Controlling metal-insulator transition in the hetero-epitaxial VO ₂/TiO₂ bilayer grown on Al₂O₃

Hetero-epitaxial VO₂/TiO₂ bilayers were synthesized on Al₂O₃ substrates by using pulsed laser deposition, and their physical properties with the changes of oxygen pressure and the substrate orientation were investigated. A metalinsulator transition of the VO₂ was observed only in a narrow oxygen pressure range of 520 mTorr. As the oxygen pressure increased, X-ray diffraction peak for the (2 0 0)VO₂ shifted to a lower 2θ position, while the metalinsulator transition temperature (T_MI) decreased by ∼7 K. On the other hand, T _MI was largely varied with substrate orientation. The (2 0 0)VO ₂/TiO₂ on the c-plane sapphire showed the highest T _MI of about 350 K, while the (0 0 2)VO₂/TiO₂ on the m-plane sapphire displayed the lowest T_MI of about 310 K. The (1 0 1)VO₂/TiO₂ on the r-plane and the a-plane exhibited T_MI∼340 and 330 K, respectively. The observed variations of T_MI with the oxygen pressure and substrate orientation were presumably due to the change in oxygen content of the VO₂ layer and/or in lattice strain.