| Abstract [eng] |
In this work, TiO2/Al2O3, TiO2/HfO2, TiO2/Ta2O5 and TiO2/Nb2O5 mixture and pure titanium oxide (TiO2) films were deposited on fused silica (FS) glass substrates by ion beam sputtering (IBS). The TiO2 concentration of different sample mixed films was approximately 50%. Index of refraction, extinction coefficient and film thickness were examined modelling spectrophotometric data. Volumetric fractions of mixture films were estimated using effective medium theory (EMT). The Stoney's formula was applied to evaluate the residual stress of layers. Surface roughness was investigated using atomic force microscopy (AFM). Lastly, impact of different temperature post deposition annealing of formed films characteristics was investigated. The ion beam sputtered mixed films have lower optical losses than the ion beam sputtered pure TiO2 films. It was found, post deposition annealing of titanium oxide sample at the annealing temperature of 300 ºC increases extinction coefficient and surface roughness. Observed phenomenon could be explained because of phase transition from amorphous to polycrystalline at high temperature. Titanium oxide composite samples with high refractive index materials were able to withstand higher annealing temperatures before phase transition. Temperature of phase modification has increased to 600-700 ºC. It is advantageous to add high refractive index materials into the ion beam sputtered TiO2 films because the mixed films can sustain higher annealing temperature while reducing the optical absorption and yet avoiding increase the scattering loss. High compressive stress could be reduced and changed into tensile by sputtering composite material and using annealing in high temperature, too. Additional experiment was performed to evaluate impact of changing sample volumetric fraction. For this purpose TiO2/Nb2O5 mixtures were selected because of the highest possible index of refraction between all previously investigated mixtures. It was shown that the temperature when phase modification occurred depended on TiO2 amount in TiO2/Nb2O5 mixture films. It decreased for higher TiO2 amount. Theoretical calculations for high reflective (R≥99,9%) dielectric mirrors for 1064nm wavelength were performed. It was estimated that TiO2/Nb2O5 mixed layers with 70-80% TiO2 concentration could replace pure TiO2 layers in multilayer coatings without optical performance degradation. Better mechanical properties like compensated mechanical stress with simple annealing techniques could be achieved for mirrors with mixture layers without significant fall in optical performance. With the properties of high-temperature amorphous structure and high refractive index, the ion-beam sputtered TiO2/Nb2O5 mixed films can be used as low-loss, low-stress, high-refractive-index materials in optical thin films applications. |
