Optical and mechanical properties of TiO₂/SiO₂/organically modified silane composite films prepared by sol-gel processing

Thin films of TiO₂/SiO₂/organically modified silane (ORMOSIL) composite material were prepared by the sol-gel spin coating technique. Acid catalyzed solutions of y-glycidoxypropyltrimethoxysilane mixed with tetraethoxysilane and tetrapropylorthotitanate were used as precursors. Ellipsometry, thermal gravimetric analysis (TGA), differential thermal analysis (DTA), UV-visible spectroscopy (UV-vis), Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy have been used to investigate the optical and structural properties of the composite films. Ellipsometry results showed that the film became thinner as the titanium content increased. A refractive index in the range of 1.52-1.61 at the optical wavelength of 633 nm was obtained by varying the molar ratio between silica and titanium. UV-vis transmission spectroscopy results showed that low absorption and high transparency (in the visible range) waveguide films could be obtained at a low temperature. Raman spectroscopy results indicated that the composite film with high titanium content contained amorphous carbon phases after being baked in air directly at a temperature of 500 °C or higher. The hardness and Young's modulus of the films were characterized by a nanoindenter and they showed a dependence on the heat-treatment temperature as well as the titanium molar fraction. A film hardness of 9.0 GPa was observed in a high titanium content film heat-treated at 800 °C.