Transmittance change with thickness for polycrystalline VO2 films deposited at room temperature

In this work, polycrystalline VO2 thin films have been prepared on BK7 glass substrates by DC magnetron sputtering at room temperature with a post-annealing treatment at 450 °C, and relative thickness dependence of the semiconductor-metal transition (SMT) properties have been systematically investigated. It revealed that the prepared VO2 films exhibited unique surfaces composed of particles with various sizes, which became larger with increased film thickness. Different luminous transmittance between the metallic phase and semiconductor phase can be obtained by changing the film thickness based on the optical interference theory. The optimum thickness of VO2 films was around 90 nm with high ΔTsol of 9.7% and relatively high Tlum of 36.4% (higher than theoretical 29.1%), which is a recommendable optical property for practical application. The critical SMT temperature fluctuated when film thickness increased, and the transition speed of the heating process got slower overall with thickness decreasing, which might be caused by mechanical stress between films and substrates. In addition, the transition speed of cooling process turned slower with thickness of VO2 films increasing, which can be ascribed to the slower heat dissipation of thicker films than that of thinner films during the cooling process. These results might give an indication for creating or designing potential materials based on the thickness dependence of phase transition properties of VO2 films.