Effect of oxygen partial pressure on properties of asymmetric bipolar pulse dc magnetron sputtered TiO_2 thin films

Abstract

A set of titanium dioxide thin films have been deposited on fused silica substrates by the novel asymmetric bipolar pulsed dc reactive sputtering technique using different oxygen partial pressures in the sputtering ambient in the range of 0%–21%. For investigating long-range structural properties of the samples, grazing incidence x-ray diffraction (GIXRD) measurements and for probing local structure surrounding Ti sites, extended x-ray absorption fine structure (EXAFS) measurements have been carried out. Optical properties of the films have been investigated by transmission spectrophotometry in UV–visible–near IR range and it has been observed that as oxygen partial pressure in the sputtering ambient is increased, refractive index of the films varies in a nonlinear fashion. Microscopically, it has been found that this nonlinear variation can be explained by the local structure tool EXAFS, while GIXRD which works on average long-range order fails to explain this. Such a variation of optical properties with increase in oxygen partial pressure during deposition of the films is attributed to the competition between the two processes, viz., improvement in the stoichiometry of the films and reduction in the mobility of the adatoms on the surface of the growing films.