Geiger-Mode Operation of AlGaN Avalanche Photodiodes at 255 nm

We report the Geiger mode operation of back-illuminated AlGaN avalanche photodiodes. The devices were fabricated on transparent AlN templates specifically for back-illumination to leverage hole-initiated multiplication. The spectral response was analyzed with a peak detection wavelength of 255 nm with an external quantum efficiency of ~14% at zero bias. Low-photon detection capabilities were demonstrated in devices with areas 25 μm x 25 μm. Single photon detection efficiencies of ~5% were achieved.