A visible-light phototransistor based on the heterostructure of ZnO and TiO2with trap-assisted photocurrent generation

Abstract

Visible-light phototransistors have been fabricated based on the heterojunction of zinc oxide (ZnO) and titanium oxide (TiO2). A thin layer of TiO2was deposited onto the spin-coated ZnO filmviaatomic layer deposition (ALD). The electrical characteristics of the TiO2layer were optimized by controlling the purge time of titanium isopropoxide (TTIP). The optimized TiO2layer could absorb the visible-light from the sub-gap states near the conduction band of TiO2, which was confirmedviaphotoelectron spectroscopy measurements. Therefore, the heterostructure of TiO2/ZnO can absorb and generate photocurrent under visible light illumination. The oxygen-related-states were investigatedviaX-ray photoelectron spectroscopy (XPS), and the interfacial band structure between TiO2and ZnO was evaluatedviaultraviolet photoelectron spectroscopy (UPS). Oxygen-related states and subgap-states were observed, which could be used to generate photocurrent by absorbing visible light, even with TiO2and ZnO having a wide bandgap. The optimized TiO2/ZnO visible-light phototransistor showed a photoresponsivity of 99.3 A W−1and photosensitivity of 1.5 × 105under the illumination of 520 nm wavelength light. This study provides a useful way to fabricate a visible-light phototransistor based on the heterostructure of wide bandgap oxide semiconductors.