Fabrication, Structure And Optical Properties Of Polycrystalline TiO₂ Films

TiO₂ films are widely used in optical coating, microelectronic devices, and protective coatings owning to their high refractive index, transparent in the visible and near–infrared wavelength regions, high dielectric constant, very good wear resistance and stability. Polycrystalline TiO₂ films were fabricated using DC facing–target reactive sputtering. The influence of post–annealing and metallic ion doping on the structure and optical properties of the polycrystalline TiO₂ films was systematically studied.The as–deposited TiO₂ films consist of pure anatase phase or a mixture of anatase and rutile, and increasing rutile content to some extent deteriorates the crystallinity of the anatase. The roughness of the films increases monotonically with the increase of the sputtering pressure, which can be ascribed to the decrease in the mobility of the impinging particles. Hermite interpolation was used to calculate the envelope curves of the transmission spectra and the refractive of the films was determined by Swanepoel's envelope method. The refractive index of the TiO₂ films is in the range of 2.20?2.42 at wavelength of 550 nm, and increases with decreasing sputtering pressures, which can be explained as the densification of the films. Due to the increase in the fraction of rutile phase, the optical band gap value decreases from ³.35 eV to ³.29 eV.The effect of annealing on the crystallinity, morphology, and optical properties has been investigated. Thermal treatment improves the crystallinity of anatase and rutile phases. After annealing, the grain size of the films with mixed phase of anatase and rutile increases sharply, which confirms that the increase of the content of rutile phase suppresses the crystallization of the anatase. Besides, annealing also smoothes the film surface and enhances the density and refractive index of the films. The optical band gap of the TiO₂ films was increased by annealing, but for the films with a mixture of anatase and rutile phases, the gap still keeps a relative small value.Fe-doped TiO₂ films with different doping concentration (Fe/Ti ratio) have been fabricated by DC facing–target reactive sputtering. XPS investigation indicates that iron doping does no influence the chemical state of the TiO₂ and the film is a mixture of Fe2O3 and TiO₂. Iron doping prefers the formation of anatase and reduces the roughness...