Modeling the optical properties of WO3 and WO3–SiO2 thin films

The optical properties and surface morphology of sol–gel spin coated WO3 and WO3–SiO2 composite films annealed at 250 and 400∘C are investigated. For the purpose of extracting the optical parameters of the films, a novel form for the dielectric function is introduced, consisting of two Tauc–Lorentz oscillators and an Urbach tail component, which is suited for amorphous multi-transition materials with substantial subgap absorption. The evolution of the refractive indices, transmittances, and band gaps with doping is marked by sizable shifts at 2.0–2.5% SiO2 doping for the 250∘C films, and 4.0–4.5% doping for the 400∘C films. In addition, pronounced changes in the surface roughness of the films occur at these doping values.