Electronic structure of b-axis oriented VO₂ thin film with mixed-valence states probed by soft-X-ray spectroscopy

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(Formula presented.), a_1g, and e(Formula presented.) bands. The bandwidths and spectral weights of the a_1g and e(Formula presented.) 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.