Optical properties of VO2 thin films in their dielectric and metallic states

Thermochromic VO2 thin films have been deposited by reactive RF cathodic sputtering, using a V2O4 target. They were characterized by XRD, AFM and optical measurements. We show that the microstructure influences the optical response of the material, i.e. its hysteresis cycle as well as the values of the optical constants n and k. We performed ellipsometric measurements from the UV to the far infrared, combining two kinds of ellipsometers, in both the dielectric and the metallic states. The n and k constants are described by a dispersion law based on Lorentz oscillators, and an additional Drude contribution for the metallic state. The results are confirmed by XPS analysis. Contrary to the semiconductor phase, the metallic phase appears to be strongly dependent of the microstructure, as far as the optical response is concerned.