Dielectric function of vanadium oxide thin films by thermal annealing

The dielectric function of ${{\rm{VO}}_x}$ and ${{\rm{V}}_2}{{\rm{O}}_5}$ thin films is determined with the use of a spectroscopic Mueller matrix ellipsometer from 1.5 to 5.0 eV. The complex dielectric function of the films is calculated using the measured Mueller matrices filtered with the Cloude decomposition. ${{\rm{VO}}_x}$ shows high absorption in the UV region, a Tauc-Lorentz gap around 2.4 eV, and non-vanishing absorption in the visible. ${{\rm{V}}_2}{{\rm{O}}_5}$ shows a high absorption band centered at 2.87 eV, an indirect optical band gap at 1.95 eV, and a direct optical band gap at 2.33 eV. The ellipsometric characterization is supported by Raman, x-ray photoelectron, and photoluminescence spectroscopy.