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

Tungsten oxide thin films with protons injected during deposition (H_xWO₃) 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 H_xWO₃, was evaluated by electrochemically ejecting protons from the films. The complex dielectric functions (? = ?₁ + i?₂) 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 ?₂, 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.