Crystallization and conductivity of large-domain Nb-doped TiO2 films prepared by electron beam evaporation

Abstract

Thin solid films of titania (TiO2), undoped and doped with niobium (Nb), are prepared by reactive electron-beam evaporation at 250 °C and post-heated at temperatures up to 750 °C. Nucleation starts at 375 °C and Nb-doped films largely crystallize at 420 °C, to anatase, as detected by X-ray diffraction (XRD), Raman spectra, and Raman phonon maps. Nb incorporation favors the anatase phase over other crystalline titania phases and leads to an increase in its unit cell volume and a blue shift of the Raman Eg(1) peak. In Raman phonon mapping, films with good conductivity show large domains with similar Raman signals, more than 1 µm in diameter, larger than the crystalline grain diameter of 70 nm obtained from XRD. Such films exhibit a unit cell volume indicating effective Nb incorporation, and a minimum electrical resistivity of about 200 µΩcm, sometimes reached already upon annealing at 350 °C. These findings suggest that the phonon domains consist of about 200 grains with similar crystalline orientation separated by small-angle boundaries.