Mid-infrared properties of a VO2 film near the metal-insulator transition

A VO2 film was grown on a sapphire(0001) substrate using pulsed laser deposition. The film showed a first-order metal-insulator (MI) transition acid its de conductivity started to increase drastically near 68 degrees C and changed by three orders of magnitude. Mid-infrared transmittance and reflectance spectra of the VO2 film were measured between 20 degrees C and 90 degrees C. Using the intensity transfer-matrix method, the frequency-dependent dielectric constant epsilon(f)(omega) and the conductivity sigma(f)(omega) of the film were obtained between 1600 and 4000 cm(-1) from the measured transmittance and reflectance spectra. With the epsilon(f)(omega) and sigma(f)(omega) spectra, mid-infrared properties of the VO2 film near the MI transition region were investigated in detail. Above 78 degrees C, epsilon(f)(omega)<0 and d epsilon(f)/d omega>0, which is a typical metallic behavior. In particular, epsilon(f)(omega) and sigma(f)(omega) at 88 degrees C were analyzed in terms of extended Drude model in which the frequency-dependent scattering rate and the effective mass could be obtained. The mean free path of charge carriers in the de limit was estimated to be larger by an order of magnitude than the previously reported value, i.e., 4 Angstrom. Below 74 degrees C, epsilon(f)(omega)>0 and d epsilon(f)/d omega approximate to 0, which is characteristic of an insulator. Interestingly, epsilon(f) in the insulating region increased as the temperature approached the MI transition temperature. To explain this anomalous behavior, the MI transition of the VO2 film was modeled with coexistence of metallic and insulating domains and their dynamic evolution. Then the behaviors of epsilon(f)(omega) and sigma(f)(omega) were explained using the effective medium approximation, which is a mean-field theory predicting a percolation transition. This work clearly demonstrates that the transport and optical properties near the MI transition region are strongly influenced by the connectivity of the metallic domains.