Composition evolution and electrical properties of VO2 thin films induced by annealing temperature

In this study, vanadium dioxide thin films were fabricated on Al2O3 (0001) substrates by sol-gel method and subsequent annealing process. The effects of annealing temperature on the structure, chemical state, surface morphology and electrical properties of the films were systematically studied. The results displayed that with the annealing temperature increasing up to 520 degrees C, the composition evolution experienced four processes: the reduction of V5+ to V4+ (or V3+), the comproportionation of V5+/V3+ to V4+, the oxidation of V3+ to V4+, and the secondary reduction of V5+ to V3+. Accompanied with the secondary reduction at 520 degrees C, the disappearance of grain boundaries was observed on the film surface. Furthermore, as the annealing temperature increased from 420 degrees C to 470 degrees C and then 520 degrees C, the prepared films showed good phase transition property with the resistance change up to nearly four orders of magnitude, and the phase transition temperatures were 62.8 degrees C, 61.8 degrees C and 61.1 degrees C respectively, showing a slow downward trend. The results not only revealed the mutual transformation of various vanadium oxides during the annealing treatment, but also supplied some clues for optimizing the parameters for VO2 film preparation with high quality.