Significant performance improvement of AlGaN solar-blind heterojunction phototransistors by using Na2S solution based surface treatment

Abstract

Deep ultraviolet (DUV) photodetectors are of tremendous interest in applications requiring solar-blind sensing. Here, solar-blind Al0.5GaN/Al0.4GaN heterojunction phototransistors with significantly improved performance have been fabricated using Na2S solution based surface treatment. Compared with the usual (NH4)2S-based treatment, the dark current of the HPTs fabricated with Na2S-based treatment is reduced by more than an order of magnitude, and the optical gain is increased from 2.6 × 102 to 9.6 × 103. A high photo to dark current ratio of more than four orders of magnitude was obtained at zero bias and is higher than two orders of magnitude in the whole measuring bias range. Characterizations on the surface morphology, optical properties, surface chemical bonding states, and surface potential reveal that the Na2S-based solution is very effective in removing III-oxides, reducing the deep-level defects associated with VIII, and thus forming SN donors on the dry-etched and thermally-annealed n-AlGaN layer. As a result, the Ohmic contact properties of the HPTs are significantly, contributing to the photoresponse enhancement. Moreover, the surface leakage is suppressed by the reduction of deep-level acceptors, thereby improving the electron injection and optical gain of the HPTs.