Avalanche photodiodes on silicon photonics

Silicon photonics technology has drawn significant interest due to its potential for compact and high-performance photonic integrated circuits.The Ge-or III-V material-based avalanche photodiodes integrated on silicon photonics provide ideal high sensitivity optical receivers for telecommunication wavelengths.Herein,the last advances of monolithic and hetero-geneous avalanche photodiodes on silicon are reviewed,including different device structures and semiconductor systems.

An advanced Ⅲ-Ⅴ-on-silicon photonic integration platform

In many application scenarios,silicon(Si)photonics favors the integration of Ⅲ-Ⅴ gain material onto Si substrate to real-ize the on-chip light source.In addition to the current popular integration approaches of Ⅲ-Ⅴ-on-Si wafer bonding or dir-ect heteroepitaxial growth,a newly emerged promising solution of epitaxial regrowth on bonded substrate has attracted a lot of interests.High-quality Ⅲ-Ⅴ material realization and successful laser demonstrations show its great potential to be a promising integration platform for low-cost,high-integration density and highly scalable active-passive photonic integra-tion on Si.This paper reviews recent research work on this regrowth on bonded template platform including template de-velopments,regrown material characterizations and laser demonstrations.The potential advantages,opportunities and challenges of this approach are discussed.

Ⅲ/Ⅴ-on-Si MQW lasers by using a novel photonic integration method of regrowth on a bonding template

Silicon photonics is becoming a mainstream data-transmission solution for next-generation data centers,highperformance computers,and many emerging applications.The inefficiency of light emission in silicon still requires the integration of a Ⅲ/Ⅴ laser chip or optical gain materials onto a silicon substrate.A number of integration approaches,including flip-chip bonding,molecule or polymer wafer bonding,and monolithic Ⅲ/Ⅴ epitaxy,have been extensively explored in the past decade.Here,we demonstrate a novel photonic integration method of epitaxial regrowth of Ⅲ/Ⅴ on a Ⅲ/Ⅴ-on-SOI bonding template to realize heterogeneous lasers on silicon.This method decouples the correlated root causes,i.e.,lattice,thermal,and domain mismatches,which are all responsible for a large number of detrimental dislocations in the heteroepitaxy process.The grown multi-quantum well vertical p-i-n diode laser structure shows a significantly low dislocation density of 9.5×104 cm^(−2),two orders of magnitude lower than the state-of-the-art conventional monolithic growth on Si.This low dislocation density would eliminate defect-induced laser lifetime concerns for practical applications.The fabricated lasers show room-temperature pulsed and continuous-wave lasing at 1.31μm,with a minimal threshold current density of 813 A/cm^(2).This generic concept can be applied to other material systems to provide higher integration density,more functionalities and lower total cost for photonics as well as microelectronics,MEMS,and many other applications.

Mechanisms of enhanced sub-bandgap absorption in high-speed all-silicon avalanche photodiodes

All-silicon (Si) photodiodes have drawn significant interest due to their single and simple material system and perfect compatibility with complementary metal-oxide semiconductor photonics. With the help from a cavity enhancement effect, many of these photodiodes have shown considerably high responsivity at telecommunication wavelengths such as 1310 nm, yet the mechanisms for such high responsivity remain unexplained. In this work,an all-Si microring is studied systematically as a photodiode to unfold the various absorption mechanisms.At-6.4 V, the microring exhibits responsivity up to0.53 A∕W with avalanche gain, a 3 dB bandwidth of25.5 GHz, and open-eye diagrams up to 100 Gb/s. The measured results reveal the hybrid absorption mechanisms inside the device. A comprehensive model is reported to describe its working principle, which can guide future designs and make the all-Si microring photodiode a promising building block in Si photonics.

64 Gb/s low-voltage waveguide SiGe avalanche photodiodes with distributed Bragg reflectors

We demonstrate low-voltage waveguide silicon-germanium avalanche photodiodes(APDs)integrated with distributed Bragg reflectors(DBRs).The internal quantum efficiency is improved from 60%to 90%at 1550 nm assisted with DBRs while still achieving a 25 GHz bandwidth.A low breakdown voltage of 10 V and a gain bandwidth product of near 500 GHz are obtained.APDs with DBRs at a data rate of 64 Gb/s pulse amplitude modulation with four levels(PAM4)show a 30%–40%increase in optical modulation amplitude(OMA)compared to APDs with no DBR.A sensitivity of around-13 d Bm at a data rate of 64 Gb/s PAM4 and a bit error rate of 2.4×10^-4 is realized for APDs with DBRs,which improves the sensitivity by^2 d B compared to APDs with no DBR.

Ultra-power-efficient heterogeneous Ⅲ–Ⅴ/Si MOSCAP(de-)interleavers for DWDM optical links

We discuss the design and demonstration of various Ⅲ–Ⅴ/Si asymmetric Mach–Zehnder interferometer(AMZI)and ring-assisted AMZI(de-)interleavers operating at O-band wavelengths with 65 GHz channel spacing. The wafer-bonded Ⅲ–Ⅴ/Si metal-oxide-semiconductor capacitor(MOSCAP) structure facilitates ultra-low-power phase tuning on a heterogeneous platform that allows for complete monolithic transceiver photonic integration.The second-and third-order MOSCAP AMZI(de-)interleavers exhibit cross-talk(XT) levels down to -22 dB and -32 dB with tuning powers of 83.0 nW and 53.0 nW, respectively. The one-, two-, and three-ring-assisted MOSCAP AMZI(de-)interleavers have XT levels down to -27 dB,-22 dB, and-20 dB for tuning powers of 10.0 nW, 7220.0 nW, and 33.6 nW, respectively. The leakage current density is measured to be in the range of 1.6–27 μA∕cm^(2). To the best of our knowledge, we have demonstrated for the first time, athermal Ⅲ–Ⅴ/Si MOSCAP(de-)interleavers with the lowest XT and reconfiguration power consumption on a silicon platform.