Refractive index and degree of inhomogeneity of nanocrystalline TiO2 thin films: Effects of substrate and annealing temperature

Nanocrystalline TiO2 thin films, deposited on single crystal Si (100) substrates under different temperature conditions by the sol–gel dip coating method, have been investigated for their optical properties using ultraviolet-visible spectroscopic ellipsometry. A gradual increase in refractive index, n, with increasing annealing temperature up to 600 °C, and thereafter a sharp increase in n at 800 °C of annealing temperature have been observed. For the heat-treated and low temperature (400 °C) annealed films, n is found to be higher at the film–substrate interface than at the film surface and the refractive index gradient slightly increases for the annealed sample. However, for the 600 °C temperature annealed film, the refractive index gradient significantly decreases and the film appears to be almost homogeneous. These results are in sharp contrast with those for the films deposited on a vitreous silica substrate where n was found to be higher at the film surface than at the film–substrate interface and the refractive index gradient increased with increasing annealing temperature. For the high temperature (800 °C) annealed sample on the Si substrate, formation of a thick SiO2 interfacial layer has been observed and the degree of homogeneity deteriorates severely.