Correlation between Symmetry and Phase Transition Temperature of VO₂ Films Deposited on Al₂O₃ Substrates with Various Orientations

Abstract

The structural aspects of the insulator–metal transition (IMT) characteristics of VO2 are sensitive to the octahedral symmetry variation of VO6. By varying substrate orientation (c‐, a‐, and m‐plane Al2O3), the correlation between IMT temperature and local symmetry is investigated. For a VO2 film deposited on m‐plane Al2O3, which has high symmetry due to fewer domain boundaries induced by m‐plane Al2O3, the IMT temperature is low (326.47 K). In contrast, for a film deposited on c‐plane Al2O3 (having lower symmetry), the IMT temperature is the highest (336.74 K) among the films used in this work. Furthermore, temperature‐dependent Raman spectra reveals that the structural phase transition temperature decreases in the order of the VO2 film deposited on c‐, a‐, and m‐plane Al2O3, suggesting that the symmetrical structure reduces the activation energy for IMT by decreasing thermodynamic energy. These results demonstrate that structural symmetry plays a crucial role in lowering the transition temperature.

Role of Surface Termination in the Metal–Insulator Transition of V2O3(0001) Ultrathin Films

“…47 We note that for the rec-O 3 and (√3 × √3) R30°(the underlying surface below the removed VO groups) structures, the surface has a stacking sequence of −V 2 O 3 − V−O 3 V 3 O 3 , which may be considered an O 3 V 3 O 3 trilayer structure with a formal VO 2 stoichiometry on top of the single V-terminated bulk structure (see Figure 1e). As VO 2 is known to show MIT below 320 K, 48 both the 300 and 108 K EDCs corresponding to these structures or at least the 108 K EDCs (as the underlying structure is still V 2 O 3 , which only shows MIT at low temperature) are expected to show insulating behavior, while our data show metallic behavior at both temperatures (Figure 4f,h). This behavior suggests that the chemical composition alone cannot determine the MIT properties but a combination of chemical composition and the crystal structure does.…”

Nanoscale mapping of temperature-dependent conduction in an epitaxial VO2 film grown on an Al2O3 substrate

“…2(c) , are good indicators for the SPT of VO 2 . 37 The two peaks in the low-frequency region ( i.e. , ω V1 and ω V2 ) are related to the vibration of V–V dimers.…”

“…We conclude that the M1 and M2 phases coexist in the wide temperature ranges in our VO 2 thin film owing to the existence of the V–O peaks around 610 cm −1 at least up to 353 K. This coexistence in the wide temperature range is ascribed to the epitaxial strain induced by the c -Al 2 O 3 substrate. 37 …”

Role of surface termination in the metal-insulator transition of V$_2$O$_3$(0001) ultrathin films

“…We note that for rec-O 3 and ( √ 3 × √ 3)R30 o (underlying surface below the removed V=O groups) structures, the surface has a stacking sequence -V 2 O 3 -V-O 3 V 3 O 3 which may be thought of as an O 3 V 3 O 3 trilayer structure with a formal VO 2 stoichiometry on top of the single V terminated bulk structure (see figure 1(e)). As VO 2 is known to show MIT below 320 K [48], both the 300 K and 108 K EDCs corresponding to these structures would show insulating behavior or at least the 108 K EDCs, as the underlying structure is still V 2 O 3 which only shows MIT at low temperature. While our data show metallic behavior in both temperatures (figure 4(f) and (h)).…”