Interfacial Strain-Insensitive Thermal and Electrical Stability of (K,Na)NbO₃-Based Lead-Free Ferroelectric Films

Despite the inherent strain relaxation in perovskite-type oxide ferroelectric films that occurs beyond a certain thickness, the interfacial states, including carrier and strain, can still influence their electrical properties. This study explores the impact of different (La0.67,Sr0.33)MnO3 (LSMO)-coated perovskite-type oxide substrates on the electrical performance of (K,Na)NbO3 (KNN)-based ferroelectric films. Specifically, 200 nm-thick epitaxial KNN-based films were grown on four distinct substrates: (001)-oriented KTaO3, SrTiO3, LaAlO3, and YAlO3. Notably, the lattice constants of the KNN-based films remained unchanged, despite the varying lattice constants of their substrates. The electrical properties of these films were found to be insensitive to the underlying substrate. All films displayed comparable hysteresis loops and frequency-dependent dielectric curves, with a consistent remnant polarization of ∼63 μC/cm2 and a dielectric constant of ∼1250 at 1 kHz. Furthermore, the KNN-based films demonstrated exceptional thermal stability. They showed resilience under multiple switching cycles and maintained stable ferroelectricity up to a high temperature of 100 °C. These observations underscore the high electrical and thermal stability of KNN-based ferroelectric films, suggesting their promising potential for applications in electronic devices. This research highlights the robustness and versatility of KNN-based ferroelectric films, which could pave the way for their integration into advanced electronic technologies.