The origin of the enhanced photoresponsivity of the phototransistor with ZnO_1-xS_x single active layer

We applied zinc oxysulfide (ZnO_1-xS_x) 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 ZnO_1-xS_x active layer. The incorporation of S atoms in the ZnO_1-xS_x 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 ZnO_0.9S_0.1 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 ZnO_1-xS_x 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 ZnO_1-xS_x active layer as a function of S content using band alignment. Consequently, we successfully demonstrated the feasibility of the ZnO_1-xS_x film as a single active layer in a phototransistor to enhance the photoresponsivity in the UV and visible regions.