Structural and electrical properties of Nb-doped TiO2 films sputtered with plasma emission control

Abstract

A technique to deposit Nb-doped films of TiO2 by radio-frequency magnetron sputtering was investigated. Nb wires are put onto the sputter track of a metallic titanium target and the oxidation state of the target is controlled by the intensity of a Ti line from the plasma emission. The sputtered films are analyzed with X-ray diffraction, RBS (Rutherford back scattering), energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and SEM (scanning electron microscopy). After post-heating at 400°C, all films exhibit the anatase structure. The films are polycrystalline with a Nb/[Nb+Ti] content from 2 to 17at.%. RBS and SEM indicate that the films consist of two sub-layers with the surface layer containing more Nb and exhibiting a clearer columnar structure. There is an optimum oxidation state of the target in the transition region between metallic and oxidic modes where the lowest resistivity of 7×10−4Ωcm is achieved. Nb is incorporated as Nb5+ into the anatase lattice, but in suboptimal films, Ti vacancies (acceptors) may compensate the donor effect of Nb. The oxygen content in the films is higher than for stoichiometric TiO2. O interstitials may increase the mass density of the films. The lattice parameter a and the unit cell volume increase with the Nb content in a similar manner as observed for single crystals of TiO2:Nb.