Simulation of a back-side illuminated Ge-on-Si single-photon avalanche diode with a light-trapping structure

Germanium-on-silicon single-photon avalanche diodes (SPADs) leverage the advantages of germanium for absorption and silicon for multiplication, offering excellent CMOS compatibility and significant potential for short-wave infrared (SWIR) detection. However, their detection efficiency at 1310 nm and 1550 nm remains limited by the device structure. In this work, we propose a back-side illuminated (BSI) Ge-on-Si SPAD with a light-trapping structure, comprising a nano-cone array on the substrate back side and an aluminum reflector on the device front side. This configuration substantially extends the effective optical path within the device. To evaluate the performance of the designed structure, finite-difference time-domain (FDTD) and TCAD simulations were conducted to optimize the nanostructure design. Simulation results demonstrate that the proposed Ge-on-Si SPAD achieves single-photon detection efficiencies (SPDEs) of 37.7% and 21.2% at 1310 nm and 1550 nm, respectively. Moreover, by employing a masked implantation technique to control the electric field distribution, the device exhibits a 61.2% reduction in dark count rate (DCR) at room temperature, with further suppression expected when operated at cryogenic temperatures. This work paves the way for larger SPAD arrays and higher integration levels in SWIR applications.