Investigation of the optical effect of stable VO₂ thin films for smart materials applications

In this study, the metallic vanadium thin film was coated on a p-type silicon substrate by e-beam evaporation and then heat treatment was performed in a controlled gas atmosphere. By this method, our aim was to achieve growth of solid metallic films on single crystal substrates followed by the formation of a thin VO₂ thin film with +4 valence value for V and obtain a metal/dielectric bilayer with tunable reflectivity. The composition/oxidation state of the surface, crystal structure of the deposited films were analyzed by XPS, SEM, XRD and ellipsometer techniques respectively. The thermochromic properties of VO₂ thin film samples were investigated by carrying out temperature dependent reflection measurements where we observed and analyzed the strong dependence of reflectivity on the incoming radiation energy with the upper VO₂ layer acting like a filter of reflection. We observe a clear and large hystereses in the reflection specta during heating and cooling near the transition temperature, indicative of a first order transition in contrast to what one would expect in a clamped film as clamping is often thought to convert the transition into second order. This behavior is discussed in the light of 2-phase coexistence for clamped film structures.