Orientation control and metal–insulator transition of VO₂ thin film deposited on Cd_0.9Mg_0.1O buffered soda-lime glass

Abstract

We prepared (001) oriented Cd_0.9Mg_0.1O thin films on soda-lime glass substrates using pulsed laser deposition (PLD). Use of the (001) oriented Cd_0.9Mg_0.1O thin film as a buffer layer was attempted to prepare VO₂(R) (rutile) thin film. Because the lattice mismatch between the a-axis of VO₂(R) (a = 0.4555 nm) and Cd_0.9Mg_0.1O (a = 0.4651) is slight (−2.06 %), we expected a (001) oriented VO₂(R) thin film on Cd_0.9Mg_0.1O buffered glass. The results indicate (110) orientation, which is likely attributable to the influence of the surface energy of VO₂(R). In the rutile structure, the (001) plane is unstable because the coordination number of the cations at the outermost surface is low. In contrast, the (110) plane has the lowest surface energy. Therefore, the surface energy of VO₂(R) is considered to exert a stronger effect than the interaction between the substrate and the thin film. The temperature dependence of resistivity of VO₂ thin films with thickness deposited on Cd_0.9Mg_0.1O buffered glass substrates revealed that no metal–insulator transition was observed for thinner VO₂ thin film (thickness less than 59 nm). For large thickness (greater than 82 nm), then a clear metal–insulator transition was observed. This phenomenon was explained in terms of stress and oxygen vacancy. A similar result was obtained from measurement of the temperature dependence of IR transmittance with thickness. For 94-nm-thick VO₂ thin film, IR transmittance change as high as 53.9 % was obtained.