Unstrained and defect-free VO2 single crystals undergo structural (from high-temperature tetragonal to low-temperature monoclinic phase) and electronic phase transitions simultaneously. In thin films, however, in the presence of unrelaxed strains and defects, structural (Peierls) and electronic (Mott) transitions are affected differently, and are separated. In this paper, we have studied the temperature dependence of structural and electrical transitions in epitaxially grown vanadium dioxide films on (0001) sapphire substrates. These results are discussed using a combined kinetics and thermodynamics approach, where the velocity of phase transformation is controlled largely by kinetics, and the formation of intermediate phases is governed by thermodynamic considerations. We have grown (020) VO2 on (0001) sapphire with two (001) and (100) in-plane orientations rotated by 122°. The (100)-oriented crystallites are fully relaxed by the paradigm of domain-matching epitaxy, whereas (001) crystallites are not relaxed and exhibit the formation of a few atomic layers of thin interfacial V2O3. We have studied the structural (Peierls) transition by temperature-dependent in situ X-ray diffraction measurements, and electronic (Mott) transition by electrical resistance measurements. A delay of 3 °C is found between the onset of structural (76 °C) and electrical (73 °C) transitions in the heating cycle. This temporal lag in the transition is attributed to the residual strain existing in the VO2 crystallites. With this study, we suggest that the control of structural and electrical transitions is possible by varying the transition activation barrier for atomic jumps through the strain engineering.
Keywords: VO2 thin films; activation barrier; domain-matching epitaxy; epitaxial strain; structural and electrical transitions; thermodynamic and kinetic models.