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Integrated Si-Based Photonics

Integrated Si-Based Photonics. James S. Harris Stanford University. Peking University Summer School Beijing, China July 19, 2013. Demands for Optics at Shorter Distances. Can evolution of telecom technology address Inter/Intra chip applications?. Adapted from IBM Research.

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Integrated Si-Based Photonics

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  1. Integrated Si-Based Photonics James S. Harris Stanford University Peking University Summer School Beijing, China July 19, 2013

  2. Demands for Optics at Shorter Distances Can evolution of telecom technology address Inter/Intra chip applications? • Adapted from IBM Research

  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

  4. Communications Challenge Intel Microprocessor, 2005 Broadway, New York City, 1887

  5. 100 10 1 100 Gate Delay FO = 4 ITRS Roadmap 2005 Local Global W Repeaters Global interconnects Global WO Repeaters RC limited Relative Delay CMOS device Local connects 250 190 130 90 66 45 32 Process Technology Node (nm) On-chip Interconnects What is required to solve this challenge? LOW COST, LOW POWER, INTEGRATED, CMOS COMPATIBLE, OPTICAL TRANSCEIVERS

  6. Interconnect Performance Energy/bit Latency Wmin for Cu CNT from ITRS for optics = 0.6µm Cdet=Cmod=10fF • 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 • 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 Koo, Kapur and Saraswat, IEEE Trans. Electron Dev., Sept. 2009

  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

  8. Silicon-Compatible Photonics: A Materials Challenge Integrate the required photonic devices on silicon Can a new material be engineered to suit our needs? Intel http://www.research.ibm.com/photonics/images/soi_phwire.jpg 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

  9. E k Band Structures of GaAs, Si & Ge E Global Minima at zone center E Local Minima at zone center [111] [100] [100] [100] k [111] [111] k Si GaAs Ge Efficient Absorption Poor Emission & Absorption Efficient Emission & Absorption Silicon Based Germanium

  10. Deep direct band gap,  QW Ec,G ∆EC, direct = 0.4 eV e- <1ps tunneling >100 GHz modulation Ec,L Lower, shallow indirect band L minima h+ Ev,lh ∆EV = 0.1 eV for heavy hole Ev,hh Strain causes valence band splitting Unique Multiple Band Ge/SiGe QW Direct band gap transition Relaxed Si1-yGey buffer Strained Si1-xGex barrier Strained Ge QW

  11. Quantum-Confined Stark Effect • Electro-absorption and electro-optic modulation by tuning electron-hole coupling in quantum wells • More pronounced for excitons (bound electron-hole pairs) • Red shiftof absorption edge • Smaller wave function overlap – lower α • Change of n through Kramers-Kronig relationship

  12. SiGe and GE QW Growth on Si • 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 Ge or SiGe Graded SiGe 10µm High Dislocation Density High-T Ge or SiGe Anneal 400nm Ge or SiGe 400nm Low-T SiGe Si Si Si Single-Tgrowth direct growth Graded buffer Two-Tgrowth direct growth

  13. SiGe Surface Morphology QWs require surface roughness ≤ 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 • As-grown roughness RMS ~ 0.2nm

  14. growth direction Ge/SiGe MQWs Compressive Tensile Strain force ε Strain-balanced Structure Average Si concentration in Ge/SiGe QWs equals that of SiGe buffer Y.-H. Kuo, et al, Nature 437, 1334 (2005)

  15. Ge/SiGe Modulator on Si Ge 10nm/ Si0.15Ge0.85 16nm Y.-H. Kuo, et al, Nature 437, 1334 (2005) Materials, Processes and Temperature are all CMOS-compatible

  16. Strong QCSE in Ge/SiGe QWs Y.-H. Kuo, et al, Nature 437, 1334 (2005)

  17. Fabrication Process

  18. Small Signal Modulation Bias: 2.5V Device top view size: 6µm *6 µm Response limited by contact resistance

  19. Modulator Light source Photodetector SiGeSn cap layer GeSn QWs SiGeSn buffer layer Si waveguide SiO2 Si substrate N-SiGe Ge Quantum Well(s) P-SiGe SiO2 P-Si Waveguide modulator Integrated WaveguideModulator, Detector and Laser Source, Modulator and Detector have identical QWs Function determined by bias polarity

  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

  21. GeSn material - Sn is semi-metal - Reported direct bandgap for SnxGe1-x is between 10% and 20% Sn - Lattice relaxed or compressive strained layer 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 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)

  22. Ge Laser Si-Ge Laser Spectrum Si-Ge Laser Structure Camacho-Aguilera-MIT OptExp20 11317 (2012) Good News—Ge can be made to lase Bad News—Insanely high threshold current

  23. Role of Heterostructures and Dimensionality on Lasers Impact of epitaxy, improved materials 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

  24. Highly Strained Ge Nano-bridge Nano-bridge Structure Calculated Gain & Loss Süess-PSI Nature Photon 10 1038 (2013) Free carrier absorption increases with carrier densities and creates high laser threshold current More sophisticated band engineering & QWs are essential

  25. The Potential of Ge/GeSn: Direct Bandgap • Advantages of Ge: • Si-compatible material • Low effective mass in Γ (0.038m0) • Nearly direct-bandgap and band engineer-able Γ Large effective mass (0.22m0) Inefficient optical transitions L Eg = 0.8eV Eg = 0.664eV Simplified Ge Bandstructure

  26. The Potential of Ge/GeSn:Direct Bandgap • Advantages of Ge: • Si-compatible material • Low effective mass in Γ (0.038m0) • Nearly direct-bandgap and band engineer-able The Biaxaial Tensile-Strain Effect Γ ~1.5% Strain Required Large effective mass (0.22m0) Inefficient optical transitions L Eg = 0.8eV Eg = 0.664eV Y. Huo, et al., APL (2011) Simplified Ge Bandstructure

  27. The Potential of Ge/GeSn:Direct Bandgap The Sn-Alloying Effect Γ ~6-8% Sn Required L R. Chen, et al., Applied Physics Letters 99 (2011) Simplified Ge Bandstructure

  28. 20 nm Ge 100 nm GaAs substrate Tensile strained Ge (TEM) In0.3Ga0.7As 10nm tensile strained Ge 10nm In0.3Ga0.7As 300nm In0.15Ga0.85As 200nm • InGaAs buffer layers: Defects are terminated at interface • Ge layer: 2.46% in-plane tensile strain Ge 10 nm

  29. Normalized intensity (a.u.) Strained Ge InGaAs Bulk Ge Raman shift (cm-1) In0.1Ga0.9As In0.2Ga0.8As In0.3Ga0.7As In0.4Ga0.6As Strain (Raman) Photoluminescence Measured Strain & PLin Ge/InGaAs

  30. 40% InGaAs 40% InGaAs GaAs Photoluminescence Ge/In0.4Ga0.6As Strained Ge 27% InGaAs 13% InGaAs Strained Ge/In0.4Ga0.6As is a Type II Heterojunction

  31. The Issue for GeSn:Solid-Solubility 3.5% Sn Attempted High-Temperature Segregation 400oC >4.5% Sn Increasing Sn SGTE Alloy Database, http://www.crct.polymtl.ca/fact/phase_diagram.php?file=Ge-Sn.jpg. High-Strain  Precipitation Y. Shimura, et. al. Jpn. J. Appl. Phys. 48 (2009)

  32. MBE Ideal for Investigative Tool GeSn and SiGeSn Challenges: • Low solid-solubility (1%) of Sn in Ge • MBE can decouple source and substrate growth temperatures • Challenges in finding precursors that decompose at low temperature • Very high-purity (99.999% or better) solid sources available for evaporation • Lattice constant changes greatly with Sn or Si alloying, adversely affecting the bandstructure and film quality Group IV Stack III-V Chamber (InGaAs/GaAs) Group IV Chamber (GeSiSn, GeSn) Strain Control with III-V

  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 InGaAs Our Method: MBE Growth on GaAs/lattice relaxed InGaAs GeSn InGaAs • 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) GaAs Anneal

  34. GaAs substrate TEM of 7% GeSn Layers Ge0.07Sn0.93 7 X greater than equilibrium solubility strained Ge or GeSn 10% InGaAs Buffer InxGa1-xAs 200nm GaAs Substrate 5 nm • High quality Ge93%Sn7% epi layer: • No defects • No precipitation (phase segregation) H. Lin, et al., Thin Solid Films 520 (2012)

  35. Surface Quality Maintainedw/High Sn Fraction Increasing Sn percentage 4.5% Sn, 100oC RMS = 0.529nm 7.0% Sn, 200oC RMS = 0.403nm 8.8% Sn, 100oC RMS = 0.626nm 4.5% and 7.0% Samples grown on In0.10Ga0.90As, ~50nm GeSn 8.8% Sn Sample grown on In0.25Ga0.75As Surface RMS roughness changes only slightly with increasing Sn %. H. Lin, et al., Thin Solid Films 520 (2012)

  36. Great Material Quality Possible with MBE GeSn with 10.5% Sn, low-T growth GeSn Film InGaAs Buffer H. Lin, et al., Thin Solid Films 520 (2012) RMS=0.519nm

  37. Where Does GeSn Become Direct Bandgap? R. Chen, et al., APL 99 (2011) Bowing = 2.1 eV, ~7% Sn 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!! Bowing = 2.4 eV, ~6.5% Sn

  38. SiGeSn/GeSn/SiGeSn Quantum Well Glue SiGeSn 30nm Ge GeSn 30nm Intensity for Ga and Ge (a.u.) Intensity for Si (a.u.) Si STEM-EDX Ga GeSn/SiGeSn Position (arb. unit) SiGeSn 50nm InGaAs buffer InGaAs buffer GaAs substrate

  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 Direct Indirect 1) H. Lin, et al., Appl. Phys. Lett. 100 141908 (2012) In-plane tensile strain 2) H. Lin, et al., Appl. Phys. Lett. 100 102109 (2012)

  40. GeSn Low-Temperature Photoluminescence T=20K T=294K

  41. Lattice-Matched Options for GeSn QWs Direct Bandgap Energy (eV) Unstrained Quantum Wells possible with the addition of Si

  42. The Stage Is Set – What About Lasers? Mirror Mirror Gain Region Photon Emission > Photon Absorption Onset of lasing when optical gain ≥ loss Require low-threshold lasers LOW LOSS • Optical 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 fabrication • Carrier 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

  43. Challenges for a GeSn Laser High Carrier Concentration Produces Free Carrier Absorption • 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)

  44. Gain Spectra for GeSn QWs Gain Spectrum for p=n=2.4e19 cm-3 Gain Spectrum for 8% Sn (GeSn) Increasing Sn Increasing Carrier Concentration Pure Ge 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-3for 1000 cm-1 ofgain for 8% Sn

  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

  46. The Benefits of Direct-Gap Materials ΔEc GeSn Photoluminescence Γ L R. Chen, et al., Applied Physics Letters 99 (2011) Increase in PL with Sn because more carriers occupy the direct Γ-valley! Sn alloying results in increased optical efficiency

  47. Carrier Confinement (for Ge/SiGe) Bias = 0.76V • Simulation: • 10nm Ge QW in Si0.2Ge0.8 pn junction • Carrier concentration: • 1.5E19 in QW and <1E18 in barrier • Carriers are confined in QW (15 – 50 X) • Calculated net gain of 200cm-1 • Experiment: • Ge 14nm*3QW • 200nm Ge grown on SiGe buffer • PL signal: • Stronger PL from QWs • Carrier confinement in QWs Xiaochi Chen et al. “Room Temperature Photoluminescence from Ge/SiGe Quantum Well Structure in Microdisk Resonator” [2012]

  48. Can Lase, but not easy with Q=100 100nm Ge 15% of TE mode experiences GeSn Gain and FCA 75% of TE mode experiences Ge FCA (no band-to-band absorption) 20nm GeSn (8%) 90nm Ge ~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

  49. GeSn Photoluminescence 5% GeSn Mostly Relaxed 3% GeSn Compressive 1% GeSn Compressive 0% GeSn Relaxed

  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

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