Physics-Based Optoelectronic Simulation of Geometry-Engineered Ge-on-Si SPADs for SWIR Photon Detection

Ge-on-Si single-photon avalanche diodes (SPADs) are promising candidates for silicon-compatible short-wave infrared (SWIR) photon-counting arrays, but pixel scaling toward the 10-µm class intensifies the tradeoff among optical absorption, electric-field control, and defect-related dark-count generation. This work presents a physics-based optoelectronic simulation study of geometry-dependent field-absorption-breakdown-probability coupling in compact back-side-illuminated Ge-on-Si SPADs with geometry-engineered Ge absorbers. A process-inspired top-wide/bottom-narrow Ge absorber is used as a representative structure and compared with uniformly narrow and uniformly wide Ge absorber baselines under the same SACM stack, field-termination configuration, defect-parameter set, and illumination condition. The upper-wide Ge region enhances the optically active absorption volume, whereas the narrowed bottom Ge/Si interface and tapered Ge/SiO 2 boundary reduce the spatial overlap between defect-sensitive regions and high breakdown probability. Dark-count decomposition further clarifies the relative contributions of bulk generation, threading-dislocation-related generation, point-defect generation, and sidewall generation. At a 5% excess bias ratio, the top-wide/bottom-narrow structure is calculated to achieve an SPDE of 16.9% at 1310 nm and a DCR of 19.36 MHz. The corresponding NEP is reduced from 11.05 and 7.82 fW Hz -1/2 for the uniformly narrow and uniformly wide baselines, respectively, to 5.59 fW Hz -1/2 . Representative defect-and sidewall-parameter variations indicate that this NEP advantage is robust and does not rely on a single nominal defect-parameter set. These results provide a simulation-based physical guideline for absorber-geometry engineering in compact Ge-on-Si SPAD pixels and support future experimental validation and optimization.