Role of epitaxial microstructure, stress and twin boundaries in the metal–insulator transition mechanism in VO2/Al2O3 heterostructures

The microstructures of epitaxial polycrystalline VO2 thin films grown on (0 0 1) sapphire were investigated by means of X-ray diffraction, Cs-corrected scanning transmission electron microscopy (STEM), in both plane and transverse geometry, in relation to its metal–insulator transition (MIT) properties. It is shown that the epitaxial relationship between the thin film and the substrate can be defined as out-of-plane twofold twinning symmetry {0 2 0}M//(0 0 6)S (where subscripts “M” and “S” denote the monoclinic phase of VO2 and the sapphire α-Al2O3 substrate, respectively) with in-plane threefold twinning structure (2 0 0)M//{1 1 0}S. The origin of these twinning structures is discussed: the in-plane threefold twinning structure comes from the threefold symmetry of the Al2O3 (0 0 1) plane, and the twofold twinning symmetry is induced by the MIT phase transition. The STEM planar view observations of the thin film demonstrate the presence of elongated grains down to nanoscale, with a high density of twin boundaries (TB). These TB are highly orientated into two sets of families. STEM low-angle annular dark-field imaging and STEM dark-field atomic displacement measurements evidence very different strain behaviors for these two TB families. Most of the TB and some of the smaller grains with typical dimensions of only a couple of nanometers exhibit locally an enhanced tetragonality. They are proposed to act as nucleation centers during the MIT process and then to influence the dynamics of the transition.