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

Abstract

Nanocrystalline ${\mathrm{TiO}}_{\mathrm{2}}$ 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 ${\mathrm{SiO}}_{\mathrm{2}}$ interfacial layer has been observed and the degree of homogeneity deteriorates severely.