Effects of Process\nGas Pressure on Properties of WO<sub>3</sub> Thin Films Deposited\nby RF Sputtering

This study investigated the effects of process gas pressure\non\nthe properties of WO<sub>3</sub> thin films deposited by RF magnetron\nsputtering using a 50%O<sub>2</sub>/50%Ar process gas. The deposited\nfilm consisted of an amorphous phase at pressures of >2.0 Pa and\na\nmonoclinic γ-WO<sub>3</sub> phase at lower pressures. The energy\nband gap of γ-WO<sub>3</sub> decreased upon lowering the pressure\nfrom 1.0 to 0.50 Pa. Moreover, a concurrent stretching of the <i>c</i>-axis due to lattice expansion was observed because the\ncrystallite size decreases at low pressures. Based on first-principles\ncalculations, energy band gap reduction was attributed to lattice\nexpansion. Although sputter deposition typically produces (001)-oriented\nfilms, a (111)-oriented film was achieved at 0.20 Pa. This is explained\nby the destabilization and stabilization of (001) and (111) orientations,\nrespectively, at reduced oxygen activity in the local atmosphere,\nbecause the (001) and (111) orientations expose oxygen and tungsten\natoms, respectively. However, these changes in the films did not affect\nthe rate of coloration due to proton incorporation and diffusion,\npossibly because coloration is dominated by proton diffusion at grain\nboundaries rather than within the grains.