Thickness dependence of optical properties of VO2 thin films epitaxially grown on sapphire (0 0 0 1)

Vanadium dioxide (VO2) films were epitaxially grown on a-Al2O3 (0 0 0 1) by rf reactive magnetron sputtering. The effects of film thickness ranging from 3 to 150 nm on optical properties were investigated. It revealed that the semiconductor–-metal phase transition temperature considerably decreases as film thickness decreases, in particular for the film with thickness less than 10 nm. On the other hand, we found that the difference in visible transmittance between the two phases of VO2 also varies with film thickness. For the films with thickness less than 50 nm, the semiconductor phase exhibits lower visible transmittance than its metallic phase, while for those with thickness larger than 50 nm the situation is reversed.