Over-Strain-Relaxation State and Dislocation-Governed Polar Topologies in Ferroelectric Superlattices

Polar topologies in ferroelectric films can be stably constructed by tensile strains through coherent growth. How potential polar textures in ferroelectric films grown under compressive strain substrate will evolve is still elusive. Here, we have grown high-quality PbTiO₃/SrTiO₃ superlattices on NdGaO₃ substrates, where the nominal mismatch of this system is about -1.0% and thus the superlattices may be subjected to compressive strain. Dislocation arrays between the substrate and the films were identified. Amazingly, these misfit dislocations were found to release the mismatch far exceeding the -1.0% nominal mismatch, approaching -1.7% and forming actually a final tensile strain state of about 0.7%. This strain state was driven by the competition between thermal mismatch dynamics and ferroelectric phase transition of PbTiO₃, which triggers the formation of polar vortex arrays. In particular, the polar topology here has the tendency of transition from vortex to periodic dipole waves under the influence of threading dislocations. Our research provides possibilities of integrating and regulating ferroelectric topologies on nominal compressive strain substrates. The novel over-strain-relaxation behavior was unrealized previously, and will stimulate more strain manipulations for oxide epitaxial films. More efforts could be focused on some practical substrates, such as direct growth of polar topologies on silicon.