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Kazumi WadaUniversity of Tokyo
2013 Peking University Summer School on Si Photonics Technology and Applications
July 11, 2013
Si Microphotonics for WDM implementation
-Three Myths in Ge-锗硅有源器件单片集成的进展
Lecture Notes
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Chair: Kazumi Wada (Univ. of Tokyo)Kyoto Japan | November 16-19 2011
Location: Kyoto University, Katsura Campus BRoom: Cluster Administration Bldg. 3F
StructuresDay 1: Si Photonics: History and Si Platform
3pm Opening, 6pm- ReceptionDay 2: Cutting Edges of Devices and MaterialsDay 3: Emerging Technologies
Lab tourDay 4: Systems
1pm Closing
General Information
Invited Lecturers
Web site: http://bit.ly/core-to-core2011sRegistration Fee: FreeE-mail to: [email protected]
World Network of Si Photonics• North America: Lionel C. Kimerling (MIT)
• Europe: Roel Baets (Ghent Univ.)
• Japan: Kazumi Wada (Univ. of Tokyo)
Lionel C. Kimerling (MIT, History and the cutting edge of Si photonics)Laurent Vivien (Univ. of Paris, Toward carbon nanotube photonics)Zhiping (James) Zhou (Peking Univ., Si microring and optical biosensors)Hideo Isshiki (Univ. Electro-Communications, ErSiO materials system and emitter)Pieter Dumon (IMEC, Foundry and university research)Susumu Noda (Kyoto Univ., Recent progresses in photonic crystals)Thomas Krauss (Univ. of St. Andrews, Light generation and control in Si photonic crystal)Gianlorenzo Masini (Luxtera, Silicon Photonics at work: from process and devices to system applications.
The case of Luxtera's 40Gbps QSFP AOC)Haisheng Rong (Intel, Silicon Photonics platform for converged high-speed optical I/O)Yurii A. Vlasov (IBM, Key application and Si photonics for computation)
Sponsored by JSPS Core-to-Core Program
Kyoto
Tokyo
��
In corporation with AP, MRS, IEEE
Co-sponsored by PESEC, PECST
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About me
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Electronicfeedback
Burst packets
Our Research History1 2 3 4 5 6 7 8
INtegrated Si VOA0Ge PD: Lateral p-i-n structure
Impact of integrated VOA + PD
p+ n+
SiO2
Si substrate
SiO2
n+ p+
Si rib WG core600 x 200 nm
Al
Geエピ成長装置@東大
Outline
• Introduction
• High performance computing and communication
• Problems and Solutions
• Three myths in Ge research in Si photonics
• Epitaxy, detection limit, and lasing
• Summary
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World Economy
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8
Important is to understand where the bottlenecks are.Bruce and Fine @Sloan “02
IT Value Chain
Moore’s Law
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Where we are?
History of Computers
Development of Supercomputers
• 1000x in 10 years, ~16 yrs from top to note.
6-8 yrs 8-10 yrs
500th
Top
Notebook
From ECOC 2008 tutorial by U. Vlasov
http://www.top500.org/
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Tianhe-233.86 PFLOPs
• However, everything has limitation in human history.
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CPU Performance Evolution
1970 1980 1990 2000 2010
105
104
103
102
10
1
0.1
Clo
cksp
eed
(MH
z)
Year
DA. Muller, Nature Mat. 4, 645, 2005
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Saturation
Speed of Aircrafts
15
1900 1925 1950 1975 2000
Year
CommercialMilitary
1,000
100
10
Spe
ed (
m.p
.h.)
D.A. Muller
How to Stop “Saturation”
• Many core architecture and parallelism.
• Many core architecture is current "Holy Grail"
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• The enhancement is limited by a fraction must have serially performed.
Amdahl’s Law
17€
Overall speedup =1
F +1− FN
,
where F - fraction of a program to serially perform, N - number of cores
G.M. Amdahl, In AFIPS Conference Proceedings vol. 30 AFIPS Press, Reston, Va., 1967, pp. 483.
5%
10%
20%
50%
F=0%
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光通信
Optical Fiber Comm.
Light is f
ast and h
as colors
LC. Kimerling, ECS Interface, p.28, Summer, 2000.
Short Summary
• High Performance Computers get faster exponentially. • Top to notebook takes ~16 years.
• Optics is “MUST HAVE” on Si to suppress “saturation”. • On-chip WDM(Wavelength Division Multiplexing)
should be the clue.
• The key is Light emitter (EMT), modulator(MOD), detector(DET), multiplexor/demultiplexor(MUX/DEMUX) on a chip.
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History of Device Development
ProcessSi-LSIs
MaterialsMagnetic disk
PrincipleOptical disk
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Innovation by Materials
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Innovation by Processing
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Si Crystal
• Si ingot• Dislocation-free• 400mm
• made by Super Silicon research Lab in Japan (2000).
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Innovation by Principle
Auto Piano Optical Disk
Innovation in Si Photonics
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Si Electronics Si Photonics
Materials Si CMOS Si CMOS + Bonding
Process Si CMOS Si CMOS
Principle Electronics Electronics + Photonics
Electronics + Photonics
• Current targets
• Signal processing by electronics (transistors)
• Signal transmission by photonics
• Future
• Both by photonics (Optical Computing)
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Hierarchy of Information Processing
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• Do not fight with transistor.
• Jump up one hierarchy.
Main stream Candidates
Algorithmprocessing
Neumann Architecture
Cell automaton,
Representation of logic
Binary, Boolean Algebra
Many values, ..
Implementation of device
Chronic switch= transistor
Various
Three algorithms
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A) Boolean Algebra
f=(X1X2+X1X2)X3X4-+ (X1X2+X1X2)X3X4+ X3X4
! ! !X1X2
00 01 10 1100 0 1 1 001 0 0 0 010 1 1 1 111 1 0 0 1
X3X4
B) Truth table
0 1
X1
X2
X3
X4
X2
0 1
1
1
0
0
0
0
1
1
Root
Node
Branch
Leaf
C) BDD
Binary Decision Diagram
Representation of logic Implementation of deviceImplementation of device
Boolean Algebra Logic gates Transistors
Truth table Lookup tables ROMs
Binary decision diagram (BDD) Trees Nodes
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Optical Switching
• Binary Decision Diagram• Switch: Directional switch
with a gate• The gate determines the
outport, 0 or 1.
Inport
Outport
Gate, Xi
0 1
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• Binary Decision Diagram prototyped. • 100 times faster than CMOS
based electronic 2 bit adders.
Optical Computing
2 bit adder
Shiyun Lin, Y. Ishikawa, and K. Wada, Optics Express, v 20, n 2, p 1378-1384, January 16, 2012.
Optical Fiber and Laser
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You
Sea Shore
Sea
Your friend
Fast
Slow
Optical Fiber Fab. (VAD)
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Optical Fiber and III-V Lasers
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Photon Energy (eV) → 2.76 1.55 1.1eV 0.41Wavelength (µm) → 0.45 0.8 1.12µm 3
Si Bandgap
• Weak EO coeficient• transparent 1.3-1.6 μm• Cost-effective • Weak light emission• CMOS Compatibility • Weak detection <1.1 μm
Communications
Si as a Photonic Material
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Moore’s Law
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World’s First E + P Convergence
Collaboration with Intel 2000Collaboration with Intel 2000
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E + P Convergence
• Short distance by electrons
• Long distance by photons
Frontier in Si Photonics
IBM (Thanks to Y. Vlasov)
EU HELIOS Project (Thanks to L. Fulbert)
最先端研究支援「荒川プロジェクト」「光電子融合システム基盤技術」http://www.pecst.org/press/press20110919.pdf
Kazumi WadaUniversity of Tokyo
Three Myths in Ge-Si Photonics-
Ge-based Devices
Devices Materials
Waveguide Si, SiON, SiOx
Modulator Si(ER), Ge(EA)
Photodetector Ge, Si
Light emitter III-V on Si (bonding), Ge
MUX/DEMUX Si, SiON, SIOx
Fiber coupler Si, SiON, SiOx
Ge as an active photonic material b/c ofhigh quality and strain.
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Si Waveguide
• Core:Si
• 0.2 x 0.5 µm2.
• Cladding: SiO2 (or air)
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World’s Smallest Ring
1 2 3 4
8 μm
In Lim 2000Thru port
1x4 WDM in Silicon
Three Myths in Ge Research
1.Epitaxy needs lattice matching.
2.Ge only works below 1550 nm to detect.
3.Laser needs direct transition type semiconductors.
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First Myth
1.Epitaxy needs lattice matching.
• High quality epitaxial layers are only available when the lattice constant matches with that of its substrate.
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Lattice Matching in Epitaxy
Lattice constant (A)
Lattice Mismatch: Growth
Ge flat epilayers A two-step (low-high temp.) growth process.
300˚C
550˚C
Islandingsdue to long migration to get energy minima.
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• Annealing Ge on Si
• Dislocations reduced from 109 to 107 cm-2.
0.5µm
109 /cm2
2x107/cm2
Annealing of Dislocations
Ge
Si1 µmLuan
Annealing
Lattice Mismatch: Defects
• Dislocations: • 4%-mismatch generates
1 misfit dislocation every 25 Si atoms.
Ge
Si
SiO
2
SiO
2
Dislo
catio
n
10μmGe SiO2
1 cycle
10 cycles
Dislocation-free Ge on SiLuan
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Second Myth
• Ge works below 1550 nm to detect. • The typical communication wavelength
range is 1530-1620 nm (C+L band) because of Er doped fiber amp.
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Absorption Spectra
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• Direct bandgap of bulk Ge is 0.8 eV (1550nm).
• Absorption is weak when light is beyond 1550 nm in wavelength.
Extended Absorption of Ge Epi
• Red shift of Ge absorption
• Beneficial for detection of C+L band.
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Thermal expansion coefficientGe > Si
Ishikawa et al. (MIT), APL 82, 2044 (2003); JAP 93, 13501 (2005).
Strain by Thermal “Shrinkage” Mismatch
0.2-0.3% strain-tensile
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Direct Bandgap under Strain
0.2%30 meV
• Ge direct gap shrinks under biaxial tensile strain.
• At 0.2% tensile, that shrinks by 30 meV. • 0.80 eV (1550 nm) to
0.77 eV (1610 nm)
• Thus, Ge on Si covers ~C+L band.
Third Myth
• Laser needs direct transition type semiconductors. • Indirect semiconductors never lase.
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Ge: indirect-gap Semiconductor
0.80 eV0.66 eV
(M. L. Cohen)
• Direct gap is 0.80 eV while Indirect gap 0.66eV.
• Optical transition: needs phonons to conserve k.
• Thus, it never lases.
Three Strategies for Ge Lasing
• The Γ valley to populate electrons via, 1.heavily n doping
2.highly strain-tensile Ge
3.high carrier injection
• No scattering of electrons to the L valley.
• Quasi-direct gap.
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Monolithic; n+Ge Laser
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Jifeng Liu, J. Michel, et al., Opt. Lett. 35, 5 (2010).
Ge lasing reported mid September 2009. !
• n+Ge Lasing• Optical pumping
• Rib structure• Multimode
Camacho-Aguilera et al, Opt. Express 20, 11316 (2012)
Laser Diodes
• Heavily doped n type required because of electrode absorption.• 4e19cm-3
1.0 0.6
0.8
0.4 0.6 1.5 nm(Resolution limited) 0.4
0.2 0.2
0.0 0.0
1600 1620 1640 1660 1680 0.0 0.2 0.4 0.6 0.8 1.0
Current (A) Wavelength (nm)
n+ Ge Laser Diodes
• Lasing around 1550-1650nm• Clear threshold behavior
Opt
ical
Pow
er (m
W)
270kA/cm2
Short Summary
Three myths and the breakthroughs1.Epitaxy needs lattice-matching.
• Dislocation-free Ge on Si using sheer stress.
2.Ge cannot detect wavelength beyond 1550 nm that InGaAs can. • Strain-tensile Ge to detect 1610nm.
3. Indirect semiconductors never lase. • Quasi-direct Ge via n-type and strain-tensile to
suppress electron scattering to the L valley.
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On-going Challenges
• WDM to further enhance signal transmission. • Increase the bit rate by 1000.
• In 1260 nm (O-band) to 1675 nm (U-band) : Δλ~400nm, 1000 signals available.
• On-chip Injection LD should be the major challenge.
• Wavelength locking of photonics on a chip (Fourth Myth)
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carrier injection�
Carrier density from n-type doping (cm-3)�
Net
gai
n (c
m-1
)�
Toward Ge Injection LDs
• Heavily doping• n type carriers ~1020
cm-3
• Here, 0.25% tensile strain assumed.
• Challenge: such high n+ doping.
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Net gain: Gain - Free carrier absorption
0.25 % tensile
Larger Net Gain (Ge) when:
5e18
1e192e19
in cm-3
5e19
Takinai, in this school
Toward Ge Injection LDs
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• Large strain• Strain tensile ~1%,
• Here, n-type carriers can be low 1019 cm-3
• Challenge is how to strain Ge that high.
• No worry about wavelength.
The Fourth Myth (potential)
• LSIs and on-chip D(dense)WDM do not coexist under uncooled chip architecture (LSIs). • Thermal fluctuation on a chip malfunctions
DWDM because of wavelength fluctuation.
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Hot chip and DWDM
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From ECOC 2008 tutorial by U. Vlasov
Wanted:
Wavelength Locking
Computing: ENIAC (1946)
• Devices• 17468 vacuum tubes
• 70000 resistors
• 10000 capacitors
• metal interconnects
• Size: 30 tons
• Power consumption: 150kW
Thanks to Wikipedia
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Principle of Wavelength Locking
• Temperature fluctuation resulting in:• EMT(emitter) grain spectrum shift: dG/dT
• MUX/DEMUX, EMT, MOD(Modulator) wavelength shift: dω/dT
• Locking by local strain ε; • dG/dT =-dG/dε (dEg/dT = - dEg/dε), • dλ/dT =- dλ/dε (dn/dT = - dn/dε)
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• Si microbeam structure can be fabricated using SOI wafers.
• A few % strain at surface of the Si beam by a few μm pushing down.
Approach: Local Strain
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Si
Stress-Bandgap relation
• Si bandgap shrinks under tensile and/or compressive stress.
• 1.3 μm under ~1 % strain-tensile.
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Silicon
µ-Photoluminescence(PL)
• Excitation• 1 μm dia. Nd YAG at
beam edge (X).
• 2.6mW@ 457nm
• Detector• InGaAs@-100°C
• Detection limit ∼1750nm
• Stressing point• Bent 1, 2, 3.
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Typical Si beam
Bent 3 2 1x
µ-PL
Evidence of Bandgap shift
• Red shifts in PL spectra under strain (Bent-1, -2, and -3).
• It reaches ~1.3 μm. • 1 % tensile strain in
the Si beam.
74
K. Yoshimoto, Opt Express 25,26492, 2010
Si beam
• However, no shifts in sharp (FP) peaks. • Refractive index does
not change with this level of strain.
75
Lattice Matching in Epitaxy
Lattice constant (A)
GaAs on Ge on Si
76
Si
Ge
GaAs
• This opens up new fields; • Integrated light emitters, cost-
effective GaAs solarcells, and more.
• XTEM• Dislocation-free GaAs on Ge
• Off-cut (100) Si substrate used.
Horie, in this school
Reconfigurable Emitter
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GaAs on Ge on Si beam• One emitter emits various λ’s. • Reduce # of
emitters on a chip.
• GaAs on Si beams should cover from the whole optical communication wavelength range (O- to U-band).
Horie, in this school
Reconfigurable Emitter
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O band U band
• One emitter emits various λ’s. • Reduce # of
emitters on a chip.
• GaAs on Si beams should cover from the whole optical communication wavelength range (O- to U-band).
AI and human beings
• IBM “Deep Blue” won the world chess champion, G. Kasparov, 1997.5• Understanding the rule of chess.
• 0.5 TFLOPs
• IBM’s Watson defeated two former Jeopardy champions, K. Jennings and B. Rutter (“Jeopardy”, 2011.2.16). • Understanding human language and
searching stored informations.
• 5 PFTLOPs.
Development of Supercomputers
• After 2027 we might have little chance of winning over notebooks, besides creativity.
6-8 yrs 8-10 yrs
500th
Top
Notebook
From ECOC 2008 tutorial by U. Vlasov
http://www.top500.org/
On-going Challenges
• WDM to further enhance signal transmission. • Increase the bit rate by 1000.
• In 1260 nm (O-band) to 1675 nm (U-band) : Δλ~400nm, 1000 signals available.
• On-chip Injection LD should be the major challenge.
• Wavelength locking of photonics on a chip (Fourth Myth)
Summary
• The three myths and the breakthroughs in early Si photonics reviewed.
• Challenges of WDM implementation in Si electronics reviewed.• Wavelength locking via strained Si, Ge, and
GaAs.
• Demands on on-chip WDM: • Injection Ge laser: by n+ and strain+
• Reconfigurable(Tunable) emitter: GaAs on Ge on Si beam.
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What we have learned
• There are always Myths in research and the ways to breakthrough (not always).
• Shortcut to breakthrough is to ask questions.
• The bigger the better. You will get big answers.
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Francis CrickJune 8, 1916July 28, 2004
