The voltage dependent spectral response in the 200–400 nm range was measured in n+-n--p SiC avalanche photodiodes with ultrathin n+ layers varying in the thickness from 60 to 120 nm. The unity gain responsivity in the far ultraviolet (200–250 nm) spectral region increases significantly with bias, leading to a 2–3 times enhancement at 200 nm for devices with 120 nm and 60 nm n+ layers, respectively. At the onset of gain, new spectral response peaks appear at 226 nm and 240 nm for the devices with the 60 nm and 120 nm thick n+ layers, respectively, both significantly blue shifted from the zero bias peak at 265 nm. Modeling indicates that the enhanced far ultraviolet unity gain responsivity results from the improved collection of carriers photogenerated near the illuminated surface of the device as the depletion of the n-region extends toward the surface, thereby mitigating surface recombination. The emergence of the peaks at higher bias and their larger blue shift with decreasing n+ layer thickness can be explained by the longer transit distance across the multiplication region for the holes with improved collection photogenerated near the illuminated surface, which leads to a larger enhancement in a multiplication gain with increasing bias for 200 nm photoexcitation over that for 380 nm. Devices employing the thicker 120 nm n+ layer achieve a multiplication gain greater than 5 × 106 at 12 pW 240 nm illumination, and ∼ 12 nA/cm2 dark current at gain of 1000 suitable for single photon counting.

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