Bias- and Geometry-Dependent Ionization Effects on Waveguide-Integrated Germanium-on-Silicon p-i-n Photodiodes

Abstract

This work investigates the waveguide-integrated vertical p-i-n (VPIN) germanium-on-silicon (Ge-Si) photodiodes under 10-keV X-ray irradiation to evaluate the impact of geometrical features, implant design, and bias during irradiation on total ionizing dose (TID) response. The use of localized p+ contact implantation for high efficiency leads to heightened TID sensitivity, due to ionization-induced oxide trapped charges at the top Ge/SiO2 interface. This more than doubles the dark current, as compared to designs with continuous contact doping. The TID-induced response of each photodiode design is strongly influenced by bias during irradiation, with increases in dark current up to ~100% for −2 V bias, compared to grounded test conditions. This increase is due to electric field enhancement near the Ge/SiO2 interface, as demonstrated by technology computer-aided design (TCAD) simulations. The application of forward bias after irradiation leads to swift annealing of TID effects and partial performance recovery due to injection-enhanced tunneling near the Ge/SiO2 interface. The observed dark current sensitivities and mitigation by forward bias annealing suggest that Ge-Si photodiodes are well-suited for applications in high-radiation environments.