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Hybrid III-V on Silicon Lasers T. Ferrotti, A. Descos, D. Bordel, H. Duprez, S. Menezo,
and B. Ben Bakir CEA, LETI, Minatec Campus, IIIV Lab, Grenoble, France
STMicroelectronics, Crolles, France
A laser on silicon?
Silicon (Ge, Si-nc+Er…) is a poor light emitter Monolithic integration: No efficient integrated laser sources achievable in the short-medium term Electrically pumped Ge on silicon lasers : promising results, but still require
developments
• III-V materials exhibit excellent laser properties: Direct growth of III-V materials has been studied for decades, but no
convincing results up to now Flip-chip bonding of lasers is a mature but rather expensive technology. Less
flexibility in the laser design
Heterogeneous integration by direct bonding: offers the best compromise between performances/ functionality/ manufacturability
CONFIDENTIAL 8-10 October 2013 | 2
Heterogeneous integration Growth of the III-V wafers
III-V die or wafer bonding on processed SOI
InP substrate removal
Processing of SOI wafers (modulators, detectors, passive waveguides, etc.)
Metallization of lasers, modulators and detectors
Processing of III-V dies/wafer
200mm fab
200mm fab
Back-end: 100mm fab
Heterostructure 3µm- thick
CONFIDENTIAL 8-10 October 2013 | 3
Processing of SOI wafers Growth of the III-V wafers
III-V die or wafer bonding on processed SOI
InP substrate removal
Processing of SOI wafers (modulators, detectors, passive waveguides, etc.)
Metallization of lasers, modulators and detectors
Processing of III-V dies/wafer
200mm fab
200mm fab
Back-end: 100mm fab
CONFIDENTIAL 8-10 October 2013 | 4
Processing of SOI wafers
Waveguide DBR Fiber-coupler
200mm SOI wafer: 500nm-Si / 2µm-BOX Typical thickness of silicon waveguides for efficient coupling with III-V waveguides Laser cavity (DBRs): hard mask/litho/partial etching (10nm) Waveguide-to-fiber couplers: hard mask/litho/partial etching (125nm) Waveguides and tapers: hard mask/litho/partial etching (250nm) Mesas: hard mask/litho/full etching (500nm) SiO2 encapsulation and planarization CMP (100nm)
Resist deposition
193nm DUV litho / hard mask etching
Resist stripping /Si RIE etching
SiO2 encapsulation (PECVD)
CMP
CONFIDENTIAL 8-10 October 2013 | 5
Growth of III-V wafers for hybrid integration
Growth of active layer in a reverse order compared to classical InP devices
Example of a 6 QW MBE grown wafer
Strained MQW InGaAsP/InP for 1.31 and 1.55 µm operation
CONFIDENTIAL 8-10 October 2013 | 6
Molecular bonding
O2 O2
1- Processed SOI substrate
2- PECVD silica deposition
3- CMP planarization
4- Surface Cleaning
1- Surface cleaning
2- PECVD silica deposition
3- O2 plasma activation
Thin layer 10 nm
(roughness < 0.5 nm RMS)
Low temperature bonding
Anneal and Substrate removing
Laser processing
III-V heterostructure SOI substrate
CONFIDENTIAL 8-10 October 2013 | 7
Bonded III-V wafers/dies on SOI Wafer to wafer bonding Die to wafer bonding
2’’
2’’ InP wafer(100nm thick SiO2 spacing layer)
Heterostructure of 3µm-thick Bonding yield > 90%
CONFIDENTIAL 8-10 October 2013 | 8
Bonding of III-V dies on SOI wafer
SiO2 BOX Si waveguide
InP
Active region (InGaAsP)
2’’ InP wafer(100nm thick SiO2 spacing layer)
SiO2 spacing layer: 100nm 250nm
500nm
CONFIDENTIAL 8-10 October 2013 | 9
III-V back-end process (100mm fab) Top view
Cross-sectionnal view
CONFIDENTIAL 8-10 October 2013 | 10
Single-mode DBR laser Gain III-V active waveguide Si-circuit supports all optical functions
CONFIDENTIAL 8-10 October 2013 | 11
Single-mode DBR laser: cavity design Structural parameters of the DBRs:
Width=10µm, Etching depth=10 nm, Period=237nm, Duty cycle=50%
κ=83cm -1
Front mirror: L=100µm R=46.4%, 3dB-BW ≈ 4nm
Back mirror: L=300µm R=97.3%, 3dB-BW ≈ 2.6nm
1,540 1,542 1,544 1,546 1,548 1,550 1,552 1,5540,0
0,2
0,4
0,6
0,8
1,0
R=46.4% @ 1547nm
R=97.3% @ 1547nm
Refle
ctiv
ity
Wavelength (µm)
Period : 237nmDBR length :
10µm 100µm 300µm 500µm
SEM micrograph
CONFIDENTIAL 8-10 October 2013 | 12
Wafer scale optical and electrical testing Courtesy of Ph. Grosse
CONFIDENTIAL 8-10 October 2013 | 13
Experimental results
0 20 40 60 80 100 120 140 160 180 2000,00,51,01,52,02,53,03,54,04,55,05,5
0,0 0,8 1,7 2,5 3,3 4,2 5,0 5,8 6,7 7,5 8,3
0,001,753,505,257,008,7510,5012,2514,0015,7517,5019,25
Si-w
aveg
uide
pow
er (m
W)
J(kA/cm²)
Fibe
r cou
pled
opt
ical p
ower
(mW
)
Current (mA)
10°C 20°C 30°C 40°C 50°C 60°C 65°C
20 40 60 80 100 120 140 1600
1
2
3
4
Fibe
r cou
pled
powe
r (m
W)
Current (mA)
1545
1546
1547
1548
1549
Wav
eleng
th (n
m)
-80-70-60-50-40-30-20-100
A.U.(dB)
CW operation (>60°C) @ λ ~ 1.55µm
Ith: 17-60mA (0.8-2.5 kA.cm-2) for T: 10 to 60°C
Rs= 7.5 Ω Lasing turn-on voltage : 1 V P-Si-waveguide > 14 mW (20°C) P-fiber > 4 mW (20°C) SMSR > 40 dB
CONFIDENTIAL 8-10 October 2013 | 14
DBR Laser-array
1520 1540 1560 1580 1600-80
-70
-60
-50
-40
-30
-20
-10
0
10 d
B/di
v
Wavelength (nm)
235 237 239 241
DBR period (nm):
4 DBR laser-array SMSR > 40dB ∆λ ∼ 12nm
CONFIDENTIAL 8-10 October 2013 | 15
Tunable hybrid DBR laser
Same architecture with heaters placed on the top of the DBRs
Resistive NiCr heater
+ Resistive heater
CONFIDENTIAL 8-10 October 2013 | 16
Tunable hybrid DBR laser Preliminary results (measurements not performed on the best design)
Tunability over 20nm can be achieved
Next step: SG-DBR (Vernier effect) to extend tunability range and to reduce heating power budget
CONFIDENTIAL 8-10 October 2013 | 17
DBR laser: Direct modulation Modulation Bandwidth ~7GHz (RT)
Eye diagram: 7Gb/s and 12.5Gb/s ER> 4.5 dB (17mW RF Power)
Eye diagram for 5Gb/s modulation Small signal modulation response
170mA@12,5Gbps
150mA@7Gbps
0 2 4 6 8 10 12 14-30
-25
-20
-15
-10
-5
0
5
10
EO R
espo
nse (
dB)
Frequency (GHz)
80mA 100mA 125mA 131mA 150mA7,22GHz@-3dB
CONFIDENTIAL 8-10 October 2013 | 18
Perspectives
Exploration of new designs/concepts DFB, SG-DBR Slow-wave structures Photonic crystals, double-racetrack….
Slow wave structure or Photonic crystals
Adiabatic mode transformer
Fiber-coupler
SOA
Fiber-coupler
Si
InP III-V waveguide region
CONFIDENTIAL 8-10 October 2013 | 19
Perspectives Improve performances
Improve output power level, external efficiency (cavity design, current confinement)
Reduce threshold current and Extend the operating T° range up to 80°C Increase wavelength tunability using optimized design
Development of a fully 200mm/300mm process
Integration with other optical and electrical functions, packaging (Si modulators, Ge photodetectors, 3 metallization levels…)
Integrated transceivers on CMOS
CONFIDENTIAL 8-10 October 2013 | 20
Thank you for your attention
CONFIDENTIAL 8-10 October 2013 | 21
APPENDIXES
CONFIDENTIAL 8-10 October 2013 | 22
Hybrid DBR lasers: the best design
0 20 40 60 80 100 120 140 160 180 200 2200
2
4
6
8
10
12
Fibe
r Opt
ical P
ower
(mW
)
Current (mA)
Optical power Tension
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
Threshold = 12mA
Max. fiber coupled power= 11,3mW
SMSR = 51dB
1530 1532 1534 1536 1538 1540 -80
-70
-60
-50
-40
-30
-20
-10
0
Puis
sanc
e op
tique
(dB
m)
Longueur d'onde (nm)
51dB
CONFIDENTIAL 8-10 October 2013 | 23
DESIGN
CONFIDENTIAL 8-10 October 2013 | 24
Adiabatic transition III-V Si
Active region (MQW=InAsP)
W-Si: Tuning parameter
Mode transformation:
IN OUT
SiO2 GAP = 100nm (+-20nm) 250nm 500nm
Wsi Silicon rib waveguide
n-doped layer (n-contact)
n/p++/p doped epilayers stack (p-contact) n graded
p graded TJ
III-V heterostructure:
CONFIDENTIAL 8-10 October 2013 | 25
Adiabatic transition III-V Si W=6µm OUT
CONFIDENTIAL 8-10 October 2013 | 26
IN W=0.5µm
Passive Si -
W-Si
W=0.80µm W=0.9µm W=1.10µm
Power transfer ensured by the supermode A
Adiabaticity criterion
( ) ( )( )
( ) ( ) ( )( )0
0
1 20tan arcsin 2
Si
zz
W z f z
zz z z
γ
δγ κ ε
κ
=
= = −
δ: mismatch of propagation constants between the individual uncoupled waveguide modes
z0: phase matching point (δ=0) κz0: coupling strength between waveguides at
the phase matching point ε: fraction of power scattered in the unwanted
supermode (odd mode)
Universal criterion for designing adiabatic mode transformers Criterion relates ε The shortest possible length of an adiabatic mode transformer Taper shape:
X. Sun, H.-C. Liu, and A. Yariv, Opt. Lett., 34, 280-282 (2009).
CONFIDENTIAL 8-10 October 2013 | 27
Adiabaticity criterion
CONFIDENTIAL 8-10 October 2013 | 28
Adiabaticity criterion
CONFIDENTIAL 8-10 October 2013 | 29
γ(z)-shaped adiabatic taper ε∼2%, Lcmin=100µm Taper length >80µm:
η>94%
Robust design: Lc=100µm: ∆WSi = ±50nm η>90%
Win= 430 nm
Wout=1930 nm
~100nm x 100nm
Taper length=100µm
CONFIDENTIAL 8-10 October 2013 | 30