Study of nanocrystalline VO₂ thin films prepared by magnetron sputtering and post-oxidation

Nanocrystalline VO₂ thin films were deposited onto glass slides by direct current magnetron sputtering and post-oxidation. These films undergo semiconductor–metal transition at 70 ^\circC, accompanied by a resistance drop of two magnitude orders. The crystal structures and surface morphologies of the VO₂ films were characterized by x-ray diffraction (XRD) and atomic force microscope (AFM), respectively. Results reveal that the average grain size of VO₂ nanograins measured by XRD is smaller than those measured by AFM. In addition, Raman characterization indicates that stoichiometric VO₂ and oxygen-rich VO₂ phases coexist in the films, which is supported by x-ray photoelectron spectroscopy (XPS) results. Finally, the optical properties of the VO₂ films in UV-visible range were also evaluated. The optical band gap corresponding to 2p–3d inter-band transition was deduced according to the transmission and reflection spectra. And the deduced value, E_opt2p–3d=1.81 eV, is in good agreement with that previously obtained by theoretical calculation.