High-responsivity germanium photodetector enabled by dual corner reflectors

Germanium waveguide photodetectors are a fundamental building block for high-speed silicon photonic transceivers. While the responsivity is normally proportional to the area of the absorbing germanium layer, too large an active region increases the dark current and sacrifices the bandwidth. It is thus essential to improve the responsivity per unit area to achieve an overall optimal performance. Here, we demonstrate a vertical germanium photodetector with Si/Ge dual corner reflectors that recycle unabsorbed light in both the Si and Ge layers. The active region occupies a compact chip area of only 4.8 μm × 3.7 μm. It achieves a high responsivity of 0.93 A/W at 1550 nm and a low dark current of 1.7 nA at -2 V. This corresponds to a significantly high responsivity per unit area. Owing to the small active region, the measured 3 dB bandwidth reaches 77 GHz, supporting reception of 112 Gbps non-return-to-zero and 224 Gbps 4-level pulse-amplitude signals.