Simulation of photonic crystal enhanced Ge-on-Si single photon avalanche diodes

Simulations of single photon avalanche diodes (SPADs) based on the Ge-on-Si material platform are presented, highlighting the potential performance enhancement achievable with nano-hole array photonic crystal structures. Such structures can be used to enhance photon absorption and therefore increase single photon detection efficiencies (SPDE). However, there is yet to be a study of these structures in application to Ge-on-Si SPADs to determine if the optical enhancements can be realized as SPDE or to evaluate the change in dark count rate due to the nano-holes that form the photonic crystal. This work establishes an optimization and analysis platform for investigating photonic crystal structures on SPAD devices. Both a direct Ge etch method, and an etched amorphous Si design are compared to a reference device with an optimized anti-reflection coating. Finite difference time domain simulations were used to optimize the photonic crystal parameters for these structures, finding a potential absorption of up to 37.09 % at wavelengths of 1550 nm for a 1 µm absorption layer, compared to 11.33 % for the reference device. Subsequently, TCAD simulations and custom code were used to calculate the effective enhancement to SPAD performance metrics, as a function of material and passivation quality, showing up to 2.41x higher SPDE and 2.57x better noise-equivalent power is achievable provided etched surfaces are sufficiently well passivated.