Effects of Process Gas Pressure on Properties of WO3 Thin Films Deposited by RF Sputtering

This study investigated the effects of process gas pressure on the properties of WO3 thin films deposited by RF magnetron sputtering using a 50%O2/50%Ar process gas. The deposited film consisted of an amorphous phase at pressures of >2.0 Pa and a monoclinic γ-WO3 phase at lower pressures. The energy band gap of γ-WO3 decreased upon lowering the pressure from 1.0 to 0.50 Pa. Moreover, a concurrent stretching of the c-axis due to lattice expansion was observed because the crystallite size decreases at low pressures. Based on first-principles calculations, energy band gap reduction was attributed to lattice expansion. Although sputter deposition typically produces (001)-oriented films, a (111)-oriented film was achieved at 0.20 Pa. This is explained by the destabilization and stabilization of (001) and (111) orientations, respectively, at reduced oxygen activity in the local atmosphere, because the (001) and (111) orientations expose oxygen and tungsten atoms, respectively. However, these changes in the films did not affect the rate of coloration due to proton incorporation and diffusion, possibly because coloration is dominated by proton diffusion at grain boundaries rather than within the grains.