64 Gb/s low-voltage waveguide SiGe avalanche photodiodes with distributed Bragg reflectors
Data communications in data centers and high-performance computing (HPC) have grown tremendously due to emerging applications in social media, video streaming, artificial intelligence (AI), and the internet of things (IoT). Hyperscale data centers and Exascale HPC require high-bandwidth and energy-efficient optical interconnects. In this context, silicon photonic interconnects are attractive, thanks to the high integration and low cost. To further increase the data rate, advanced modulation formats are preferred which can relax the bandwidth limitation of integrated circuitry or optical devices. However, the optical link may suffer degraded signal-to-noise ratio (SNR), which requires either a higher-power laser or a higher-sensitivity receiver to retain the bit error rate (BER). Using a receiver with better sensitivity can yield a lower total link power consumption compared to using a high-power laser and consequently improve the energy efficiency. Particularly, high-sensitivity detectors relax the link budget requirements for on-chip lasers with limited output power. An avalanche photodiode (APD) with internal gain is the ideal candidate to increase the receiver sensitivity. Since APDs introduce the excess noise while bringing the multiplication gain, a novel device structure with optimum layer thickness and doping profile is necessary for higher gain and lower noise. Typically, there are trade-offs among APD design metrics: breakdown voltage, quantum efficiency, multiplication gain, bandwidth, and excess noise. It is a big challenge to decouple these trade-offs and optimize the overall performance.
The APD with a DBR proposed by Dr. Binhao Wang from Hewlett Packard Labs in Photonics Research, Vol. 8, Issue 7, 2020 (Binhao Wang, Zhihong Huang, Yuan Yuan, et al. 64 Gb/s low-voltage waveguide SiGe avalanche photodiodes with distributed Bragg reflectors[J]. Photonics Research, 2020, 8(7): 07001118) breaks the trade-off between quantum efficiency and bandwidth while retaining high gain, low breakdown voltage and low excess noise without additional fabrication steps. Compared to most III-V compound devices, SiGe APDs have lower noise and higher bandwidth due to the low impact ionization coefficient ratio in silicon. The proposed SiGe APD is with a separate absorption and charge multiplication (SACM) structure to take advantages of the large absorption in Ge in the near-infrared region and the low multiplication noise in Si. The waveguide APD design is employed rather than normal incidence due to the smaller parasitic capacitance as well as the decoupling of quantum efficiency and carrier transit time, achieving higher quantum efficiency and bandwidth. In addition, waveguide APDs can be integrated in complex photonic integrated circuits (PICs) such as wavelength division multiplexers (WDMs) for many applications. With the help of the integrated distributed Bragg reflector (DBR), the APD quantum efficiency is improved from 60% to 90% in C band. APDs with DBRs can still achieve a 25 GHz bandwidth, which is comparable to APDs with no DBR. A low breakdown voltage of 10 V and a gain bandwidth product of near 500 GHz are obtained. Error-free transmission at 64 Gb/s is successfully demonstrated.
Dr. Zhihong Huang from Hewlett Packard Labs believes that the creative APD design with excellent performance in breakdown voltage, quantum efficiency, multiplication gain, bandwidth, and excess noise will play an important role for next generation high-bandwidth and energy-efficient optical interconnects in mega data centers and high-performance computing. Next step, we will demonstrate a high-speed silicon photonic WDM transceiver with the SiGe APD.
Schematic of a waveguide SiGe APD integrated with a DBR