High-resolution spatial responsivity of PureB single-photon avalanche diodes using focused-laser scanning

PureB single-photon avalanche diodes (SPADs) integrate ultra-shallow p⁺ⁿ junctions formed by nanometer-thin boron-layer deposition, providing high sensitivity to shallow-penetration photons and particles. PureB SPADs have achieved low dark count rates below 1 Hz/µm² and were designed with an implicit guard ring configuration to increase the fill factor. The fabricated devices include SPADs both with and without an n⁺ buried layer, where contamination during processing was identified as a source of bulk defects in the silicon. In this work, we present a spatial responsivity analysis of PureB SPADs using a focused-laser scanning optical-setup. The setup was calibrated on conventional PureB photodiodes using continuous-wave lasers at 520 nm and 638 nm, and pulsed lasers at 660 nm and 1060 nm, with spot sizes down to 2 µm as determined by using the knife-edge method. High-resolution spatial scanning of PureB SPADs identified regions of decreased responsivity across the SPAD entrance window that were attributed to the presence of bulk defects and lower carrier lifetimes resulting from device processing. Additionally, line scans and two-dimensional maps revealed increased responsivity before onset of multiplication localized at the diode perimeter related to spreading of the depletion region and diffusion of photogenerated carriers from the quasi-neutral region. Optoelectrical line-scan simulations of the SPAD responsivity were performed using technology computer aided design (TCAD) to analyze the impact of perimeter carrier collection, defects and lifetime on spatial responsivity.