Characteristics of ion-beam-sputtered high-refractive-index TiO_2-SiO_2 mixed films

The ion-beam-sputtering method was used to deposit TiO2-SiO2 mixed films on silica substrates. The SiO2 concentration ranged from 0 to 17%, and the refractive index ranged from ∼2.7 to ∼2.3 in visible wavelength. All the structures of the as-deposited films were amorphous. The refractive index and the extinction coefficient decreased with increased SiO2 concentration. High temperature annealing reduced the optical absorption for all films. There was a phase transition from amorphous to polycrystalline anatase at high temperature. Surface roughness, and thus optical scattering, increased drastically with the appearance of the phase transition. The phase transition temperature was higher for films with higher SiO2 concentration. The ion-beam-sputtered TiO2-SiO2 mixed film could sustain higher temperature annealing, resulting in lower extinction coefficient, than that of the ion-beam-sputtered pure TiO2 film. The phase diagram of the mixed film system was given.