Metal-insulator transition of valence-controlled VO₂ thin film prepared by RF magnetron sputtering using oxygen radical

We have prepared b-axis-oriented VO₂ thin films by RF magnetron sputtering using oxygen radicals as the reactive gas. The VO₂ thin films consist of a mixed-valence V³⁺/V⁴⁺ state formed by oxygen vacancies. The V3+ ratio strongly depends on the film thickness and the oxygen partial pressure of the radical gun during deposition. The lattice constant of the b-axis increases and the metal-insulator transition (MIT) temperature decreases with decreasing V³⁺ ratio, although the VO₂ thin films with a high V³⁺ ratio of 42% do not exhibit MIT. The bandwidths and spectral weights of V 3d a1g and e g bands at around the Fermi level, which correspond to the insulating phase at 300 K, are smaller in the VO₂ thin films with a low V³⁺ ratio. These results indicate that the control of the mixed-valence V³⁺/V⁴⁺ state is important for the MIT of b-axis-oriented VO₂ thin films.