Temperature dependence of the optical properties of VO2deposited on sapphire with different orientations

Spectroscopic ellipsometry studies are reported for vanadium dioxide grown on $c$-, $m$-, and $r$-plane sapphire substrates. The crystallographic orientation of the VO${}_2$ depends strongly on the substrate, producing diverse strains in the layers which affect the interband transition energies and the phase transition temperatures. These structural differences correlate with distinct variations of the optical transitions observed in the ellipsometry results. For the $m$- and $r$-plane substrates, the VO${}_2$ appears to transform abruptly from the monoclinic phase to the rutile $R$ structure as temperature is increased. In contrast, VO${}_2$ deposited on $c$-plane sapphire exhibits a sluggish transformation. For the $m$-plane sample, the energy gap collapses over a narrow temperature range. For the $c$-plane case, a broad temperature range is obtained between the onset and completion of the transformation. Raman studies of the vibrational structure show that internal stresses due to expansion and contraction across the phase transitions impacts the observed phonon energies.