09:15 〜 09:30
▼ [12a-S203-2] Lattice Strain and Insulator-to Metal Transition of VO2 / TiO2 Epitaxial Bilayer Films Grown on M-plane Sapphire Substrates
キーワード：film, phase transition
Introduction
VO2 shows first-order insulator-to-metal (IMT) transition at 68 °C with thermal hysteresis. Since the optoelectronic properties can be largely modulated around the critical temperature (Tc), VO2 has attracted much attention as the active materials of sensors. Although the Tc can be reduced to room temperature if rutile TiO2 is used as the substrate, the use of TiO2 substrate is not realistic because of the size limitation of Verneuil-grown TiO2 crystal. In this study, we focused on the use of M-plane sapphire as the substrate and use of TiO2 epitaxial film as the buffer layer. We studied the relationship between the lattice strain and the insulator-to-metal transition of VO2 / TiO2 epitaxial bilayer films.
Experimental
The bilayer films of VO2 / TiO2 were grown on (10-10) α-Al2O3 substrate by PLD method.
Results and discussion
Out-of-plane XRD patterns of the resultant VO2 / TiO2 bilayer films revealed that the out-of-plane lattice parameter is insensitive to the thicknesses. However, the in-plane lattice parameters are highly sensitive to the thickness; when the thickness is thin, the distortion b/a is large due to the asymmetric oxygen lattice of (10-10) α-Al2O3 substrate. The Tc can be reduced to ~305 K when the TiO2 lattice is relaxed and the VO2 film is coherently grown on it. The thermopower data (data not shown here) reveals that the lattice distortion would induce carrier generation and therefore the IMT behavior is suppressed when there is large in-plane lattice distortion. These results suggest that M-plane sapphire substrate would be used as the VO2 that shows IMT near room temperature.
VO2 shows first-order insulator-to-metal (IMT) transition at 68 °C with thermal hysteresis. Since the optoelectronic properties can be largely modulated around the critical temperature (Tc), VO2 has attracted much attention as the active materials of sensors. Although the Tc can be reduced to room temperature if rutile TiO2 is used as the substrate, the use of TiO2 substrate is not realistic because of the size limitation of Verneuil-grown TiO2 crystal. In this study, we focused on the use of M-plane sapphire as the substrate and use of TiO2 epitaxial film as the buffer layer. We studied the relationship between the lattice strain and the insulator-to-metal transition of VO2 / TiO2 epitaxial bilayer films.
Experimental
The bilayer films of VO2 / TiO2 were grown on (10-10) α-Al2O3 substrate by PLD method.
Results and discussion
Out-of-plane XRD patterns of the resultant VO2 / TiO2 bilayer films revealed that the out-of-plane lattice parameter is insensitive to the thicknesses. However, the in-plane lattice parameters are highly sensitive to the thickness; when the thickness is thin, the distortion b/a is large due to the asymmetric oxygen lattice of (10-10) α-Al2O3 substrate. The Tc can be reduced to ~305 K when the TiO2 lattice is relaxed and the VO2 film is coherently grown on it. The thermopower data (data not shown here) reveals that the lattice distortion would induce carrier generation and therefore the IMT behavior is suppressed when there is large in-plane lattice distortion. These results suggest that M-plane sapphire substrate would be used as the VO2 that shows IMT near room temperature.