Compositional Tailored Sol-Gel SiO₂–TiO₂ Thin Films: Crystallization, Chemical Bonding Configuration, and Optical Properties

Thin films of SiO₂–TiO₂ composite oxides with various SiO₂:TiO₂ compositions were prepared by the sol-gel method, using tetraethylorthosilicate (TEOS) and titanium tetraisopropoxide (TTIP) as precursors. The composition, crystal structure, and chemical bonding configuration of the as-deposited and annealed SiO₂–TiO₂ thin films were analyzed using Rutherford backscattering spectrometry (RBS), glancing incident angle x-ray diffraction (GIAXRD) and Fourier transform infrared spectroscopy (FTIR), respectively. Optical properties of the films were characterized by spectroscopic ellipsometry and ultraviolet-visible spectrophotometry. The Si/Ti ratios in the SiO₂–TiO₂ films agree with the TEOS/TTIP molar ratio in the sol-gel precursor. When the TEOS/(TEOS + TTIP) ratio is greater than 40%, the SiO₂–TiO₂ thin films remain amorphous (without formation of TiO₂ crystalline phase) after annealing at temperatures as high as 700 °C. FTIR spectra indicate that the quantity of Si–O–Ti bonding can be maximized when the TEOS:TTIP in the precursor is 80%:20%. The refractive index of the SiO₂–TiO₂ films increases approximately linearly to the mixing ratio of TTIP/(TEOS + TTIP). However, SiO₂-rich films possess higher ultraviolet-visible transmittance than the TiO₂-rich films. The modification of microstructure and chemical bonding configuration in the SiO₂–TiO₂ films by the composition and its influence on the optical properties are discussed.