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

Abstract

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)/( )-oriented VO2 films on the a-plane sapphire showed the lowest T-MI of about 329.3 K (56.3 A degrees C) while the (020)/(002)-VO2 films on the c-plane sapphire displayed the highest T-MI of about 339.6 K (66.6 A degrees C). The VO2 films showed reversible changes in the resistivity as large as 1.19 x 105 and a hysteresis of similar to 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 (sigma) 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.