Contribution of thickness dependent void fraction and TiSixOy interlayer to the optical properties of amorphous TiO2 thin films

•Amorphous TiO2 thin films fabricated on Si substrate by electron beam evaporation•The refractive index and band gap are obtained from spectroscopic ellipsometry.•The refractive index decreases with decreasing film thickness.•Effective medium approximation theory and effective series capacitance model introduced•A band gap increases gradually with an increase in film thickness.

The optical properties of TiO2 thin films prepared by electron beam evaporation were studied by spectroscopic ellipsometry and analyzed quantitatively using effective medium approximation theory and an effective series capacitance model. The refractive indices of TiO2 are essentially constant and approach to those of bulk TiO2 for films thicker than 40 nm, but drop sharply with a decrease in thickness from 40 to 5.5 nm. This phenomenon can be interpreted quantitatively by the thickness dependence of the void fraction and interfacial oxide region. The optical band gaps calculated from Tauc law increase with an increase of film thickness, and can be attributed to the contribution of disorder effect.