Time- and strain-dependent nanoscale structural degradation in phase change epitaxial strontium ferrite films

Topotactic phase transition between metallic, perovskite SrFeO₃ and insulating, Brownmillerite SrFeO_2.5 has been extensively studied due to the potential applications in resistive switching devices for neuromorphic computing. However, its practical utilization as memristors has been hindered by the structural instability of SrFeO₃, which is often ascribed to the generation of oxygen vacancies to form SrFeO_3-δ. Here we reveal that the dominating defects generated in SrFeO₃ epitaxial thin films are atomic scale gaps generated as a result of interfacial strain. Our correlated time- and strain-dependent measurements show that tensile strained SrFeO₃ films form vertical, nanoscale gaps that are SrO-rich, which are accountable for the observed metal-to-insulator transition over time. On the other hand, compressively strained or small lattice mismatched SrFeO₃ films mainly yield horizontal gaps with a smaller impact on the in-plane transport. The atomic scale origin of such defects and their impact on device performance need to be further understood in order to integrate phase change materials in oxide electronics.