Topology of conductive clusters in sputtered high-quality VO2 thin films on the brink of percolation threshold during insulator-to-metal and metal-to-insulator transitions

Abstract

Electrical conductivity (σ) and optical transmittance of high quality VO2 thin films deposited by DC reactive magnetron sputtering on r-cut sapphire substrates (at 650 °C) have been measured simultaneously as a function of temperature by heating and cooling scans through the phase transition region. The partial concentration of the metallic phase (Xm) has been calculated from the optical transmittance, and the σ(Xm) dependence has been analyzed through an insulator-to-metal transition (IMT) during heating and through a metal-to-insulator transition (MIT) during cooling. The results have shown to be consistent with the Efros–Shklovskii percolation theory, predicting the formation of two-dimensional infinite conductive cluster (ICC) during IMT and the preservation of three-dimensional ICC during MIT. The critical concentrations (Xc) corresponding to the appearance of ICC at IMT and the disappearance of ICC at MIT were found to be very different, 0.57 and 0.06, respectively. A mathematical model explaining very small Xc at MIT was developed. The dissimilarity of the ICC topology during IMT and MIT is connected with the appearance and disappearance of local mechanical stresses imminent in VO2 phase transitions.