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

Abstract

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.

Substrate-induced microstructure effects on the dynamics of the photo-induced metal–insulator transition in VO2thin films

“…This, and the more ordered mono-crystalline structure of the VO 2 /TiO 2 samples seems to favor the formation of the more uniform nucleation sites during MIT, as evident by a the narrower thermally-induced MIT in this sample, happening at lower critical temperature. The signif-icantly broader width of the MIT transition in the VO 2 /Al 2 O 3 sample implies the broader distribution26 of the metallic nucleation cluster sizes compared to VO 2 /TiO 2 samples, as the connection between the width of the thermally-induced MIT and the structure of the film have been demonstrated previously 7,[26][27][28]. These structural differences may also play a critical role in explaining the observed differences in temperature dependence of the full MIT threshold Φ f ull−M IT for the two samples.The absence of the temperature sensitivity of Φ f ull−M IT for VO 2 /TiO 2 sample indicates that once the critical density of the photo-electrons is reached, the whole film undergoes the MIT uniformly, independently of its original temperature.…”

Modification of electronic structure in compressively strained vanadium dioxide films

“…Raman spectroscopy performed on films grown by the same method also shows negligible shift in the phonon center frequencies compared to bulk VO 2 . 36 Remarkably, the lattice dynamics are virtually unchanged relative to bulk VO 2 in a film where the T c is so significantly depressed.…”

Oxidizing annealing effects on VO2 films with different microstructures

“…It was proved that the SMT transition characteristics and light transmittance of VO 2 films strongly depend on nature of boundaries, grain size and defect content etc. [8][9][10][11][12]. Sahana etc.…”