Optical properties of mixed TiO2-SiO2 films, from infrared to ultraviolet

Mixed oxides are useful for obtaining the intermediate refractive indices needed in the realization of graded-index thin films. Coevaporated TiO2-SiO2 mixtures are studied for a large range of concentrations via W-VIS, IR ellipsometry and XPS. An understanding of the nature of these mixtures and their air exposure stability is important for further applications. At low TiO2 concentrations, Ti4+ ions are inserted into the silica tetrahedral network, as shown by the IR peak at 945 cm(-1) At higher concentrations, an evolution from TiO4 tetrahedra to TiO6 octahedra is presumed The behavior of the O-1s, core level peak indicates that a least two phases coexist (silica-rich and titania-rich phases). Comparison between concentrations determined using XPS and RES shows a deficit in TiO2 at the surface of the films, especially at high TiO2 concentrations. The evolution of the mixtures optical constants will be presented in a large wavelength range, going from IR to UV. Particular attention will be paid to the variations with respect to the frequency of the vibration modes in the IR range, to the refractive index in the transparency region, and to the extinction coefficient at the absorption threshold In addition, AFM measurements show the variation of the grain size as a function of the TiO2 concentration.