V-Pits-Induced Photoresponse Enhancement in AlGaN UV-B Photodetectors on Si (111)

We demonstrate the influence of surface terminated V-pits in tuning dark current and spectral responsivity of Al0.25Ga0.75N-based UV-B photodetectors with metal-semiconductor-metal geometry on Si (111) substrate. We show that the V-pit morphological defects contribute to a large internal gain in these photodetectors, thereby leading to a substantial enhancement in external quantum efficiency (EQE) at relatively low applied biases. For photodetectors fabricated on metal organic chemical vapor deposition grown Al0.25Ga0.75N epilayers with a surface pit density of 2 x 10⁸ cm⁻₂, an EQE of 100% was measured at a meager bias of 1.7 V, which increased significantly with bias. The EQE, photo-to-dark current ratio, and UV-to-visible rejection ratio measured 5 x 10⁴%, 1.2 x10⁴, and 2 x 10₃, respectively, at 5 V. The evidence of localized enhancement of photoresponse at the surface terminations of V-pits is exemplified by UV-assisted conductive atomic force microscopy. Temperature-dependent carrier transport analysis under dark and UV illumination revealed cumulative contributions of pit-induced thermionic field emission and hole-trapping-induced gain to the observed large EQE. This work presents the highest value of responsivity for III-nitride UV-B detectors at a given bias.