Influence of thermal cycling on structural, optical and electrical properties of vanadium oxide thin films

Vanadium oxide films with different thicknesses were deposited on quartz substrates at room temperature by thermal evaporation technique. For investigating the effect of thermal cycling on the properties of the films, the as-deposited films were heated from the room temperature up to 300 °C and then cooled down to the room temperature. The structure and the surface morphology of the samples were studied by X-ray diffraction, Raman spectra and atomic force microscopy. The transmittances of the samples were measured by spectrophotometer. Electrical resistance during thermal cycling was recorded by dual probe method. The experimental results show that the amorphous as-deposited film changes to crystalline structure after thermal cycling. The crystalline film consists of V2O5 and a little of VO2. A reversible semiconductor–metal phase transition, with decrease of electrical resistance in a factor of 103, is observed at temperature about 230 °C during the heating process. An obvious thermal hysteresis in electrical resistance is observed during the thermal cycling. After thermal cycling, the transmittance of the film decreases, but the refractive index and extinction coefficient increase, which are the results of structural change from amorphous to crystalline phase.