Correlation between metal-insulator transition characteristics and electronic structure changes in vanadium oxide thin films

We correlate electron transport data with energy band structure measurements in vanadium oxide thin films with varying V-O stoichiometry across the VO$_{2}$ metal-insulator transition (MIT). A set of vanadium oxide thin films were prepared by reactive DC sputtering from a V target at various oxygen partial pressures resulting in films with different MIT strength as determined from the electrical resistance measurements. The results of the near edge X-ray absorption fine structure spectroscopy (NEXAFS) of the O K-edge in identical VO films are presented. Redistribution of the spectral weight from $\sigma $* to $\pi $* bands is found in the vanadium oxide films exhibiting stronger VO$_{2}$ MIT. This is taken as evidence of the strengthening of the metal-metal ion interaction with respect to the metal-ligand and indirect V-O-V interaction in vanadium oxide films featuring sharp MIT. We observe also a clear correlation between MIT and the width and the area of the lower $\pi $* band which is likely to be due to the emergence of the d$_{\vert \vert}$ band overlapping with $\pi $*. The strengthening of this d$_{\vert \vert}$ band near the Fermi level only in the vanadium oxide compounds displaying the MIT points out the importance of the role of the d$_{\vert \vert}$ band and electron correlations in the phase transition.