Optical charge transfer absorption in proton injected tungsten oxide thin films analyzed with spectroscopic ellipsometry

Tungsten oxide thin films with protons injected during deposition (HxWO3) were prepared using reactive direct-current magnetron sputtering in a mixture of argon, oxygen, and hydrogen gases. The concentration of injected protons, given by the x-values in HxWO3, was evaluated by electrochemically ejecting protons from the films. The complex dielectric functions (ɛ = ɛ1 + iɛ2) of the films were 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 imaginary part of the dielectric function ɛ2, which represents optical absorption, was composed of two Lorentz oscillator terms whose peak positions were about 1.0 eV (L1) and 1.6 eV (L2). The two terms with L1 and L2 are assumed using the modified site-saturation model to be due to optical charge transition between W6+ and W5+ sites, and between W6+ and W4+ sites, respectively.