Surface versus bulk characterizations of electronic inhomogeneity in a VO2 thin film

Abstract

We investigated the inhomogeneous electronic properties at the surface and interior of VO2 thin films that exhibit a strong first-order metal-insulator transition (MIT). Using the crystal structural change that accompanies a VO2 MIT, we used bulk-sensitive x-ray diffraction (XRD) measurements to estimate the fraction of metallic volume p(XRD) in our VO2 film. The temperature dependence of the p(XRD) was very closely correlated with the dc conductivity near the MIT temperature and fitted the percolation theory predictions quite well: sigma similar to(p-p(c))(t) with t=2.0 +/- 0.1 and p(c)=0.16 +/- 0.01. This agreement demonstrates that in our VO2 thin film, the MIT should occur during the percolation process. We also used surface-sensitive scanning tunneling spectroscopy (STS) to investigate the microscopic evolution of the MIT near the surface. Similar to the XRD results, STS maps revealed a systematic decrease in the metallic phase as temperature decreased. However, this rate of change was much slower than the rate observed with XRD, indicating that the electronic inhomogeneity near the surface differs greatly from that inside the film. We investigated several possible origins of this discrepancy and postulated that the variety in the strain states near the surface plays an important role in the broad MIT observed using STS. We also explored the possible involvement of such strain effects in other correlated electron oxide systems with strong electron-lattice interactions.