Surface-growth-mode-induced strain effects on the metal-insulator transition in epitaxial vanadium dioxide thin films

A series of high-quality vanadium dioxide (VO2) epitaxial thin films on (0001)-oriented sapphire substrates with various thicknesses were fabricated using radio frequency (RF) magnetron sputtering techniques. Structural analysis revealed that an out-of-plane tensile strain (similar to+0.035%) in the thinner VO2 epitaxial films was induced by epitaxial lattice mismatch between the monoclinic VO2 films and Al2O3 substrates. However, an anomalous compressive strain (similar to+0.32%) was accumulated along the out-of-plane direction in the thicker VO2 films. This result contradicts with the conventional epitaxial lattice-mismatch mechanism for strain formed in epitaxial films. We attribute this anomalous strain to the surface growth mode (island growth) in the thicker VO2 films, especially those sputtered from the metal target at low pressure. Furthermore, the metal-insulator transition (MIT) temperature shifted to lower temperature with decreasing thickness, which is attributed to modulation of the orbital occupancy through the epitaxial strain and growth-mode-induced strain in the VO2 epitaxial films. Moreover, the very large resistance change (on the order of magnitude similar to 10(3)) in the VO2/Al2O3 epitaxial heterostructures is promising for electrical switch applications.