Influence of substrate temperature and salt concentration on the properties of Vanadium dioxide thin films synthetized by spray pyrolysis

Vanadium oxide thin film has been grown on a glass substrate utilizing the spray pyrolysis technique. In the present work, by using Vanadyl acetylacetonate (C10H14O5V) as a salt source, we have succeeded, for the first time, to the best of our knowledge in the formation of VO_2 instead of the commonly produced V_2O_5 . The influence of salt concentration and substrate temperature is investigated. Structural, morphological, optical and electrical properties of the synthesized VO_2 thin films were characterized by several techniques such as ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmittance and electrical resistivity. The XRD analysis revealed that the effect of substrate temperature is more significant at low salt concentrations. At high salt concentrations, the crystallite size varies slightly with increasing the substrate temperature. The Raman spectroscopy comforts the presence of a small amount of the V_2O_5 especially at higher salt concentration. The whole synthesized films exhibit a continuous pinholes-free and dense structure. The influence of the substrate temperature on the film morphology depends on the used salt molarity. Increasing the substrate temperature causes the formation of needle-like features, at a low molarity of 0.05M, while, at a high molarity of 0.1M, spherically shaped grains are formed. A similar trend of the influence of the substrate temperature on the optical properties of the film is observed. At low molarity, increasing the substrate temperature causes an increase in the vanadium thin film band gap. On the contrary, at higher molarity, the reverse behavior is observed i.e. the band gap reduction. The semiconductor metal transition has been observed in the prepared VO_2 films. However, this transition is not too drastic due, on one hand, to the presence of disorder in the film network and on the other hand to the presence of vanadium vacancy defects.