Optical properties of tungsten oxide thin films with protons intercalated during sputtering

Tungsten oxide thin films with protons intercalated during deposition $(HxWO3)$ were prepared using reactive direct-current-magnetron sputtering in a gas mixture of argon, oxygen, and hydrogen. The as-deposited films fabricated under suitable conditions were colored due to the formation of tungsten bronze. The concentration of intercalated protons, given by the $x$ values in $HxWO3$⁠, was evaluated by ejecting protons electrochemically from the films. The $x$ value of the films prepared at a constant working pressure was found to be proportional to the hydrogen flow ratio during deposition. On the other hand, the $x$ value of the films prepared at a constant hydrogen flow ratio decreased sharply with increasing working pressure during deposition. The dispersion of the extinction coefficient $(κ)$ of the films was estimated by analyzing the experimental spectra of $Ψ$ and $Δ$ measured with spectroscopic ellipsometry using the model composed of a homogeneous tungsten bronze layer with an additional surface roughness layer. As a result of this analysis, the $κ$ value was found to increase sharply with the number of intercalated protons. There was a linear dependence between the $κ$ value and the $x$ value for $x<0.2$⁠, while for $x>0.3$⁠, the absorption saturated. This indicates that it is possible to evaluate the $x$ value of $HxWO3$ films using spectroscopic ellipsometry.