Raman spectra analysis of vanadium dioxide thin film deposited by polymer-assisted deposition method

Raman microscopy was widely utilized to study the temperature dependent evolution of vibration modes of vanadium dioxide thin films in the vicinity of phase transition. In this study, the temperature dependent Raman spectra characterization was performed on epitaxial growth vanadium dioxide (VO₂) thin film, which was fabricated on m-sapphire substrate using polymer-assisted deposition method with a thickness of 50 nm. The results show that the characteristic phonon modes of VO₂ monoclinic phase got weakened with the increase of temperature and finally disappeared well above the phase transition. Then, by comparing the Raman spectra in heating and cooling cycles under dependent temperatures, a phase diagram across the metal-insulator transition was proposed identifying monoclinic, intermediate, and rutile regimes. The intermediate region was attributed to the co-existence of monoclinic and rutile phase, which might be caused by strains and defects within the grains of the film. Moreover, MIT of epitaxial growth VO₂ film observed in this study is much sharper than that of polycrystalline film reported by other workers, indicating the MIT property is closely related to the crystal quality of VO₂ thin film.