Ge-on-Si Avalanche Photodiodes With Photon-Trapping Nanostructures for Sensing and Optical Quantum Applications

High-sensitivity Ge/Si avalanche photodiodes (APDs) have recently gained attention for their application in sensing and optical communication due to their low cost and CMOS-compatible process. However, compared to commercial III–V compound APDs, Ge/Si APDs usually suffer from the issue of relatively low primary responsivity. In this article, we report Ge-on-Si separate absorption, charge, and multiplication APDs (SACM-APDs) with photon-trapping nanostructures to enhance light absorption. Besides, by optimizing the depth of the holes, the photon-trapping structure could reduce the dark current without compromising the avalanche effect as confirmed by both simulations and experimental results. As a result, the responsivity of the photon-trapping APDs increases by 20%–50% from that of control APDs at the 1550 nm wavelength band. Furthermore, a quantum efficiency higher than 80% could be achieved at 1550 nm when the photon-trapping Ge-on-Si APD is on Si-on-insulator (SOI) platforms as predicted by simulations. Our results demonstrate that the photon-trapping Ge/Si APDs exhibit superior dark current, light absorption, and gain than those of the control devices, which have potential applications in sensing and optical quantum communications.