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

Visible-light phototransistors have been fabricated based on the heterojunction of zinc oxide (ZnO) and titanium oxide (TiO). A thin layer of TiO was deposited onto the spin-coated ZnO film atomic layer deposition (ALD). The electrical characteristics of the TiO layer were optimized by controlling the purge time of titanium isopropoxide (TTIP). The optimized TiO layer could absorb the visible-light from the sub-gap states near the conduction band of TiO, which was confirmed photoelectron spectroscopy measurements. Therefore, the heterostructure of TiO/ZnO can absorb and generate photocurrent under visible light illumination. The oxygen-related-states were investigated X-ray photoelectron spectroscopy (XPS), and the interfacial band structure between TiO and ZnO was evaluated ultraviolet photoelectron spectroscopy (UPS). Oxygen-related states and subgap-states were observed, which could be used to generate photocurrent by absorbing visible light, even with TiO and ZnO having a wide bandgap. The optimized TiO/ZnO visible-light phototransistor showed a photoresponsivity of 99.3 A W and photosensitivity of 1.5 × 10 under 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.