Improving the photodetection and stability of a visible-light QDs/ZnO phototransistor via an Al2O3 additional layer

Abstract

We have developed a visible-light phototransistor with excellent photodetection characteristics and stability via atomic layer deposition (ALD) to add a thin layer of aluminum oxide (Al₂O₃) to quantum dot (QD)/zinc oxide (ZnO) films. Al₂O₃ was used to passivate the surface of the QDs and fill the interspace of the QDs. This led to a reduction in the number of QD surface traps and prevented damage due to the migration of aggressive metal atoms during electrode deposition. Additionally, some parts of Al₂O₃ deposited on the surface of ZnO triggered a doping effect by interacting with ZnO. This enhanced the electrical properties of ZnO TFTs by providing an extra electron and allowed the efficient transfer of photoelectrons from the QDs to ZnO by reducing the conduction band minimum of ZnO. Therefore, upon 520 nm laser irradiation Al₂O₃/QD/ZnO TFTs exhibited excellent photodetection characteristics (e.g., photosensitivity: 3.8 × 10⁷, photoresponsivity: 46.8 A W⁻¹, and photodetectivity: 4.5 × 10¹⁴ Jones) at a gate voltage of −5 V. Excellent electrical properties, such as saturation mobility of 1.72 cm V⁻¹ s⁻¹ and on/off current ratio of 1.8 × 10⁶, were also realized. Furthermore, the photo-excited charge transfer mechanism was investigated using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and time-resolved photoluminescence. Our experimental results suggest a method for realizing enhanced optoelectronic characteristics and stability of QD/ZnO TFTs via ALD-Al₂O₃ deposition, which is useful for developing high-performance visible phototransistors.