MOCVD growth and structure of Nb- and V-doped TiO2 films on sapphire

Nb- and V-doped TiO2 thin films at a doping level up to 20 and 40 at%, respectively, have been grown on sapphire (0 0 0 1), (1 1 (2) over bar 0), and (0 1 (1) over bar 2) by metalorganic chemical vapor deposition. The Nb-doped TiO2 films are epitaxial rutile films, but the V-doped TiO2 films exhibit phase separation. The epitaxial orientation relationships for the Nb-doped TiO2 films were determined by X-ray diffraction (phi scans). Rutherford backscattering and X-ray theta rocking curves reveal that the atomic alignment in the growth direction is much better for the Nb-doped TiO2 grown on sapphire (0 0 0 1) than on sapphire(1 1 2 0) and (0 1 I 2). On the other hand, the in-plane alignment for the latter films is better than that for the former. Rutherford backscattering also shows that Nb atoms substitutionally incorporate at cation sites in the rutile lattice. X-ray photoelectron spectroscopy reveals that the oxidation state of both Ti and Nb is 4 +. In contrast, XPS shows Ti4+ and V5+ for the V-doped TiO2 films. X-ray diffraction and atomic force microscopy indicate that the V-doped TiO2 films are comprised of epitaxial TiO2 rutile islands in a V2O5 matrix. (C) 2000 Elsevier Science B.V. All rights reserved.