High speed silicon-based optoelectronic devices Delphine Marris-Morini Institut d Electronique Fondamentale, Université Paris Sud

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1 High speed silicon-based optoelectronic devices Delphine Marris-Morini Institut d Electronique Fondamentale, Université Paris Sud

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

3 Optical telecommunication link I t Photodétecteur detectors CMOS module Laser Source laser Modulateur Optical modulator optique U t I t Virage 90 turn 90 Diviseurs splitter 2 Optoelectronic devices = key building blocks For this presentation : focus on optical modulator

4 Optical modulation in silicon I 0 Optical intensity t Electrical driver Optical modulator t I max I min Modulated optical intensity t Figures of merit V p L p IL f c ER Modulation efficiency Insertion loss -3dB bandwidth Extinction ratio Voltage swing Power consumption

5 Optical modulation in silicon I 0 Optical intensity t Electrical driver Optical modulator t I max I min Modulated optical intensity t Electroabsorption Absorption coefficient variation with electric field Intensity modulation Electrorefraction Refractive index variation with electric field In silicon : index variation by free carrier concentration variation

6 Outline Introduction Carrier depletion based silicon optical modulators fabricated on 300 mm SOI wafers Ge/SiGe quantum well electroabsorption modulator Conclusion

7 Silicon optical modulator Metal PIPIN diode Metal P-doped region N-doped region Europe: Univ. Paris Sud, CEA Leti, Univ. of Southampton, IMEC, Gent Univ. Asia: A*Star, Petra, AIST, Chinese Academy of Sciences, Samsung Electronics, Tokyo Institute of Technology North America: Intel, IBM, Cornell, Luxtera, Ligthwire, Kotura, Oracle

8 Fabrication : 200 nm versus 300 nm 200mm wafer 300mm wafer High volume Sub-65nm CMOS node 193nm immersion photolithography Sub-50nm resolution Zoom ~750 Million! Wafer thickness uniformity < ±5nm on 300-mm ~ ±10nm on 200-mm >55 Million ring resonators! Fabrication Crolles 2 (Fr)

9 Silicon optical modulator: phase shifter Carrier depletion in interleaved pn diode

10 DC transmission characterisation Interleaved pn diode Phase shifter efficiency (V p L p ) Active region 0V MMI loss Waveguide loss V.cm ~ 2.1 db/mm 0.7 to 0.9 db < 1 db/cm

11 RF characteristics Mach Zehnder interferometer : 950 µm active region -3dB cut-off frequency > 20 GHz

12 40 Gbit/s data transmission Mach Zehnder interferometer : 950 µm active region 40 Gbit/s Extinction ratio (L=0.95 mm) Total on chip operating point of high extinction ratio Interleaved pn diode 7.8 db 4 db

13 Silicon optical modulator ITRS: Optical interconnect ( ) A large variety of CMOS compatible modulators have been proposed in the literature ( ) ( ) significant progress has been made ( ) The primary challenges for optical interconnects at the present time are producing cost effective, low power components. Short distance and high volume applications (electrical bottleneck) Optical cables Data-center Main challenges: Driving voltage of modulator Power consumption Electroabsorption modulator

14 Power consumption ~3 pj/bit ~0.5 to 1 pj/bit?

15 Outline Introduction Carrier depletion based silicon optical modulators fabricated on 300 mm SOI wafers Ge/SiGe quantum well electroabsorption modulator Conclusion

16 Electroabsorption in group IV materials Bandgap Bulk-Si Bulk-Ge Direct Indirect 3.3 ev 0.37µm 1.13 ev 1.1µm 0.8 ev 1.55 µm 0.66 ev 1.88µm Silicon : transparent at l > 1.1 µm, and indirect bandgap material => Electroabsorption is not possible at telecommunication wavelengths Germanium : indirect gap material with direct gap properties, at telecommunication wavelengths.

17 Electroabsorption in QW structures Quantum confined Stark effect (QCSE) a E=0 E 0 l Excitonic peak reduction and red-shift absorption spectra with electrical field

18 Active region design and growth Si 0.15 Ge nm SiGe n-type SiGe spacer QWs SiGe spacer Ge 10 nm Si 0.15 Ge nm SiGe p-type Si 0.1 Ge 0.9 relaxed buffer QW structure: 10 nm Ge well/ 15 nm Si 0.15 Ge 0.85 barrier Strain-compensated structure Average Ge in MQWs = 90% => Si 0.1 Ge 0.9 buffer Graded buffer from Si to Si 0.1 Ge 0.9 Si(100) 4.2% lattice mismatch between Si and Ge

19 Active region design and growth Si 0.15 Ge nm Si 0.1 Ge 0.9 n-type Si 0.1 Ge 0.9 spacer QWs Si 0.1 Ge 0.9 spacer Ge 10 nm Si 0.15 Ge nm Si 0.1 Ge 0.9 p-type Si 0.1 Ge 0.9 relaxed buffer Graded buffer from Si to Si 0.1 Ge 0.9 Low-rate growth (~0.3 nm/s) for optimum layer and interface control Around 1 hr Ge QW Low Energy Plasma Enhanced Chemical Vapour Deposition High-rate growth (5-10 nm/s) (LEPECVD) 40 minutes for 13 µm buffer Si(100) LEPECVD Graded buffer : TDD (Threading dislocation denstity) ~ 10 6 cm -2 G. Isella, D. Chrastina, J. Frigerio

20 Electroabsorption modulator Ge/SiGe quantum well modulator 20 periods of Ge/SiGe QW 90 µm 3 µm 1 µm Light

21 Static performance: optical transmission Ge/SiGe quantum well modulator ~6x10 4 V/cm ~7.5x10 4 V/cm Bias from 0 to 5V : Extinction Ratio (ER) > 6 db for 20 nm range Insertion Loss (IL) : 5 to 15 db Overlap with virtual substrate ~3x10 4 V/cm ~4.5x10 4 V/cm

22 Static performance: optical transmission Ge/SiGe quantum well modulator ~6x10 4 V/cm ~7.5x10 4 V/cm Bias from 0 to 5V : Extinction Ratio (ER) > 6 db for 20 nm range Insertion Loss (IL) : 5 to 15 db Overlap with virtual substrate ~3x10 4 V/cm ~4.5x10 4 V/cm Bias from 3 to 4V (1V swing) ER > 6 db for 5nm range

23 High frequency performance Ge/SiGe QW modulator -3dB cut-off frequency > 20 GHz

24 Power consumption ~3 pj/bit ~0.5 to 1 pj/bit 70 fj/bit

25 Ge/SiGe QW modulator : on-going work Design of a Ge/SiGe modulator on SOI 1.3 µm Wavelength ~ 1440 nm Towards operation at 1.3µm: Increase of compressive strain in Ge wells 10 nm Ge well/ 15 nm Si 0.15 Ge 0.85 barrier strained-balanced on Si 0.1 Ge 0.9 buffer 10 nm Ge well/ 15 nm Si 0.35 Ge 0.65 barrier strained-balanced on Si 0.21 Ge 0.79 buffer Ge well: Compressive strain Si 0.15 Ge 0.85 barrier: Tensile strain

26 Ge/SiGe QW : 1.3 µm Surface illuminated diode : photocurrent measurement

27 Ge/SiGe QW modulator : integration on SOI Si 0.1 Ge 0.9 n-type Si 0.1 Ge 0.9 spacer QWs Si 0.1 Ge 0.9 spacer Si 0.15 Ge nm Ge 10 nm Si 0.15 Ge nm Si 0.1 Ge 0.9 p-type Si 0.1 Ge 0.9 relaxed buffer Graded buffer from Si to Si 0.1 Ge 0.9 Graded buffer from Si to Si 0.1 Ge 0.9 Si(100) Si(100) Schematic description Real scale Challenge : light coupling from silicon to QW region?

28 Ge/SiGe QW modulator : integration on SOI Challenge : reduction of the buffer thickness keeping high quality structures and good material properties Thin buffer Si(100) Graded buffer from Si to Si 0.1 Ge 0.9 Si(100) New process development : 360 nm thick Si 0.08 Ge 0.92 buffer obtained by the sequentially growing of four 90 nm thick Si 0.08 Ge 0.92 films. Each growth performed at 400 followed by thermal annealing at 780 to reduce dislocation density and to form a relaxed buffer layer G. Isella

29 Growth and characterization Ge/SiGe QW : QCSE on thin buffer Surface illuminated diode : photocurrent measurement Low dislocation density Good QCSE is observed.

30 Design of the modulator Ge/SiGe QW modulator : design of the integrated modualtor Estimated performance : 4 db loss and 7.7 db extinction ratio On going work : fabrication

31 Conclusion : silicon based optical modulator Carrier depletion based silicon modulators : Fabrication in 300 mm SOI wafers Good performances : Mach Zehnder modulator : extinction ratio : 7.9 db ER and insertion loss = 4 db Ge/SiGe QW modulators : The solution to achieve energy consumption < 100 fj / bit First demonstration : extinction ration > 6 db, cut off frequency > 20 GHz Challenge : integration on SOI Preliminary results : development of thin buffer, active region design to work at 1.3 µm

32 Acknowledgments P. Chaisakul G. Rasigade M. Ziebel M-S. Rouifed D. Perez-Galacho D. Bouville S. Edmond X. Le Roux P. Crozat L. Vivien Funding S. Menezo J-M. Fédéli N. Vulliet P. Rivallin S. Olivier J-M. Hartman L. Virot A. Souhaité C. Baudot F. Boeuf G. Isella D. Chrastina J. Frigerio