Effect of nanostructured porous n+ GaAs substrates on the energy band gap of nanocrystalline TiO2 thin films analyzed by spectroscopic ellipsometry

We present the optical properties of TiO2 deposited, by sol–gel method, on porous n+type GaAs substrates with different porosities and were thermally annealed at 600 °C for 15 min. The surface topography and density of grains packing at different samples were determined based on AFM images. AFM studies showed that the structure of the TiO2 thin film was nanocrystalline with grains size dependent on the etching time and ranging between 4 and 7 nm. The optical constants (n and k) of the films, as a function of etching time of GaAs substrate before and after deposition and content, were obtained using spectroscopic ellipsometer with rotating polarizer type in the ultraviolet–visible–near infrared (UV–vis–NIR) regions. The thickness of TiO2 thin films (top layer) and the mixed layer of TiO2 and porous GaAs (bottom layer) were determined by scanning electroscopic spectroscopy (SEM) and confirmed by spectroscopic ellipsometry using a modified Cauchy dispersion law for optical index calculation and through an optical model with double layers dispersion, we were able to determine the change of the optical properties of porous GaAs with integration effect of the TiO2 particle.