Solomon Assefa, Nature, March 2010 Reinventing germanium avalanche photodetector for nanophotonic on- chip optical interconnects Jeong-Min Lee High-Speed Circuits and Systems LAB Special Topics in Optical Communications

Contents 1.Abstract 2.Nanophotonic Ge waveguide-integrated APD 3.Impulse response of an APD 4.Sensitivity and excess noise measurement 5.Conclusion High-Speed Circuits and Systems LAB Special Topics in Optical Communications

Abstract Integration of optical communication circuits directly into high- performance microprocessor chips can enable extremely powerful computer systems. Ge PD with Si transistor technique: Chip components  Infrared optical signals  Capability to detect very-low-power optical signals at very high speed  Suffer from an intolerably high amplification noise characteristic of Ge Ge layer for detection of light source & Amplification taking place in a separate Si layer  High gain with low excess noise  Thick semiconductor layer: limit APD speed (10 GHz) with high bias voltages (25 V) High-Speed Circuits and Systems LAB Special Topics in Optical Communications

Abstract A Ge amplification layer can overcome the intrinsically poor noise characteristics  Achieving a dramatic reduction of amplification noise by over 70 % By generating strongly non-uniform electric fields, the region of impact ionization in Ge (30nm)  Noise reduction effects Smallness APD  Avalanche gain: 10 dB (30 GHz, 1.5 V) Application: Optical interconnects in telecommunications, secure quantum key distribution, and subthreshold ultralowpower transistors High-Speed Circuits and Systems LAB Special Topics in Optical Communications

Nanophotonic Ge waveguide-integrated APD  For on-chip interconnects, the germanium(Ge)-based APD photodetector should be integrated into a silicon waveguide that can route near-infrared light on a silicon chip.  Ideal APD: Compact micrometer-scale foot print, operate at a 1V  Compatible with CMOS technology, high avalanche gain, detect very fast optical signals of up to 40 Gbps.  Contradiction & Innovation High-Speed Circuits and Systems LAB Special Topics in Optical Communications  A waveguide-integrated Ge APD  Thickness and width of both Ge and Si layers were optimized to ensure the highest responsibility  Thickness: Ge (140 nm), Si (100 nm)  Width: Ge (750 nm), Si (550 nm)

Nanophotonic Ge waveguide-integrated APD  Provide propagation of at most only two optical modes in the combined layer stack for the transverse electric field polarization at both the 1.3 & 1.5 um wavelenghts.  Allows efficient coupling of light from the routeing silicon waveguide High-Speed Circuits and Systems LAB Special Topics in Optical Communications  The resulting optical power resides almost completely in top Ge layer (77%)  Short absorption length (10um)  minimize the APD capacitance (10 fF)

Nanophotonic Ge waveguide-integrated APD  Problem: Growth of such a thin Ge layer directly on top of Si using epitaxial technique  Large concentration of misfit dislocations  Solution: Rapid melting growth technique (Si – SiON – Ge) High-Speed Circuits and Systems LAB Special Topics in Optical Communications

Nanophotonic Ge waveguide-integrated APD Nanophotonic Ge waveguide-integrated APD  Very thin Ge layer  Ensure fast operation up to 40 Gbps  Cu – W – Ge: W plugs are in direct contact with the Ge layer  A series of metal-semiconductor-metal Schottky diode  Strong electric fields (30 kVcm -1 ) in small thickness of Ge (2.8 V) High-Speed Circuits and Systems LAB Special Topics in Optical Communications  High E  fast acceleration of both electrons and holes to their saturation velocities  Complete electrical isolation  block unwanted slow diffusion of photo- generated carriers  fast response

Impulse response of an APD  Exponential increase: A significant current gain (M = 3.5 V)  Over 1 V: fast component makes up 70% of the pulse area  Gain is fast & broadband (inset of Fig.2b) High-Speed Circuits and Systems LAB Special Topics in Optical Communications  Total area under the impulse response  total # of carriers collected at the electrodes  0.5 ~ 1.5 V flat: all photo- generated carriers are being collected  R = 0.4 A/W (1.3 um)  R = 0.14 A/W (1.5 um)

Impulse response of an APD  Avalanche gain origin: 1)p-i-n: uniform E distribution  MSM contact: non-uniform fields (red: exceeds 120 kVcm -1 )  high probability of impact ionization 2)A series of small-signal radio-frequency measurements: High-Speed Circuits and Systems LAB Special Topics in Optical Communications  10 MHz ~ 1 GHz: flat frequency response  (Fig.3a) 3 dB BW: 5 ~ 34 GHz (0.1 ~ 1.1 V)

Impulse response of an APD  (Fig.3d) Gain flat btw 0.4 ~ 0.8 V  collection of all photo-generated carriers  Similar high M but higher voltages around 3.7 V  Higher bias  BW constant (carriers reach their saturation velocity)  However, gain x bandwidth continues to grow (because of rise in avalanche gain)  300 GHz  Saturation of the bandwidth before considerable gain is reached  carrier transport and avalanche amplification are taking place in spatially separated areas within the APD High-Speed Circuits and Systems LAB Special Topics in Optical Communications Red : 200 nm contact spacing Blue: 400 nm contact spacing

Sensitivity and excess noise measurement  A large (10 dB) avalanche gain in the APD does not necessarily guarantee a corresponding increase in the detector sensitivity  Can easily degrade as a result of the higher excess noise level High-Speed Circuits and Systems LAB.12 (Fig.4a)  sensitivity continues to improve even after the unity gain plateau is reached, at around 0.7 V

Sensitivity and excess noise measurement  (Fig.4b) Improvement of sensitivity measured at a BER of  Sensitivity: -8 dBm (Absolute)  A significant improvement of 5.9 dB at a bias of 3.2 V was achieved (Gain: 11.8 dB)  High dark current  main factor resulting in saturation of sensitivity improvement (50 a unity gain)  K eff = 0.1  Improvement in sensitivity of over Gbps  can be expect that dark current could be suppressed 10 times High-Speed Circuits and Systems LAB.13

Sensitivity and excess noise measurement  K eff : effective ratio of ionization coefficient for electrons and holes  almost equal in bulk Ge (k eff = 0.9)  large excess noise  conventional Ge APD uncompetitive for building digital optical links  Total reduction of noise can be estimated as more than 70% wrt the noise expected for a bulk Ge High-Speed Circuits and Systems LAB.14

Conclusion  Several factors can account for the dramatic reduction of excess multiplication noise in our nanophotonic APD 1)The avalanche multiplication is happening only in very close proximity to the W plug (30 nm)  Thinning the multiplication region  excess noise reduce 2)Initial energy effect  carriers entering the multiplication region have already acquired high energy  narrow the probability distribution functions and suppress excess multiplication noise 3)The large electric field gradients  further narrowing of the probability distribution functions owing to the fast acceleration of secondary carriers towards the ionization threshold. High-Speed Circuits and Systems LAB Special Topics in Optical Communications

Thank you for listening Jeong-Min Lee High-Speed Circuits and Systems Special Topics in Optical Communications