  In this paper we used Raman spectroscopy to investigate the optical
properties of vanadium dioxide (VO2) thin films during the thermally induced
insulating to metallic phase transition. We observed a significant difference
in transition temperature in similar VO2 films grown on quartz and sapphire
substrates: the film grown on quartz displayed the phase transition at a lower
temperature (Tc=50C) compared a film grown on sapphire (Tc=68C). We also
investigated differences in the detected Raman signal for different wavelengths
and polarizations of the excitation laser. We found that for either substrate,
a longer wavelength (in our case 785 nm) yielded the clearest VO2 Raman
spectra, with no polarization dependence.
