Epitaxial corundum-VTiO3 thin films grown on c-cut sapphire

Corundum structured VTiO3 has been grown as epitaxial films on c-cut sapphire by laser molecular beam epitaxy. The properties of the film were characterized by reflection high energy electron diffraction, X-ray diffraction, transmission electron microscopy (TEM), and photoemission spectroscopy. All the structural probes clearly indicate the corundum structure of the film. X-ray photoemission spectroscopy (XPS) indicates that V is in a 3 + charge state implying that Ti also should adopt a 3 + charge state in order for the corundum structure to form. However, the Ti-2p XPS, while clearly broadened to the lower binding energy side compared to TiO2, also exhibits a pronounced Ti4 + component. Similarly TEM-electron energy loss spectroscopy indicates a mixture of Ti3 + and Ti4 +. This is tentatively assigned to a combination of final state effects in XPS measurements and existence of excess (interstitial) oxygen. The valence band spectra show occupation of 3d metal states that resemble more closely those of Ti2O3 than for V2O3, suggesting that mostly the a1g molecular states are occupied.