Temperature dependence of the optical properties of VO${}_2$ deposited 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.

Figure 1

Real $〈{\varepsilon }_1〉$ and imaginary $〈{\varepsilon }_2〉$ part of pseudodielectric function of VO${}_2$ at selected temperatures for samples (a) and (b) $c$, (c) and (d) $m$, and (e) and (f) $r$. Solid and dashed lines in the $〈{\varepsilon }_2〉$ panels demark optical transitions in insulating and metallic phases of VO${}_2$, respectively.

Figure 2

Energy gap (black) and Drude energy (red) as a function of temperature for sample (a) $c$, (b) $m$, and (c) $r$. Vertical dashed lines in each panel represent $T_{\mathrm{onset}}$ and $T_{\mathrm{MIT}}$.

Figure 3

Temperature-dependent optical transitions in semiconductor phase (1, 2, and 3), intermediate and metallic phase (1′, 2′, and 3′) of VO${}_2$ samples (a) $c$, (b) $m$, and (c) $r$ obtained from SE measurement. Vertical dashed lines in each panel indicate $T_{\mathrm{onset}}$ and $T_{\mathrm{MIT}}$.

Figure 4

Band diagram of VO${}_2$ for (a) $T<T_{\mathrm{onset}}$ in insulating phase, (b) intermediated phase at $T$ ∼ $T_{\mathrm{onset}}$, and (c) metallic phase at $T>T_{\mathrm{MIT}}$.

Figure 5

Raman spectra of sample (a) $c$, (b) $m$, and (c) $r$ low temperature and near $T_{\mathrm{onset}}$.

Figure 6

Phonon energies as a function of temperature for peaks at (a) 195 and 225 cm${}^{-1}$, (b) 263 and 309 cm${}^{-1}$, and (c) 615 cm${}^{-1}$.