Structural evolution and optical properties of TiO₂ thin films prepared by thermal oxidation of sputtered Ti films

A dense rutile TiO₂ thin film was synthesized by the thermal oxidation of a sputtered titanium metal film in ambient air. The effects on optical properties of TiO₂ films of the crystal structure and microstructural evolution at various oxidation temperatures were investigated. The Ti films transformed into single-phase rutile TiO₂ at temperatures ≥ 550 °C without going through an anatase-to-rutile transformation. Instead, an additional crystalline Ti₂O phase was detected at 550 °C only. An increase in the oxidation temperatures ranging between 700 and 900 °C led to an increase in both the refractive index and absorption coefficient, but a decrease in the band gap energy (E_g). According to the coherent potential approximation model, the band gap evolution of the oxidized films was primarily attributed to the electronic disorder due to oxygen deficiency at a higher oxidation temperature rather than the presence of an amorphous component in the prepared films.