Optical and electrical properties of vanadium dioxide films prepared under optimized RF sputtering conditions

Vanadium dioxide films were prepared by DC and rf reactive magnetron sputtering of a 99.7% pure vanadium target in an Ar+O₂ plasma with a well-controlled oxygen partial pressure. The films were deposited onto normal glass substrates at 400°C. The films showed a metal–semiconductor transition at the temperature, τ_c=65–68°C. Optical and electrical properties of the films were investigated around the metal–semiconductor phase transition and found to be very sensitive to the oxygen flow rate. Sheet resistance of the films were recorded using a two-point probe over the temperature range 26τ100°C. It was observed that the sheet resistance can change by three orders of magnitude when heating the films from room temperature to temperatures above the transition. Transmittance of the films was obtained in the 300λ2500 nm wavelength range at two extreme temperatures (i.e. 26°C and 100°C). The luminous transmittance for the films was rather unaffected with heating, whereas near-infrared transmittance showed lower values. Optical constants, n and k were measured using ellipsometry. The semiconducting state optical constants were found to be 2.67 and 0.04 for n and k, respectively, while the metallic state values were 2.26 for refractive index and 0.3 for the extinction coefficient. The samples showed a slow deterioration when left in the laboratory for a period of one year.