Controllable phase-transition temperature upon strain release in VO₂/MgF₂ epitaxial films

Metal-to-insulator transition (MIT) behaviors accompanied by a rapidly reversible phase transition in vanadium dioxide (VO₂) have gained much attention from researchers. In this research, lattice-mismatched epitaxial films of VO₂ (210) on MgF₂ (111) substrates exhibited different phase-transition temperatures and developed a phase regulation of misfit dislocations. It reveals an “effective phase-transition regulation” for VO₂ films with different thicknesses in certain substrates—MgF₂. It is speculated that the dislocation density could progressively increase with the increasing thickness of the film, and the dislocation spacing distribution tends to become narrower. When the thickness of the film is close to a certain thickness, the misfit dislocation density is close to saturation for full relaxation. The misfit dislocation arrangement produces hysteresis loops of different widths upon resistance and transmittance: when the width of the transmittance hysteresis loop is narrow at 3.9 °C, and the resistance hysteresis loop can reach the largest width at 14.0 °C. Further research about the hysteresis with other substrates and the thickness-dependent transition temperature showed that the variance of hysteresis properties during the phase transition can be ascribed to the different strain states along the V–V chains and the change of the hybrid t_2g-orbital occupancy. In addition, critical thickness along the growth direction is also discussed, which could be identified by the experimental results. This research provides a comprehensive understanding of the strain effect on phase-transition behaviors and also could be a guidance for some potential applications in optoelectronic devices.