Oxidizing annealing effects on VO ₂ films with different microstructures

Vanadium dioxide (VO ₂ ) films have been prepared by direct-current magnetron sputter deposition on m-, a-, and r-plane sapphire substrates. The obtained VO ₂ films display different microstructures depending on the orientation of sapphire substrates, i.e. mixed microstructure of striped grains and equiaxed grains on m-sapphire, big equiaxed grains on a-sapphire and fine-grained microstructure on r-sapphire. The VO ₂ films were treated by the processes of oxidation in air. The electric resistance and infrared transmittance of the oxidized films were characterized to examine performance characteristics of VO ₂ films with different microstructures in oxidation environment. The oxidized VO ₂ films on m-sapphire exhibit better electrical performance than the other two films. After air oxidization for 600 s at 450 °C, the VO ₂ films on m-sapphire show a resistance change of 4 orders of magnitude over the semiconductor-to-metal transition. The oxidized VO ₂ films on a-sapphire have the highest optical modulation efficiency in infrared region compared to other samples. The different performance characteristics of VO ₂ films are understood in terms of microstructures, i.e. grain size, grain shape, and oxygen vacancies. The findings reveal the correlation of microstructures and performances of VO ₂ films, and provide useful knowledge for the design of VO ₂ materials to different applications.