Effect of a substrate-induced microstructure on the optical properties of the insulator-metal transition temperature in VO2 thin films

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Using both Raman spectroscopy and direct laser reflectivity measurements, we investigate the optical properties of vanadium dioxide (VO2) thin films deposited on different substrates as they undergo the thermally induced insulator to metal phase transition. Comparing similarly prepared VO2 films grown on quartz, sapphire, and rutile substrates, we observed a significant difference in the transition temperatures without hysteresis loop broadening after heating and cooling the samples. We attribute these different transition temperatures to differences in the VO2 microstructure, mainly the difference in average grain sizes. We also observed variations in the contrast of the detected Raman resonances using different wavelengths for the excitation laser, and found that in all cases a longer wavelength (in our case 785 nm) yielded the clearest VO2 Raman spectra.