Spectroscopic ellipsometry analysis of vanadium oxide film in Vis-NIR and NIR-MIR

Abstract

The monoclinic phase (M phase) VO2 film is prepared on quartz glass substrate by a model MSP-3200 three-target co-sputter coater with RF magnetron reactive sputtering. The optical properties in incident energy ranges of 0.5-3.5 eV (350-2500 nm) and 0.083-0.87 eV (1400-15000 nm) of VO2 film are investigated by spectroscopic ellipsometry with variable temperature attachment. The good results are determined point by point with the three Lorentz harmonic oscillator dispersion models in the range of 0.5-3.5 eV and four Gaussion harmonic oscillator dispersion models in the range of 0.083-0.87 eV in the state of semiconductor below the transition temperature, while adding seven Lorentz harmonic oscillator dispersion models in the high temperature metallic state film results in the characteristic absorption peaks. The results show that the refractive index of the semiconductor state of VO2 film is maintained at maximum 3.27 and extinction coefficient k is close to zero in the near infrared-mid infrared, which is due to the fact that the absorption of semiconductor thin film in the VIS-NIR range is derived from the free carrier absorption and d// orbital of the semiconductor film has less electron density. The refractive index n of high temperature metallic state VO2 film has an obviously increasing trend in the near infrared-the mid infrared which is larger than the refractive index of the semiconductor state when the incident light energy is 0.45 eV. Extinction coefficient k increases rapidly in the near infrared, which is because the density of free carrier increases in the range of 0.5-1.62 eV and electron transition absorption augments within the V3d band. When the incident energy less than 0.5 eV, k value changes gently in the film because free carrier concentration and flow rates are stable.