Highly enhanced visible light photodetection properties of a ZnO phototransistor via an additional solution processed thin Al2O3 layer

Abstract

Zinc oxide (ZnO) is one of the most widely used oxide semiconductors as an active layer of ultra-violet (UV) phototransistors due to its wide band gap of 3.3 eV. Recently, it was found that a ZnO phototransistor can generate photo-excited electron–hole pairs by absorbing visible light as well as UV light due to its intrinsic defect states, such as oxygen vacancies. Here, we fabricated a visible-light phototransistor by adding a solution-processed thin Al₂O₃ layer under the ZnO layer to improve the visible-light photodetection. Photo-generated holes were efficiently induced under a negative gate bias in the Al₂O₃ layer, and they contributed to the photogating effect. Moreover, the induced electrons in the conduction band contributed to the channel current of the ZnO phototransistor. The Al₂O₃/ZnO phototransistor exhibited excellent photodetection characteristics (photoresponsivity: 113.67 A W⁻¹ and photosensitivity: 2.31 × 10⁹) under 520 nm wavelength light exposure. High-resolution transmission electron microscopy (HR-TEM), electron dispersion spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS), and UV-vis spectroscopy measurements were used to investigate the origin of the improved visible light photodetection characteristics.