Creation of a two-dimensional conducting path based on the engineering oxidation state of WO3 with Al2O3 at the interface

Abstract

We report the dramatic manipulation of an oxide interface in Al2O3/WO3 from insulating to metallic state by creating a two-dimensional conducting path. The WO3 thin film is deposited on the SiO2 grown Si substrate using radio frequency magnetron sputtering and the amorphous Al2O3 layer is formed by atomic layer deposition on the WO3 layer. After rapid vacuum annealing at 400 °C, the interfacial conducting path of Al2O3/WO3 reveals an abnormally high sheet carrier density of 4.91 × 1016 cm−2 compared with <109 cm−2 for bare WO3. The X-ray photoelectron spectroscopy (XPS) shows that WO3 is partially reduced; and a W5+ oxidation state is observed at the interface. The schematic electronic band diagram of the interfacial region obtained by combined XPS and ultraviolet photoelectron spectroscopy analyses indicates that the downward band bending at the surface of WO3 induces a confined electron accumulation at the interface. Synergetic effects between the increased unpaired W 5d electrons in the W5+ oxidation state and the accumulative charge confinement at the Al2O3/WO3 interfaces lead to a two-dimensional conducting path. The metallic conduction is distinct from the insulating bulk properties. This study demonstrates an example of a short-range-ordered two-dimensional conducting path at two insulating oxide interfaces formed at low temperature, which is applicable to flexible oxide conductors with great conduction path stability.