Vanadium dioxide (VO2 ) exhibits a reversible semiconductor-to-metal phase transition around 341 K. The low temperature monoclinic P21 /c structure transforms into the tetragonal P42 /mnm rutile structure. The associated sharp changes in optical and electrical properties are used in various thin film switching devices. The present paper describes a study on the optical and structural changes during the synthesis of VO2 thin films using Infra-Red (IR) reflectance spectroscopy and Transmission Electron Microscopy (TEM).
The synthesis was derived from a sol-gel process by Guzman et al. Solutions of vanadyl isopropoxide (VO(OC3 H7)3 ) in n-propanol were spin-coated on a Si wafer provided with 300 nm thermal oxide. Subsequently, the wafer was transferred to an oven and heated to 773 K in a reducing H2/N2 atmosphere using a heating rate of 5 K/min. After a variable time (Δt) at 773 K, the samples were cooled down to room temperature.
Figure 1 shows the IR reflectance of thin (∽50 nm) VOx films on oxidised Si at λ,=3 μm and an angle of incidence ϕ of 30°.