In situ X-ray diffraction study of the controlled oxidation and reduction in the V–O system for the synthesis of VO2 and V2O3 thin films

Abstract

VO₂ and V₂O₃ thin films have been prepared by controlled oxidation and reduction reactions in the vanadium–oxygen system. During these reactions, crystalline phase formation and stability were characterized by means of in situ X-ray diffraction. Oxidation of vanadium thin films was carried out over a wide range of oxygen partial pressures between 0.2 and 200 mbar and temperatures between 430 °C and 615 °C. Depending on the oxygen partial pressures and temperatures, VO₂, V₆O₁₃ and V₂O₅ could be obtained as pure or mixed phases. Reduction of V₂O₅ in 50 mbar H₂ resulted in a continuous reduction to V₂O₃. Stabilization of the VO₂ phase was obtained by adding low O₂ concentrations in the range from 0.2 to 2 mbar to the H₂ gas, a method which proved to be successful also for the controlled oxidation of vanadium to VO₂. The semiconductor−metal transition was observed by means of temperature dependent sheet resistance measurements. VO₂ films prepared by the oxidation of vanadium at low oxygen partial pressures were characterized by a 3 orders of magnitude decrease in resistance during transition. Annealing in air only yielded comparable switching ratios when the annealing time was carefully optimized. Both the VO₂ films prepared by oxidation of vanadium or reduction of V₂O₅ in the mixture of H₂ and O₂ showed 4 to 5 orders of magnitude switching, which is close to the best reported values for bulk, single-crystal VO₂. This excellent switching performance is believed to originate from a decreased level of defects at grain boundaries and in the bulk. In addition, the V₂O₃ films prepared by reduction of V₂O₅ showed a 3 orders of magnitude increase in resistance near −100 °C. Our results provide methods for transforming vanadium oxide phases into VO₂ and V₂O₃ with high resistance switching ratios.