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 and its dc conductivity started to increase drastically near 68 °C and changed by three orders of magnitude. Mid-infrared transmittance and reflectance spectra of the VO₂ film were measured between 20 °C and 90 °C. Using the intensity transfer-matrix method, the frequency-dependent dielectric constant ε_f(ω) and the conductivity σ_f(ω) of the film were obtained between 1600 and 4000 cm^-1 from the measured transmittance and reflectance spectra. With the ε_f(ω) and σ_f(ω) spectra, mid-infrared properties of the VO₂ film near the MI transition region were investigated in detail. Above 78 °C, ε_f(ω)<0 and dε_f/dω>0, which is a typical metallic behavior. In particular, ε_f(ω) and σ_f(ω) at 88 °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 dc limit was estimated to be larger by an order of magnitude than the previously reported value, i.e., 4 Å. Below 74 °C, ε_f(ω)>0 and dε_f/dω≊0, which is characteristic of an insulator. Interestingly, ε_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 ε_f(ω) and σ_f(ω) 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. © 1996 The American Physical Society.