Comparative analysis of VO2 thin films prepared on sapphire and SiO2/Si substrates by the sol-gel process

VO2 thin films were successfully grown on sapphire and SiO2/Si substrates by the sol-gel process. The VO2 phase was well formed during simplified low pressure annealing in oxygen. The films prepared on sapphire directly crystallized to the VO2 phase without passing through intermediate phases with increasing the annealing temperature, resulting in highly [010]oriented films on Al2O3(10 (1) over bar0) substrate. In contrast, the polycrystal films grown on SiO2/Si reached the final VO2 phase with passing through several phases. Mixed phases existed at the interface between the film and substrate. For the films on sapphire, the phases with low-valent vanadium appeared drastically at the interface region along the depth, whereas the phase of the polycrystal film slowly changed into a low valence state at the initial stage of the interface and then returned to a high value. And both films showed an abrupt change in resistivity at the different transition temperature. The property of the interface may affect the crystallization and the metal-insulator transition of the films.