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

Layered perovskite-based compounds offer a range of unconventional properties enabled by their naturally anisotropic structure. Among these, the Carpy-Galy phases (A_nB_nO_3n+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.