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Reflection Insensitive Quantum Dot Lasers Grown on Silicon Substrates Reflection Insensitive Quantum Dot Lasers Grown on Silicon Substrates John Bowers Art Gossard, Songtao Liu, Justin Norman, Yating Wan, Chen Zhang, Robert Zheng Bob Herrick (Intel) Weng Chao (Sandia) Frederic Grillot (U. Paris) November 1, 2019 Team Project Team ‣ Silicon Photonics Research Group at UCSB – State of the art equipment for the characterization and packaging of a wide range of semiconductor devices and optical communication systems ‣ UCSB Nanofab – >10,000 ft2 of Class 100 and 1000 cleanroom space – Optical lithographic capability to 200 nm ‣ UCSB Growth Facilities – 30 years of pioneering MBE research – 9 MBE systems with two dedicated to III-Vs ‣ California Nanosystems Institute – Advanced material characterization tools – ECCI, TEM, AFM, XRD, SIMS, atom probe 1 Team Collaborators ‣ Frederic Grillot, ParisTech – Laser dynamics, feedback stability ‣ Robert Herrick, Intel Corp. – Laser reliability and aging ‣ Matteo Meneghini, Univ. of Padova – Laser reliability and aging ‣ Weng Chow, Sandia – Laser theory for linewidth enhancement factor and mode- locking 2 Project Objectives Achieve High Performance Epitaxial Lasers on Silicon ‣ Leverage silicon manufacturing infrastructure – Photonics in CMOS foundries – Economical, high integration density photonic circuits ‣ Indirect bandgap necessitates III-Vs for lasers – Heterogeneous integration: high cost, limited scalability – Epitaxial growth: low cost, scalable with Si wafer size 50 mm InP wafer 3 Why Quantum dot lasers? Lower threshold Higher temperature operation (220C) Lower diffusion length (enables smaller devices Less sensitivity to defects (important for growth on Si) Lower linewidth enhancement factor-narrower linewidth Lower reflection sensitivity 4 1Jung et al. ACS Photonics (2017) Project Objectives Heteroepitaxial Challenges ‣ Crystal lattice mismatch – High density of dislocations, antiphase domains ‣ Thermal expansion mismatch – Cracking, residual strain, dislocations Bulk GaAs: 푎 = 0.565 nm Bulk Si: 푎 = 0.543 nm 5 Project Objectives Quantum Dots Enable High Performance ‣ III-V/Si laser research has existed for 30 years – Most reliable QW/Si laser has ~200 h lifetime for GaAs/Si materials ‣ Quantum dots represent breakthrough for high performance on Si – >10 M hour lifetime at 35C – Ultrashort (500 nm) in-plane diffusion lengths 100M QD (UCSB) 10M GaAs/AlGaAs lasers on Si 1M 100k QD (UCL) 10k QD (Nagoya) 1k QD (UCSB) ~60 meV 100 QW (UIUC) QW limit QW (Nagoya) 10 QW (MIT) 1 Laser lifetime (hours) 100m 10m QW (Bell lab) * Time to double threshold 1m 1985 1990 1995 2000 2005 2010 2015 2020 Publication year 6 Project Objectives Advanced Capabilities ‣ High temperature operation ‣ Sidewall insensitive – Small footprint ‣ High performance mode-locked lasers – Ultrafast gain recovery, high four-wave mixing ‣ Ultralow linewidth enhancement factor – Narrow linewidth – Reflection insensitivity 7 Project Objectives Applications ‣ Low cost, small footprint, efficient transmitters – Datacenters, HPC, LIDAR, etc. ‣ Isolator-free Lasers – Save cost and footprint 40 2600 µm 1600 µm Isolator 35 1350 µm 1100 µm 850 µm 30 25 20 Wall-Plug Efficiency (%) Wall-Plug Efficiency 15 10 0 1 2 3 4 5 6 7 8 9 10 11 12 Ridge Width (µm) 8 Accomplishments Optimized III-V/Si Templates ‣ Thermal cycling and defect filter layers – Antiphase domain free on-axis (001) Si – 7×106 cm-2 dislocation density ‣ Ongoing optimization → 2×106 cm-2 9 Accomplishments Optimized Quantum Dot Active Region ‣ InAs dots in In.15Ga.85As well – O-band emission achievable from ~1260-1320 nm ‣ Inhomogeneously broadened gain spectrum – Dots form via self-assembled growth – Large, coupled parameter space to optimize Before Optimization After Optimization 28 meV 54 meV 10 Accomplishments Highly Efficient Lasers ‣ Small-footprint microring cavities – Sidewall insensitive – Sub-milliampere threshold current ‣ High wall-plug efficiency Fabry-Perot lasers 40 2600 µm 1600 µm 35 1350 µm 1100 µm 850 µm 30 25 20 Wall-Plug Efficiency (%) Wall-Plug Efficiency 15 10 0 1 2 3 4 5 6 7 8 9 10 11 12 Ridge Width (µm) Y. Wan, et al., Optica, 4, (2017). 11 Accomplishments P-type Modulation Doping ‣ Band offsets leave holes weakly confined in dots – Add active region doping to offset thermalization – ~10-30 holes per dot ‣ Significantly increases gain, differential gain – Critical to high temperature reliability & low linewidth enhancement factor – Costs higher threshold, lower slope efficiency, ~10-20% WPE p+GaAs contact layer (300 nm) p-Al0.4Ga0.6As cladding (1500 nm) 17.5 nm uid-GaAs UID-GaAs waveguide (12.5 nm) 10 nm pGaAs GaAs (37.5 nm) 5x 10 nm uid-GaAs InAs QDs in In0.15Ga0.85As QW (~7 nm) 5 nm InGaAs UID-GaAs waveguide (50 nm) 2 nm InGaAs n-Al0.4Ga0.6As cladding (1500 nm) n+GaAs contact (500 nm) GaAs/GaP/Si template (001) on-axis Si 12 10 100 Years Years Herrick et al.(Intel, UCSB) OFC 2019 13 Reliability at 60ºC ‣ Need reliable operation at elevated temperatures ‣ Datacenters & HPC applications at 60-80ºC or higher ‣ First aging test at 60 °C shows lifetime of ~2500 hours 14 Summary of 60 C Reliability (300-hour) ‣ Varied p-doping levels in active region – P=5e17 cm-3 to p=1.5e18 cm-3 ‣ Highly improved lifetimes from 60°C aging – >>10,000 hours extrapolated lifetime 170620 (5e17) 170522 (1e18) 170623 (1.5e18) 170511-UID 10 o o Aging at 60 C, LIV sweeps at 35 C Aging time (hours) LIVs at 35 C 0.1 100k 8 50 500 1000 6 1500 2000 2750 10k 4 Power (mW) 2 Extrapolated Lifetime (hours) Lifetime Extrapolated UID lasers-past >1800 hr aging 1k 0 800 1000 1200 1400 1600 1800 2000 2200 0 25 50 75 100 Aging current density (A/cm2) Current (mA) 15 Still Defect Limited 10 100 Years Years Herrick et al.(Intel, UCSB) OFC 2019 16 Accomplishments High Performance Optical Amplifiers ‣ Columbia Enlitened Project ‣ 39 dB ground state gain (>20 dB at 70°C) ‣ Noise figure as low as 6.1 dB ‣ Wall-plug efficiency up to 20% Igain = 750 mA, T = 20°C, L = 5000 µm, tapered width 5 µm → 11 µm 17 S. Liu, et al., ACS Photonics, 6, 2523-2529 (2019). Accomplishments Mode-Locked Combs for Data Transmission ‣ Quantum dots uniquely suited to MLLs – Ultrafast gain/absorber recovery – Broad, engineerable gain bandwidth 18 S. Liu, et al. Optica, 6(2), 2019 Accomplishments 4.1 Tbps from Single Laser ‣ 64 channel, 32 Gbaud Nyquist pulse shaped PAM-4 ‣ 61 channels below HD-FEC, 64 below SD-FEC ‣ 4.1 Tbps 19 Accomplishments 100 GHz Colliding Pulse MLL ‣ 5th harmonic design ‣ Wide mode-locking range – < 2.5 ps and >8 dB pulse- contrast ratio Igain = 144 mA, VSA = - 3.3 V Pulsewidth mapping L–I curve 0 Pulsewidth (ps) spectrum 5.0 1.2 4 20 -10 Linewidth 1.3 Current upward 4.5 SA voltage 1.5 1.6 0 V -20 2 4.0 1.8 15 1 V 2 V 1.9 -30 3.5 Current downward 2.1 0 SA voltage 3.0 2.2 10 0 V -40 2.4 1 V 2.5 2.5 2 V -2 Power (dBm) -50 5 Linewidth (MHz) 2.0 Reverse voltage (V) Single-side power (mW) -60 -4 1.5 0 -70 80 100 120 140 160 180 0 50 100 150 200 1264 1266 1268 1270 1272 1274 Current (mA) Current (mA) Wavelength (nm) 20 Accomplishments 0.9 Tbps from Single Laser at 100 GHz ~ 10 ps ‣ 8 channels w/56 Gbaud Nyquist 10 pulse shaped PAM-4 8 ‣ Autocorrelator confirms 100 GHz 6 4 Intensity (a.u.) 2 0 40 60 80 Time (ps) B2B 5Km Wavelength =1268.794 nm 21 Accomplishments Quantum Dots for Low Linewidth Enhancement Factor ‣ Quantum dots have inherently low LEF 푑 푑휙/푑푧 푑푁 – Symmetric density of states 훼퐻 = − 푑 퐺푀 – Identically zero from Kramers-Kronig 푑푁 Increasing 푁 22 Accomplishments Inhomogeneous Broadening Increases LEF ‣ Dot sub-populations overlap – Low alpha at gain peaks, high at tails ‣ Also depends on carrier density – Many-body effects 23 Accomplishments Experimental Ultralow Linewidth Enhancement Factor ‣ Alpha factors of QD Si lasers show ~0.15 with p-doping – Quantum wells typically ~3-5 푑푛/푑푁 4휋 푑푛/푑푁 4휋 푑푛 훼퐻 ≡ − = − = − 푑푛푖/푑푁 휆 푑푔/푑푁 휆푎 푑푁 24 Jian Duan et al. APL 2018 Accomplishments High Critical Feedback Level ‣ High damping and low 휶푯 yield high feedback tolerance 2 2 휏퐿 2 2 1 + 훼퐻 푓푒푥푡 = 2 퐾푓푅 + 훾0 4 푐푟푖푡 16퐶푒 훼퐻 25 Zhang, et al. J. Sel. Top. Quant. Electron. 25(6), 2019. Accomplishments Feedback Insensitive Operation ‣ 7 m feedback Quantum Dot Quantum Well ‣ Bias at 3×Ith, up to 100% back- reflection (-10% tap) ‣ Quantum dot device perfectly stable ‣ Quantum well undergoes coherence collapse at < 2% 26 J. Duan, et al., Photon. Technol. Lett. 2019 Accomplishments Short-Cavity Regime 푓 3 GHz ‣ P-doped laser stable in short cavity regime: 푅푂 = < 1 푓푒푥푡 5 GHz ‣ Movable mirror – Feedback up to 20.8% (coupling limited) – Cavity length 3 cm 27 Feedback Insensitive Transmission Accomplishments Quantum Dot Laser Quantum Well Laser 28 J. Duan, et al., Photon. Technol. Lett. 2019 Accomplishments Future Prospects ‣ Waveguide integration – All-epitaxial photonic integrated circuits ‣ >60°C reliability – Need lower dislocation density – Native substrate devices reliable >80°C ‣ Single-mode lasers – Feedback insensitive, narrow linewidth DFBs ‣ Achieve zero linewidth enhancement factor above threshold – Careful engineering of threshold modal gain and inhomogeneous broadening 29 Technology-to- T2M – Market Opportunity Market ‣ Silicon photonics market expected to grow to $2B by 2023 – Datacom projected 90% of market ($6B by 2025) ‣ Major players: Acacia (US): Luxtera (US), Intel (US), Cisco (US), Mellanox (Israel/US), Finisar (US), STMicroelectronics (Switzerland), Hamamatsu (Japan), IBM (US), Juniper (US), GlobalFoundries (US), Broadcom (US), Oclaro (US), NeoPhotonics (US), Ciena (US) 3 0 Technology-to- T2M – Market Opportunity Market ‣ Google, Cisco want PAM4 format now . Looking at QPSK (and higher-level modulation) in the future Datacenter Example– Google Interconnect 3 Scaling large data center interconnects…, Zhou 2017 1 Technology-to- T2M – Datacom Market Opportunity Market ‣ Facebook currently upgrading to 100 and 200 GB/s transceivers .
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