NUV-Enhanced 4H-SiC SACM APDs

Numerous applications require detectors with high gain or single-photon sensitivity while demonstrating good sensitivity throughout the ultraviolet (UV) spectrum from 200 – 400 nm; these sensors are of significance for solar-blind imaging and spectroscopy applications given their insensitivity to parasitics from longer wavelength scattered light. 4H-SiC avalanche photodiodes (APDs), with an indirect bandgap of 380 nm, have exhibited both high UV-C (< 280 nm) responsivity and high multiplication gain at low dark current density (0.75 nA/cm 2 < J dark < 63 nA/cm 2 at a gain of 1000, as well as > 10 6 peak multiplication gain [1] . These properties make 4H-SiC APDs very attractive for single photon counting applications. However, the quantum efficiency (QE) of these devices diminishes dramatically in the NUV at wavelengths longer than ~280 nm due to weak absorption of photons, where, for example, 20 μm of absorbing material is sufficient to absorb only 70% of the incident photons at 340 nm illumination. In the NUV, Shen et al. demonstrated SiC Schottky diodes with internal QE between 80–90% by using unintentionally doped 20–50 μm thick layers with doping sufficiently low to allow significant depletion at high bias [2] such that collection from the 50 μm thick layer was due to a mix of drift and diffusion. A separate-absorption charge-multiplication (SACM) APD structure may enable both improved QE and avalanche gain through extension of the absorber layer (AL) thickness to 20–50 μm while maintaining a large field in the multiplication layer (ML), ideally at a lower AL doping enabling both depletion and longer diffusion length [3] . However, using the SACM architecture to improve the NUV unity gain QE possesses unique challenges regarding the transport of photocarriers from the thick AL across the charge layer (CL) to the ML. 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.