High-performance compact vertical germanium photodiodes enabled by silicon corner reflectors

High-performance germanium photodiodes are critical components in silicon photonic systems for high-capacity data communications. By reducing the length of the photodiodes, a smaller resistance–capacitance product can be achieved, leading to a larger bandwidth and lower dark current. However, this also leads to diminished responsivity due to insufficient light absorption. Here, we introduce a silicon corner reflector (SCR) to alleviate this issue by reflecting and recycling the unabsorbed light. The process of evanescent coupling between the silicon and germanium layers is elaborately engineered to optimize the efficiency of light absorption. Experimentally, a responsivity of 0.96 A/W, which is a 21% increase compared to the one without SCR, is achieved at 1550 nm with a germanium length of 4.8 μm. Simultaneously, a remarkably low dark current of 0.76 nA and a large bandwidth of 100 GHz are achieved. Open eye diagrams of 140 Gb/s on–off keying and 240 Gb/s four-level pulse amplitude signals are obtained. To the best of our knowledge, this work achieves the lowest dark current density and noise equivalent power to date and offers a promising solution for low-cost, high-performance optical detection.