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

Abstract

We correlate electron transport data directly with energy band structure measurements in vanadium oxide thin films with varying V-O stoichiometry across the ${\text{VO}}_{2}$ metal-insulator transition. A set of vanadium oxide thin films were prepared by reactive dc sputtering from a V target at various oxygen partial pressures (${\text{O}}_{2}$ p.p.). Metal-insulator transition (MIT) characteristic to ${\text{VO}}_{2}$ can be seen from the temperature dependence of electrical resistance of the films sputtered at optimal ${\text{O}}_{2}$ p.p. Lower and higher ${\text{O}}_{2}$ p.p. result in disappearance of the MIT. The results of the near edge x-ray absorption fine structure spectroscopy of the $\text{O}\text{ }K$ edge in identical VO films are presented. Redistribution of the spectral weight from ${\ensuremath{\sigma}}^{\ensuremath{\ast}}$ to ${\ensuremath{\pi}}^{\ensuremath{\ast}}$ bands is found in the vanadium oxide films exhibiting stronger ${\text{VO}}_{2}$ 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 also observe a clear correlation between MIT and the width and area of the lower ${\ensuremath{\pi}}^{\ensuremath{\ast}}$ band, which is likely to be due to the emergence of the ${d}_{||}$ band overlapping with ${\ensuremath{\pi}}^{\ensuremath{\ast}}$. The strengthening of this ${d}_{||}$ band near the Fermi level only in the vanadium oxide compounds displaying the MIT points out the importance of the role of the ${d}_{||}$ band and electron correlations in the phase transition.