Electron-Ion Mixed Conduction of Amorphous WO_3-x Thin Film Probed by Soft-X-Ray Spectroscopy

Abstract

Amorphous WO_3-x thin films with ～200 and ～600 nm thicknesses have been prepared on Pt/SiO₂ substrates by RF magnetron sputtering. The mixed valence states of W⁶⁺ and W⁵⁺ are observed in the photoemission spectroscopy (PES) spectra of W 4d core level. The electrical conductivities exhibit the thermal activation-type behaviors in the temperature region of 100～200°C. The activation energies of 200 and 600 nm films are 0.1 and 0.6 eV, respectively. The band gaps (E_g) of 200 and 600 nm films are ～2.6 and ～2.0 eV, respectively. The expanded PES in the E_g region and X-ray absorption spectra exhibit the W 5d-DOS at the Fermi level and defect-induced state at the bottom of conduction band, respectively. The above results indicate that the conducting carrier of amorphous WO_3-x thin film is closely related to the film thickness and oxygen vacancies.