The surface growth mode can induce the anomalous compressive strain in thicker VO 2/Al 2O 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.
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 2O 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 2O 3 epitaxial heterostructures is promising for electrical switch applications.