Room temperature optical constants and band gap evolution of phase pure M₁-VO₂ thin films deposited at different oxygen partial pressures by reactive magnetron sputtering

Spectroscopic ellipsometry study was employed for phase pure VO ₂(M₁) thin films grown at different oxygen partial pressures by reactive magnetron sputtering. The optical constants of the VO ₂(M₁) thin films have been determined in a photon energy range between 0.73 and 5.05 eV. The near-infrared extinction coefficient and optical conductivity of VO₂(M₁) thin films rapidly increase with decreasing O₂-Ar ratios. Moreover, two electronic transitions can be uniquely assigned. The energy gaps correlated with absorption edge (E₁) at varied O₂-Ar ratios are almost the same (2.0 eV); consequently, the absorption edge is not significantly changed. However, the optical band gap corresponding to semiconductor-to-metal phase transition (E ₂) decreases from 0.53 to 0.18 eV with decreasing O₂-Ar ratios.