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

Abstract

Tungsten oxide thin films with protons injected during deposition (H x WO 3) 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 x WO 3, was evaluated by electrochemically ejecting protons from the films. The complex dielectric functions ( ɛ = ɛ 1 + iɛ 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 ɛ 2, which represents optical absorption, was composed of two Lorentz oscillator terms whose peak positions were about 1.0 eV ( L 1) and 1.6 eV ( L 2). The two terms with L 1 and L 2 are assumed using the modified site-saturation model to be due to optical charge transition between W 6+ and W 5+ sites, and between W 6+ and W 4+ sites, respectively.