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

Abstract

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 (∼+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 (∼−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 ∼10³) in the VO₂/Al₂O₃ epitaxial heterostructures is promising for electrical switch applications.