Comparison of the optical responses of O-poor and O-rich thermochromic VOX films during semiconductor-to-metal transition

O-poor and O-rich thermochromic vanadium oxide (VOX) nanostructured thin films were prepared by applying reactive direct current magnetron sputtering and post-annealing in oxygen ambient. UV–visible spectrophotometer and spectroscopic ellipsometry were used to investigate the optical properties of films. It was found that, when the O-poor VOX thin film underwent semiconductor-to-metal transition, the values of optical conductivity and extinction coefficient in the visible region increased due to the existence of occupied band-gap states. This noticeable feature, however, was not observed for the O-rich film, which showed a similar optical behavior with the stoichiometric crystalline VO2 films reported in the literatures. Moreover, the O-poor VOX film exhibits consistent variations of transmission values in the visible/near-infrared region when it undergoes semiconductor-to-metal transition.

► An optical absorption shoulder is found in the absorption coefficient plot of O-poor VOX film. ► Enhancement of optical conductivity is observed in the O-poor VOX film when it undergoes SMT. ► It is believed that the optical property of the O-poor film is related to occupied band-gap states.