Polarization Boost and Ferroelectricity Down to One Unit Cell in Layered Carpy‐Galy La₂Ti₂O₇ Thin Films

Abstract

AbstractLayered perovskite‐based compounds offer a range of unconventional properties enabled by their naturally anisotropic structure. Among these, the Carpy‐Galy phases (AnBnO3n+2), characterized by (110)‐oriented perovskite planes interleaved with additional oxygen layers, stand out for robust in‐plane polarization. However, the challenges associated with the synthesis of ultrathin Carpy‐Galy films and understanding the impact of strain on their properties limit their integration into devices. Here, La2Ti2O7 (n = 4) films grown on substrates imposing tensile, compressive, or negligible epitaxial strains are investigated. Surprisingly, a 3% tensile strain from DyScO3 (100) substrates facilitates layer‐by‐layer growth mode, whereas compressive (LaAlO3‐Sr2TaAlO6 (110)) or negligible (SrTiO3 (110)) epitaxial strains require post‐deposition annealing to reach comparable crystallinity. Using density‐functional theory calculations, scanning probe microscopy, X‐ray diffraction, scanning transmission electron microscopy, and polarization switching experiments, it is confirmed that these films possess exceptional ferroelectric properties, including a polarization of 18 µCcm−2 – more than three times higher than previously reported – as well as persistence of ferroelectricity down to a single‐unit‐cell thickness. This study not only advances the understanding of Carpy‐Galy phases as epitaxial thin films but also lays a foundation for their integration into advanced ferroelectric device architectures.