGe Surface Morphology

As-grown roughness RMS ~ 0.2nm

2-Temp-step Ge-on-Si by MBE Single-Temp-step SiGe-on-Si by CVD

As-grown roughness RMS ~ 2.5nm

Annealed roughness RMS ~ 0.228nm

QWs require surface roughness ≤ 0.2nm


STANFORD

JSH 14

Strain-balanced Structure

n+ SiGe cap layer

Silicon Substrate

p+ Relaxed SiGe buffer layer

Undoped SiGe buffer layer

Undoped SiGe buffer layer

Average Si concentrationin Ge/SiGe QWs equals

that of SiGe buffer

Strain force ε

Compressive

growth direction

Tensile

Ge/SiGe MQWs

Y.-H. Kuo, et al, Nature 437, 1334 (2005)


STANFORD

JSH 15

Ge/SiGe Modulator on Si

Ge 10nm/Si0.15Ge0.85 16nm

Materials, Processes and Temperature are all CMOS-compatible



STANFORD

JSH 16

Strong QCSE in Ge/SiGe QWs



STANFORD

JSH 17

Fabrication Process


STANFORD

JSH 18

Small Signal Modulation

Bias: 2.5V Device top view size: 6µm *6 µmResponse limited by contact resistance


STANFORD

JSH 19

Waveguide modulator

SiO2

P-SiP-SiGeN-SiGe

Ge Quantum Well(s)

Light source Modulator Photodetector

Si substrateSiO2

Si waveguideSiGeSn buffer layerGeSn QWsSiGeSn cap layer

Integrated WaveguideModulator, Detector and Laser

Source, Modulator and Detector have identical QWsFunction determined by bias polarity


STANFORD

JSH 20

Outline

Introduction Ge/SiGe QCSE Electroabsorption Modulator

SiGe Growth and Characterization Device Fabrication and Measurement Optical Characterization

Strained Ge and GeSn Emitters Growth & Characterization of Tensile Strained Ge Growth & Characterization of GeSn

Summary


STANFORD

JSH 21

Si Based LaserGe direct band gap engineering

Strain- Theoretically,1.8% tensile

strained Ge is direct bandgap - Thin layer of Ge - Potential buffer layer (larger

lattice constant) Relaxed GeSn, GaAsSb, InGaAs

GeSn material- Sn is semi-metal- Reported direct bandgap for SnxGe1-x

is between 10% and 20% Sn- Lattice relaxed or compressive

strained layer

M. Bauer et al., APL, 81, 2992 (2002)He and Atwater, PRL, 97(10), 1937 (1997)M.V. Fischetti et al., JAP. 80(4) 2234 (1996)


STANFORD

JSH 22

Ge Laser

Good News—Ge can be made to laseBad News—Insanely high threshold current

Si-Ge Laser Structure Si-Ge Laser Spectrum

Camacho-Aguilera-MIT OptExp 20 11317 (2012)


STANFORD

JSH 23

Role of Heterostructures and Dimensionality on Lasers

Zh. Alferov, IEEE JSTQE, 6 832 (2000)

Nobel Lecture

FOUR orders of magnitude decrease in threshold current density as a result of heterojunctions and energy band engineering

Impact of epitaxy, improved materials


STANFORD

JSH 24

Highly Strained Ge Nano-bridge

Süess-PSI Nature Photon 10 1038 (2013)

Free carrier absorption increases with carrier densities and creates high laser threshold current

Nano-bridge Structure Calculated Gain & Loss

More sophisticated band engineering & QWs are essential


STANFORD

JSH 25

The Potential of Ge/GeSn: Direct Bandgap

Γ

L

Simplified Ge Bandstructure

Eg = 0.664eV

Advantages of Ge:• Si-compatible material• Low effective mass in Γ

(0.038m0)• Nearly direct-bandgap

and band engineer-able

Large effective mass (0.22m0)

Inefficient optical transitions

Eg = 0.8eV


STANFORD

JSH 26

Advantages of Ge:• Si-compatible material• Low effective mass in Γ

(0.038m0)• Nearly direct-bandgap

and band engineer-able L

The Potential of Ge/GeSn:Direct Bandgap


Γ

Eg = 0.8eV Eg = 0.664eV

The Biaxaial Tensile-Strain Effect~1.5% Strain Required

Y. Huo, et al., APL (2011)

Large effective mass (0.22m0)

Inefficient optical transitions


STANFORD

JSH 27

L

The Potential of Ge/GeSn:Direct Bandgap


Γ

The Sn-Alloying Effect~6-8% Sn Required

R. Chen, et al., Applied Physics Letters 99 (2011)


STANFORD

JSH 28

20 nm

Ge

InGaAs buffer layers:Defects are terminated at

interface Ge layer:

2.46% in-plane tensile strain

tensile strained Ge 10nm

In0.15Ga0.85As 200nmIn0.3Ga0.7As 300nm

GaAs substrate

In0.3Ga0.7As 10nm

100 nm

10 nm

Ge

Tensile strained Ge (TEM)


STANFORD

JSH 29

Indium concentration

Ge Raman shift (cm-1)

Strain (Raman)

10% 1.05 0.26%

20% 3.63 0.91%

30% 7.13 1.78%

40% 9.60 2.35%

Raman shift (cm-1)260 270 280 290 300 310 3200

0.2

0.4

0.6

0.8

1

Nor

mal

ized

inte

nsity

(a

.u.)

Bulk Ge

InGaAs

Strained Ge

260 280 300 3200

0.2

0.4

0.6

0.8

1

raman shift (cm-1)

Nor

mal

ized

inte

nsity

(a.u

.)

In0.1Ga0.9AsIn0.2Ga0.8AsIn0.3Ga0.7AsIn0.4Ga0.6As

Measured Strain & PLin Ge/InGaAs

1300 1350 1400 1450 1500 1550 16000

0.2

0.4

0.6

0.8

1

(nm)In

tens

ity (a

.u.)

0.34%0.92%1.81%2.33%

Strain (Raman) Photoluminescence


STANFORD

JSH 30

1200 1400 1600 1800 2000 22000

0.2

0.4

0.6

0.8

1

(nm)

Inte

nsity

(a.u

.)

20K30K40K50K75K100K150K200K300K 13% InGaAs

27% InGaAs

GaAs

Strained Ge40% InGaAs

40% InGaAs

Photoluminescence Ge/In0.4Ga0.6As

Strained Ge/In0.4Ga0.6As is a Type II Heterojunction


STANFORD

JSH 31

High-Temperature Segregation

3.5% Sn Attempted

400oC

The Issue for GeSn:Solid-Solubility

SGTE Alloy Database, http://www.crct.polymtl.ca/fact/phase_diagram.php?file=Ge-Sn.jpg.

>4.5%

Y. Shimura, et. al. Jpn. J. Appl. Phys. 48 (2009)

High-Strain Precipitation

Incr

easi

ng S

n

Sn

http://www.crct.polymtl.ca/fact/phase_diagram.php?file=Ge-Sn.jpg

http://www.crct.polymtl.ca/fact/phase_diagram.php?file=Ge-Sn.jpg


STANFORD

JSH 32

MBE Ideal for Investigative Tool

GeSn and SiGeSn Challenges:

1. Low solid-solubility (1%) of Sn in Ge– MBE can decouple source and

substrate growth temperatures2. Challenges in finding precursors that

decompose at low temperature– Very high-purity (99.999% or better)

solid sources available for evaporation3. Lattice constant changes greatly with

Sn or Si alloying, adversely affecting the bandstructure and film quality

III-V Chamber(InGaAs/

GaAs)

Group IV Chamber

(GeSiSn, GeSn) Strain Control with III-V

Group IV Stack


STANFORD

JSH 33

Want High-Quality, Direct-Bandgap GeSn

Goal: Explore basic material properties and unravel competing strain and composition bandgap effects to provide basis for quantum well device design

• Ability to control strain with Indium composition

• GaAs/InGaAs & GeSn optically distinguishable

• Higher Ge strain and higher Sn incorporation using low-temperature MBE growth (200oC)

GeSn

GaAs

InGaAs

InGaAsAnneal

Our Method: MBE Growth on GaAs/lattice relaxed InGaAs


STANFORD

JSH 34

TEM of 7% GeSn Layers

5 nm

GaAs Substrate

10% InGaAs Buffer

Ge0.07Sn0.93

High quality Ge93%Sn7% epi layer:• No defects• No precipitation (phase segregation)

7 X greater than equilibrium solubility

strained Ge or GeSn

InxGa1-xAs 200nm

GaAs substrate

H. Lin, et al., Thin Solid Films 520 (2012)


STANFORD

JSH 35

Surface Quality Maintainedw/High Sn Fraction

4.5% Sn, 100oCRMS = 0.529nm

7.0% Sn, 200oCRMS = 0.403nm

8.8% Sn, 100oCRMS = 0.626nm

Increasing Sn percentage

Surface RMS roughness changes only slightly with increasing Sn %.

4.5% and 7.0% Samples grown on In0.10Ga0.90As, ~50nm GeSn

8.8% Sn Sample grown on In0.25Ga0.75As



STANFORD

JSH 36

Great Material Quality Possible with MBE

RMS=0.519nm

GeSn with 10.5% Sn, low-T growth


InGaAs Buffer

GeSn Film


STANFORD

JSH 37

Where Does GeSn Become Direct Bandgap?

Bowing = 2.1 eV, ~7% Sn

Bowing = 2.4 eV, ~6.5% Sn

R. Chen, et al., APL 99 (2011)

H. Lin, et al., APL 100 (2012) J. Mathews, et al., APL 97 (2010)

Consensus: It’s a lot less than people thought! Experimental data suggests it’s around 5.5-7% Sn – very achievable!!


STANFORD

JSH 38

SiGeSn/GeSn/SiGeSn Quantum Well

GaAs substrate

SiGeSn 50nm

InGaAs buffer

SiGeSn 30nm

GeSn 30nm

InGaAs buffer

GeSn/SiGeSn

GlueSTEM-EDX

Inte

nsity

for G

a an

d G

e (a

.u.)

Intensity for Si (a.u.)

Position (arb. unit)

Si

Ge

Ga


STANFORD

JSH 39

Strain and Compositional Analysis

• Composition and Strain measured by SIMS and XRD-RSM– SiGeSn: Si = 5.58%; Sn = 9.16% Eg = 0.785 eV– GeSn: Sn = 7.91%; strain = 0.3% Compressive

• Previous studies1,2 decoupled Sn composition and strain effects– Eg = 0.548 eV calculated

for Ge0.92Sn0.08

Indirect

Direct

In-plane tensile strain

1) H. Lin, et al., Appl. Phys. Lett. 100 141908 (2012)2) H. Lin, et al., Appl. Phys. Lett. 100 102109 (2012)


STANFORD

JSH 40

GeSn Low-Temperature Photoluminescence

T=20K

T=294K


STANFORD

JSH 41

Lattice-Matched Options for GeSn QWs

Unstrained Quantum Wells possible with the addition of Si

Direct Bandgap Energy (eV)


STANFORD

JSH 42

The Stage Is Set – What About Lasers?

Require low-threshold lasers LOW LOSSOptical Losses:• Minimize mirror losses -> Ge difficult to cleave, high-Q resonators• Free carrier absorption -> Minimize doping to reach threshold• Optical scattering and mode confinement: Good design and fabricationCarrier Recombination and Threshold Current:• Reduce SRH recombination -> Maintain high material quality,

reduce doping• Auger recombination -> Minimize doping to reach threshold

With competing L-Valley occupation, n-type doping of 2-5 x 1018 cm-3 is optimum

Onset of lasing when optical gain ≥ loss

Gain RegionPhoton Emission > Photon

Absorption

Mirror Mirror


STANFORD

JSH 43

Challenges for a GeSn Laser

Effect of FCA on Laser threshold:

• Large internal losses increase threshold since required carrier concentration at threshold is an exponential function of αi

• Even worse for threshold current, (Ideal case), (Auger Recombination dominant)

High Carrier Concentration Produces Free Carrier Absorption


STANFORD

JSH 44

Gain Spectra for GeSn QWs

Gain Spectrum for p=n=2.4e19 cm-3

Increasing Sn

Gain Spectrum for 8% Sn (GeSn)

Addition of Sn greatly increases the net material gain for fixed carrier concentration!! MUCH LOWER threshold current lasers!

Only need carrier density of ~5e18 cm-3 for 1000 cm-1 of gain for 8% Sn

Increasing Carrier Concentration

Pure Ge


STANFORD

JSH 45

High-Quality Material is Paramount

Relative Laser Threshold vs. Carrier Lifetimes in Just Direct-Bandgap GeSn (ΔEc = 0)

Relative Threshold, log10

Due to Density of States, ~98% of carriers still in the L-valley• Non-radiative lifetimes critical for both valleys• Need high-quality material to reduce defect states

• Moderate n-type doping


STANFORD

JSH 46

The Benefits of Direct-Gap Materials

Increase in PL with Sn because more carriers occupy the direct Γ-valley! Sn alloying results in increased

optical efficiency

GeSn Photoluminescence

R. Chen, et al., Applied Physics Letters 99 (2011)

Γ

L

ΔEc


STANFORD

JSH 47

Carrier Confinement (for Ge/SiGe)

Experiment:• Ge 14nm*3QW• 200nm Ge grown on SiGe

bufferPL signal:• Stronger PL from QWs• Carrier confinement in

QWs

Simulation:• 10nm Ge QW in Si0.2Ge0.8 pn junctionCarrier concentration:• 1.5E19 in QW and <1E18 in barrier• Carriers are confined in QW (15 – 50

X)• Calculated net gain of 200cm-1

Xiaochi Chen et al. “Room Temperature Photoluminescence from Ge/SiGe Quantum Well Structure in Microdisk Resonator” [2012]

450 500 550

1010

1020

carr

ier c

once

ntra

tion

(cm

-3)

Bias = 0.76V450 500 550-1

-0.5

0

0.5

1

Y (um)

Ene

rgy

(eV

)


STANFORD

JSH 48

Can Lase, but not easy with Q=100

20nm GeSn (8%)

100nm Ge

90nm Ge

15% of TE mode experiences GeSn Gain and FCA75% of TE mode experiences Ge FCA (no band-to-band absorption)

~30x higher carrier density in GeSn QW than in Ge barriers due to

heterostructure

Choose low resonator loss to hit threshold, Q of 500 ->

~100cm-1


STANFORD

JSH 49

GeSn Photoluminescence

1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 24000.00E+00

1.00E-07

2.00E-07

3.00E-07

4.00E-07

5.00E-07

6.00E-07

7.00E-07

8.00E-07

1% GeSnCompressive

0% GeSnRelaxed

5% GeSnMostly Relaxed

3% GeSnCompressive


STANFORD

JSH 50

Microdisk Ge QW Photoluminescence

• Amplified spontaneous emission pumped by 900nm pulsed laser

• Two small peaks on each fringe TE / TM or higher order

mode Ridge waveguide profile,

thick disk• PL intensity is super linear,

estimated gain of 1500 cm-1

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1350 1400 1450 1500 15500

2000

4000

6000

8000

Wavelength (nm)

PL

inte

nsity

(a.u

.)

10 mW20 mW30 mW40 mW60 mW


STANFORD

JSH 51

Summary of Initial MBE GeSn Studies

• Strain-free layers of high-Sn% GeSn alloys – Low-growth temperatures using MBE– Lattice-matched growth using InGaAs buffers

• Increased photoluminescence for higher-Sn samples– Large increase in integrated PL– Shrinking ΔEc energy with increased Sn– Bandgap mapped out for strain/Sn combinations– Only ~7% Sn necessary for direct-bandgap!

• GeSn is favorable for lasers!– Low density of states in Γ-valley reduces current to reach

transparency optical gain at low carrier concentrations– Low carrier concentrations means reduced free-carrier

absorption and Auger recombination, results in low-threshold lasers


STANFORD

JSH 52

Summary

● Strong quantum confined Stark effect and absorption shift observed in Ge/SiGe quantum well device

● Modulation demonstrated at 30 GHz & 100 GHz possible● Waveguide modulator can be integrated into SiGe

waveguides, eliminating alignment and coupling losses● Both tensile strain and GeSn alloy will be required to

achieve direct bandgap Ge and stimulated emission● Photonic crystal or optical disks will be required to

achieve high-Q cavities● Strained Ge and GeSn/SiGeSn are all CMOS compatible


STANFORD

JSH 53

Acknowledgements

STUDENTS, POSTDOCS and COLLABORATORSYu-Hsuan Kuo Shen Ren Theodore I. Kamins Yiwen Rong Jonathan E. Roth Marco Fiorentino Yijie Huo Elizabeth Edwards Michael R.T. Tan Hai Lin Rebecca Schaevitz Jae-Hoon Kim Yangsi Ge Onur Fidaner Lars Thylen Yong Kyu Lee Selcuk Yerci Guillaume Huyet Tomasz Ochalski Yiyang Gon Seongjae ChoEd Fei Suyog Gupta Edris MohammedRobert Chen Jelena Vuckovic Krishna SaraswatColleen Shang Ian Young Mark Brongersma

SUPPORTDARPAIntelSRC-IFCThank You

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