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

A series of high-quality vanadium dioxide (VO 2 ) 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 (∼+0.035%) in the thinner VO 2 epitaxial films was induced by epitaxial lattice mismatch between the monoclinic VO 2 films and Al 2 O 3 substrates. However, an anomalous compressive strain (∼−0.32%) was accumulated along the out-of-plane direction in the thicker VO 2 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 VO 2 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 VO 2 epitaxial films. Moreover, the very large resistance change (on the order of magnitude ∼10 3 ) in the VO 2 /Al 2 O 3 epitaxial heterostructures is promising for electrical switch applications. The surface growth mode can induce the anomalous compressive strain in thicker VO 2 /Al 2 O 3 epitaxial films, which can't be explained by conventional epitaxial lattice-mismatch. Strain may be an effective tool for manipulating MIT of the VO 2 films.