New advances in silicon photonics Delphine Marris-Morini

New advances in silicon photonics Delphine Marris-Morini

P. Brindel Alcatel-Lucent Bell Lab, Nozay, France New Advances in silicon photonics D. Marris-Morini, L. Virot*, D. Perez-Galacho, X. Le Roux, D. Bouville, P. Chaisakul, M-S. Rouifed, L. Vivien Institut d'electronique Fondamentale, Université Paris-Sud, France; * also with ST Microelectronics and CEA-Leti C. Baudot, A. Souhaité, F. Bœuf, ST Microelectronics, Crolles, France J-M. Fédéli, J.-M. Hartmann, N. Vulliet, S. Olivier, CEA, LETI, Grenoble, France J. Frigerio, D. Chrastina, S. Cecchi, and G. Isella L-NESS, Politecnico di Milano, Como, Italy 1. Ge photodetector and Si modulators on 300 mm SOI 2. Advanced concepts for future low power consumption communications

Data centers Optical telecommunications Environment Interconnects Silicon photonics Chemical/Biological sensors Free space communications FTTH Military

Optical communication link using silicon photonics Goal : silicon photonics for : Intra-chip communications Inter-chip communications Data-com communications Silicon photonics on 300 mm SOI Advanced concepts Si SiO 2 SiO 2 Si 40 Gbit/s modulator and photodetector in microelectronic fab Low power consumption modulator and photodetector (University clean room) 4

Silicon photonics on 300 mm platform

Germanium photodetector Lateral Ge photodiode buttcoupled with SOI waveguide Absorption coefficient of pure Ge: 9000 cm -1 at =1.3µm L 95% ABS 3.3µm (!) Low capacitance devices High frequency operation Lattice misfit with Si of about 4.2% specific growth strategies required (wafer-scale and localized) High carrier mobility 6

Germanium on silicon Two-step growth process: Direct growth of Ge on Si using a low temperature ( 350 ) CVD process thin (a few 10nm) highly-dislocated Ge layer Growth of a thick Ge layer (a few 100nm) at a higher temperature ( 600 ) high quality Ge absorbing layer Thermal annealing to reduce the dislocation density HT Ge ( 600 C, 300-500nm) LT Ge ( 350 C, 50nm) Si substrate (001) Optical absorption Close to bulk values Bandgap shrinkage (50 nm) Tensile strain Absorption up to 1.6 µm 7

Ge on Si photodetector Static properties Operational photodiodes = 98.3% Dark Current Best dark current = 3.9nA Dark Current < 1µA > 94% Dark current median value = 21.9nA Dark current mean value = 182.5nA Standard Deviation = 556.7nA (@ -1V)

Ge on Si photodetector Dynamic properties 3dB cuttoff (0V) = 14GHz (-1V) = 37GHz (-2V) > 40 GHz Al Pad Cu line W Contact Ge

Si modulator based on interleaved pn diodes The overlap between the optical mode and the depletion region can be maximized

Si modulator fabrication 300mm wafer High volume Sub-65nm CMOS node 193nm immersion photolithography Sub-50nm resolution Zoom ~ x 750 Million! Wafer thickness uniformity < ±5nm on 300-mm ~ ±10nm on 200-mm Yield Fabrication Crolles 2 (Fr)

Silicon modulator fabrication Interleaved diode phase shifter Ring resonator Mach-Zehnder Interferometer (MZI) R = 100µm Length = 950 µm

Si modulator characterization: static Ring resonator + interleaved phase shifter Phase shifter efficiency (V p L p ) Active region loss @ 0V MMI loss Waveguide loss 2.4 V.cm ~ 2.1 db/mm 0.7 to 0.9 db < 1 db/cm

Mach Zehnder modulator : RF characteristics Mach Zehnder + interleaved phase shifter -3dB cut-off frequency > 20 GHz S 11 < 20 db

Si modulator: 40 Gbit/s data transmission 40 Gbit/s ring modulator (R=100µm) ER=3.1dB On-chip loss @operating point ~1dB 40 Gbit/s Mach-Zehnder modulator (L=950 µm) ER=7.8dB On-chip loss @operating point ~4 db

Modulator s driving power ITRS Roadmap : Optical interconnect : ( ) A large variety of CMOS compatible modulators have been proposed in the literature ( ) The primary challenges for optical interconnects at the present time are producing cost effective, low power components. Mach Zehnder 3 pj/bit Transmitter specifications : ~100 fj/bit to a few fj/bit (D.A.B. Miller, Opt Exp., 2012) Ring resonator 0.5 to 1 pj/bit Electroabsorption modulator?

Electroabsorption in Group IV materials Germanium: indirect gap material with direct gap properties Ge/SiGe quantum well Electroabsorption by Quantum Confined Stark Effect (QCSE) E=0 E 0

electroabsorption Ge/SiGe QW modulator and photodetector 90 µm Ge QW LEPECVD growth (L-Ness, Politechnico di Milano) Fabrication in university clean-room (cm 2 platform) P. Chaisakul et al, Opt. Express (2013) Extinction ratio > 6 db on 20 nm spectral range Power consumption: 70 fj/bit Electro-optical bandwidth > 20 GHz Remind : Mach Zehnder 3 pj/bit Anneau 0.5 1 pj/bit Responsivity : 0.5-1A/W

Wavelength tuning Previous demonstrations : wavelength ~ 1440 nm 10 nm Ge well/ 15 nm Si 0.15 Ge 0.85 barrier strained-balanced on Si 0.1 Ge 0.9 buffer Si 0.15 Ge 0.85 15 nm Ge well 10 nm Si 0.15 Ge 0.85 15 nm Strain-compensated structure : Ge well: Compressive strain Si 0.15 Ge 0.85 barrier: Tensile strain Towards operation at 1.3µm: Increase of compressive strain in Ge wells Ishikawa et al, APL 2003 10 nm Ge well/ 15 nm Si 0.35 Ge 0.65 barrier strained balanced on Si 0.21 Ge 0.79 buffer

Active region design for operation at 1.3µm Surface illuminated diode : photocurrent measurement 20 periods : 10 nm Ge well 15 nm Si 0.35 Ge 0.65 barrier on Si 0.21 Ge 0.79 buffer Absorption variation by QCSE at 1.3 µm

Photonics integrated circuits based on Ge/SiGe QW? Si 0.1 Ge 0.9 N doped Si 0.1 Ge 0.9 QWs 2 µm Si 0.1 Ge 0.9 relaxed buffer Si 0.1 Ge 0.9 Si 0.15 Ge 0.85 Ge 15 nm 10 nm Si 0.15 Ge 0.85 15 nm Si 0.1 Ge 0.9 P doped 2 µm Si 0.1 Ge 0.9 relaxed buffer 13 µm graded buffer from Si to Si 0.1 Ge 0.9 13 µm graded buffer graduel from Si to Si 0.1 Ge 0.9 Si Schematic description Si More realistic scale Challenge : coupling the light from silicon to Ge/SiGe QW 21/61

Integration on bulk silicon 1 st option: waveguide in the relaxed SiGe layer (thanks to the graded buffer ) Ge concentration in the waveguide : compromise between Si 0.09 Ge 0.91 N doped Strain compensation Optical loss Si 0.09 Ge 0.91 QWs Si 0.09 Ge 0.91 Si 0.15 Ge 0.85 Ge Si 0.15 Ge 0.85 15 nm Si 0.09 10 Ge nm 0.91 P doped 15 nm P 1.5 µm Si 0.16 Ge 0.84 relaxed buffer 8 µm graded buffer from Si to Si 0.17 Ge 0.83 P. Chaisakul et al, Nature Photonics (2014) Si Optical loss of each device, including input/output coupling with Si 0.16 Ge 0.84 waveguide < 5dB

Integration on SOI 2 nd option : decrease the thickness of the buffer layer Ge/SiGe QW epitaxially grown on SOI Buffer homogène Si 0.1 Ge 0.9 Si SiO 2 360 nm Checking of the QW quality Good material quality deduced from photocourant measurement on surface illuminated diodes Design of integrated devices on SOI Ge/SiGe modulator integrated with SOI : estimated performance : Extinction ratio = 7.7 db, loss = 4 db M-S. Rouifed et al, JSTQE 2014

Conclusion Photonic links may replace copper links, even for very short distances Silicon photonics is more and more a mature technology with the demonstration of 40Gbit/s optoelectronic devices including optical modulators and photodetectors Development of silicon photonics on 300mm platform Advanced concepts for low power consumption optoelectronic devices : Ge/SiGe QW for modulation and photodetection