Design Challenges in Binary 4H-SiC NUV-Enhanced SACM APDs

Near-ultraviolet (NUV) Geiger-mode avalanche photodiodes (NUV-GM-APD) require high unity-gain quantum efficiency (QE), while operating above avalanche breakdown. 4H-SiC has long been established as a proven GM-APD in the UV-C (<280> $\lt 3~\mu $ m thick) to a separate-absorption charge-multiplication (SACM) architecture. However, using a SACM architecture to improve the NUV unity-gain QE has unique challenges: including deviating from existing front-side absorber SACM architectures to a very thick backside one. This is further compounded when binary semiconductor materials are used (e.g., 4H-SiC) instead of alloyed heterostructures (e.g., InGaP or HgCdTe), removing what is arguably the most versatile design parameter, the mole fraction of the alloy. To overcome these challenges, we have implemented a numerical model with a calibrated 4H-SiC material library for the development of APDs and leveraged it to design NUV-enhanced SACM structures, where both non-reach-through (NRT) and reach-through (RT) architectures have been considered. For the NRT-SACM case, it was determined that the doping profiles must be engineered such that two competing mechanisms are balanced: maximizing the minority carrier diffusion length in the absorber layer (AL, longest at lower AL doping), while minimizing the corresponding potential barrier at the AL/charge layer (CL) interface (lowest at higher AL doping). Conversely, in a RT-SACM architecture, it was determined that a narrow range of total charge in the CL properly modulated the electric field to be non-zero in the AL and sufficiently large in the multiplication layer (ML) to operate above avalanche breakdown. As such, it was determined that the CL design is exceptionally intolerant to variations in either layer thickness or doping. Leveraging design rules learned and reported in this paper, we have designed both types of SACM architectures: NRT-SACM APDs and RT-SACM APDs, with unity gain QE at 340 nm up to 32% and 71% respectively, while maintaining a large electric field in the ML required for Geiger-mode operation.