Pseudo-hexagonal in-plane alignment of rutile (100)Nb:TiO2 on hexagonal (0001)Al2O3 plane

Nb-doped TiO2 (Nb:TiO2) films were grown on a hexagonal (0001)Al2O3 substrate at 650 °C and ∼10−5 Torr. The Nb:TiO2 film had a small resistivity of ∼8×10−4 Ω cm at room temperature and a behavior of a slightly increasing resistance upon cooling. In addition, the Nb:TiO2 film had an optical transmittance of about 60% in the visible range. A careful analysis of the in-plane atomic structure suggests that the rutile Nb:TiO2 film on the hexagonal (0001)Al2O3 can be re-interpreted by a certain pseudo-hexagonal structure, which is discriminated from the in-plane rectangular one of the tetragonal (100)Nb:TiO2. The pseudo-hexagonal properties of the Nb:TiO2 film were characterized by negligible mosaic structure at the interface, the same electron diffraction pattern as the hexagonal Al2O3 substrate, and perfect six-fold symmetries in the pole figure and ϕ-scan XRD patterns.