Intrinsic evolutions of optical functions, band gap, and higher-energy electronic transitions in VO ₂ film near the metal-insulator transition region

Transmittance spectra of (011) vanadium dioxide (VO ₂) film have been studied in the temperature range of 45-80 °C. Owing to increasing carrier concentration, the near-infrared extinction coefficient and optical conductivity around metal-insulator transition (MIT) rapidly increase with the temperature. Moreover, three electronic transitions can be uniquely assigned and show the hysteresis behavior near the MIT region. It was found that the optical band gap decreases from 0.457 to 0.042 eV before the MIT, then reduces to zero for the metal state. This confirms the fact that the a _1g and e g π bands are moved close and finally overlap with the temperature.