Electronic Structure of b-Axis Oriented VO2 Thin Film with Mixed-Valence States Probed by Soft-X-ray Spectroscopy

Abstract

The electronic structure of b-axis oriented VO2 thin films in monoclinic phase with various thicknesses has been studied by soft-X-ray spectroscopy. The VO2 thin films show the mixed-valence V3+/V4+ state, which is created by oxygen vacancies. The V3+ state and oxygen vacancies increase with increasing film thickness. The thinnest film of 56 nm exhibits a metal–insulator transition (MIT) at ∼340 K, although thicker films of 126 and 155 nm do not exhibit MIT. The valence band structure around the Fermi level consists of V 3d \(e_{g}^{\pi }\), \(a_{1g}\), and \(e_{g}^{\sigma }\) bands. The bandwidths and spectral weights of the a1g and \(e_{g}^{\sigma }\) bands are smaller in the thinnest film, which corresponds to the insulating phase at 300 K. The above results indicate that the mixed-valence states with oxygen vacancies are closely related to the MIT in the monoclinic phase of VO2 thin films.