A visible-light phototransistor based on the heterostructure of ZnO and TiO 2 with trap-assisted photocurrent generation.

Visible-light phototransistors have been fabricated based on the heterojunction of zinc oxide (ZnO) and titanium oxide (TiO 2 ). A thin layer of TiO 2 was deposited onto the spin-coated ZnO film via atomic layer deposition (ALD). The electrical characteristics of the TiO 2 layer were optimized by controlling the purge time of titanium isopropoxide (TTIP). The optimized TiO 2 layer could absorb the visible-light from the sub-gap states near the conduction band of TiO 2 , which was confirmed via photoelectron spectroscopy measurements. Therefore, the heterostructure of TiO 2 /ZnO can absorb and generate photocurrent under visible light illumination. The oxygen-related-states were investigated via X-ray photoelectron spectroscopy (XPS), and the interfacial band structure between TiO 2 and ZnO was evaluated via 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 2 and ZnO having a wide bandgap. The optimized TiO 2 /ZnO visible-light phototransistor showed a photoresponsivity of 99.3 A W -1 and photosensitivity of 1.5 × 10 5 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.