Improving the visible-light photoresponse characteristics of a ZnO phototransistor via solution processable Li dopants

The wide band gap of oxide semiconductors enables them to generate photocurrents through irradiation with high-energy photons, such as by ultraviolet light. To enhance the photoresponsivity of an oxide semiconductor to visible light, we fabricate a phototransistor by doping ZnO with Li. The amount of oxygen vacancies in ZnO is controlled by changing the concentration of lithium hydroxide in ZnO solution. Li dopants increase the electron concentration in ZnO, thereby improving the electrical behavior of ZnO thin-film transistors (TFTs). The Li-doped ZnO TFTs exhibit a high field-effect mobility of 10.15 cm2 V−1 s−1 and a high on/off ratio of ∼107. These oxygen vacancy states are located above the valence band of ZnO; therefore, they can generate a photocurrent upon irradiation of Li-doped ZnO TFTs with visible light. Li-Doped ZnO phototransistors exhibit excellent photodetection characteristics (photoresponsivity: 23.11 A W−1; photosensitivity: 4.94 × 106) upon 520 nm laser irradiation on the device, when the concentration of lithium hydroxide is 5 mol%. In addition, Li doped ZnO TFTs can be operated stably without light-induced aging effects or degradation caused by negative bias stress. Ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy measurements are performed to investigate the origin of the improved visible-light photoresponsivity of the Li-doped ZnO phototransistors. Our experimental results suggest that a visible-light photodetector can be developed using wide band gap ZnO.