Nanophotonic Design of Germanium-Silicon Single-Photon Detectors for Integrated Quantum Photonic Circuits

Scalable on-chip quantum photonic systems require single-photon detectors that simultaneously provide high detection efficiency, low timing jitter, high bandwidth, and low noise while remaining compatible with silicon photonics and room-temperature operation. Germanium-silicon (Ge/Si) single-photon avalanche detectors (SPADs) offer a promising CMOS-compatible platform for telecom-band photon detection; however, existing implementations often suffer from limited photon-detection efficiency, excess timing jitter, and challenges in monolithic integration with quantum photonic circuits. Here, we present a nanophotonic design of waveguide-integrated Ge/Si SPADs engineered to address these limitations through enhanced optical confinement, engineered germanium absorption, and optimized avalanche multiplication in silicon. Finite-difference time-domain (FDTD) and carrier-transport simulations demonstrate efficient evanescent coupling from the silicon waveguide into the Ge absorber while spatially confining the highest electrostatic field within the silicon multiplication region. Under simulated operating conditions, the proposed devices exhibit near-unity internal quantum efficiency, avalanche gain exceeding 10⁶, sub-50 ps intrinsic temporal response, electrical bandwidths beyond 5 GHz, and dark current below 100 pA, corresponding to low projected dark-count-rate operation. These projected performance characteristics establish a promising pathway toward high-fidelity telecom-band on-chip single-photon detection and scalable room-temperature integrated quantum photonic circuits for quantum information processing applications.