Influence of O 2 partial pressure on the growth of nanostructured anatase phase TiO 2 thin films prepared by DC reactive magnetron sputtering

Abstract

Nanostructured titanium dioxide (TiO 2) thin films were deposited on unheated glass substrates by DC reactive magnetron sputtering. The influence of the oxygen percentage in the discharge and the sputtering power ( P) on the phase composition, crystallinity, surface morphology and optical properties of the films was investigated. The structure of the as-sputtered TiO 2 films prepared in different oxygen concentrations for P = 500 W and 1000 W varies from amorphous to crystalline. X-ray diffraction revealed that, excluding the lowest oxygen concentration (1% O 2), all annealed TiO 2 thin films at 400 °C/4 h have an anatase crystal structure and the crystallinity decreases with the oxygen percentage. However, for 5 and 10% O 2, the films are textured with the (1 0 1) crystal direction normal to the substrate surface and for O 2 ≥ 15% the films have the preferred orientation (0 0 4), which is accounted for different surface energies associated with different planes. Field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) evidence distinct modifications in the morphology of the thin films which are corroborated with the crystallite sizes estimated from the typical diffraction peaks. Agglomeration of smaller TiO 2 nano-sized grains to form bigger sized particulates is observed and enhanced by the decrease in oxygen content. With the variation in oxygen concentration, the optical properties (refractive index and optical band gap) of TiO 2 films also change strongly being dependent on crystallinity, crystallite size and oxygen vacancies.