Controlling metal–insulator transition in the hetero-epitaxial VO2/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 metal–insulator transition of the VO2 was observed only in a narrow oxygen pressure range of 5–20 mTorr. As the oxygen pressure increased, X-ray diffraction peak for the (2 0 0)VO2 shifted to a lower 2θ position, while the metal–insulator transition temperature (TMI) decreased by ∼7 K. On the other hand, TMI was largely varied with substrate orientation. The (2 0 0)VO2/TiO2 on the c-plane sapphire showed the highest TMI of about 350 K, while the (0 0 2)VO2/TiO2 on the m-plane sapphire displayed the lowest TMI of about 310 K. The (1 0 1)VO2/TiO2 on the r-plane and the a-plane exhibited TMI∼340 and 330 K, respectively. The observed variations of TMI 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.