Effects of annealing on thermochromic properties of W-doped vanadium dioxide thin films deposited by electron beam evaporation

Thermochromic vanadium dioxide (VO₂) undergoes a fully reversible semiconductor-metal transition (SMT) at a critical temperature (T_t) of ~ 68 °C with a dramatic change in electric and optical properties, which makes it an attractive candidate for its application in smart windows. Switchable VO₂ and W-doped vanadium dioxide (W_xV_{1 − x}O₂) thin films are grown over quartz substrates via electron beam evaporation technique by using VO₂/W_xV_{1 − x}O₂ as targets at room temperature (RT) followed by post annealing process at different temperatures. The as-deposited films are amorphous, and that transform to monoclinic VO₂ (VO₂(M)) with (011)-preferred orientation after annealing at 500 °C under vacuum. The (011) peak of W-doped VO₂ films shifts to a lower diffraction angle as compared with un-doped VO₂ films which confirms the incorporation of W ions into the VO₂ lattice. Temperature dependent optical transmittance measurement demonstrates the thermochromic properties, with a reduction in the phase transition temperature (T_t) as observed in W-doped VO₂ films, which is attributed to the variation of electron structure in VO₂ due to doping. The films are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and optical transmittance measurement.