The origin of the enhanced photoresponsivity of the phototransistor with ZnO1-xSx single active layer

We applied zinc oxysulfide (ZnO1-xSx) as a single active layer in a phototransistor by adjusting the S-to-O ratio. The electrical properties and photoresponsivity of the phototransistors were investigated as a function of the S content, x, of the ZnO1-xSx active layer. The incorporation of S atoms in the ZnO1-xSx layer induced a change in the electrical properties and photoresponsivity, especially in the ultraviolet (UV) and visible regions, compared with a ZnO film. The phototransistor with ZnO0.9S0.1 showed the highest performance; for 450 nm light, the photoresponsivity was 8.0 × 102 A/W, and the maximum photosensitivity was 1.7 × 104. By analyzing the chemical states of ZnO1-xSx in the phototransistor, we realized that the origin of the improvement of the photoresponsivity is due to the increase of the number of oxygen vacancies formed by the incorporation of S atoms. Based on the above results, we suggested the mechanism of electron transport in phototransistors with a ZnO1-xSx active layer as a function of S content using band alignment. Consequently, we successfully demonstrated the feasibility of the ZnO1-xSx film as a single active layer in a phototransistor to enhance the photoresponsivity in the UV and visible regions.