Electrical and optical properties of VO₂ thin films grown on various sapphire substrates by using RF sputtering deposition

VO₂ thin films were grown on a-, c-, m-, and r-plane sapphire and SiO₂/Si substrates under identical conditions by using RF sputtering deposition from a VO₂ target. The structural and the morphological properties of all VO₂ films were investigated. The grain sizes of the VO₂ films varied between 268 nm and 355 nm depending on the substrate’s orientation. The electrical and the optical properties of all VO₂ thin films were examined in detail. The metal-insulator transition temperature (T_MI) varied with the substrate’s orientation. The (200)/(2 ¯ 11 )-oriented VO₂ films on the a-plane sapphire showed the lowest T_MI of about 329.3 K (56.3 °C) while the (020)/(002)-VO₂ films on the c-plane sapphire displayed the highest T_MI of about 339.6 K (66.6 °C). The VO₂ films showed reversible changes in the resistivity as large as 1.19 × 105 and a hysteresis of ~2 K upon traversing the transition temperature. The variations observed in the T_MI with respect to the substrate’s orientation were due to changes in the lattice strain and the grain size distribution. Raman spectroscopy showed that metal (rutile) - insulator (monoclinic) transitions occurred via the M2 phase for VO₂ films on the c-plane substrate rather than the direct M1 to rutile transition. The shifts in the phonon frequencies of the VO₂ film grown on various sapphire substrates were explained in terms of the strain along the V-V atomic bond direction (c_R). Our work shows a possible correlation between the transition parameters (e.g., T_MI, sharpness, and hysteresis width) and the width (σ) of the grain size distribution. It also shows a possible correlation between the T_MI and the resistivities at the insulating and the metallic phases for VO₂ films grown on various sapphire substrates.