Spectroscopic ellipsometry studies are reported for vanadium dioxide grown on c-, m-, and r-plane sapphire substrates. The crystallographic orientation of the VO2 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 VO2 appears to transform abruptly from the monoclinic phase to the rutile R structure as temperature is increased. In contrast, VO2 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.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - Jan 31 2013|