Observation of Substrate Orientation-Dependent Oxygen Defect Filling in Thin WO3-delta/TiO2 Pulsed Laser-Deposited Films with in Situ XPS at High Oxygen Pressure and Temperature

Abstract

Substoichiometric tungsten oxide films of approximately 10 nm thickness deposited with pulsed laser ablation on single-crystal TiO2 substrates with (001) and (110) orientation show defect states near the Fermi energy in the valence-band X-ray photoelectron spectroscopy (XPS) spectra. The spectral weight of the defect states is particularly strong for the film grown on the (001) surface. In situ XPS under an oxygen pressure of 100 mTorr shows that the spectral weight of the defect states decreases significantly at 500 K for the film on the (110) substrate, whereas that of the film grown on the (001) substrate remains the same at a temperature up to 673 K. Furthermore, diffusion of titanium from the substrate to the film surface is observed on the (110) substrate, as is evidenced by the sudden appearance of the Ti 2p core level signature above 623 K and below 673 K. The film grown on the (001) surface does not show such an interdiffusion effect, which suggests that the orientation of the substrate can have a significant influence on the high-temperature integrity of the tungsten oxide films. Quantitative analysis of the O Is core level XPS spectra shows that chemisorbed water from sample storage under ambient conditions is desorbed during heating under oxygen exposure.