Polarization boost and ferroelectricity down to one unit cell in layered Carpy-Galy La$_{2}$Ti$_{2}$O$_{7}$ thin films

TL;DR

This study demonstrates that La$_2$Ti$_2$O$_7$ thin films exhibit enhanced ferroelectricity, including polarization down to one unit cell, by controlling strain and growth conditions, opening new avenues for ferroelectric device applications.

Contribution

We achieved controlled growth and confirmed robust ferroelectricity in ultrathin La$_2$Ti$_2$O$_7$ films, revealing strain-dependent effects and polarization enhancement in Carpy-Galy layered compounds.

Findings

Polarization of 18 μC/cm² in La$_2$Ti$_2$O$_7$ films

Ferroelectricity persists down to one unit cell thickness

Strain influences growth mode and ferroelectric properties

Abstract

Layered perovskite-based compounds offer a range of unconventional properties enabled by their naturally anisotropic structure. While most renowned for the superconductivity observed in the Ruddlesden-Popper phases, many of these layered compounds are also ferroelectric and exhibit a sizeable in-plane polarization. Among these, the Carpy-Galy phases (A${_n}$B${_n}$O$_{3n+2}$), characterized by 110-oriented perovskite planes interleaved with additional oxygen layers, have been debated as platforms for hosting not only a robust polarization but also multiferroicity and polar metallicity. 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. Addressing this issue, our study focuses on La$_2$Ti$_2$O$_7$, an $n$=4 (A$_2$B$_2$O$_7$) representative of the Carpy-Galy family, exploring its growth and concurrent phase stability on various substrates under different strain conditions. Remarkably, we demonstrate that a 3% tensile strain from DyScO$_3$ (100) substrates promotes a controlled layer-by-layer growth mode, while SrTiO$_3$ (110) and LaAlO$_3$-Sr$_2$TaAlO$_6$ (110), that exert negligible and compressive strains respectively, require post-deposition annealing to achieve similar results. Using scanning probe microscopy, X-ray diffraction, scanning transmission electron microscopy, and polarization switching experiments, we confirm that these films possess exceptional ferroelectric properties, including a polarization of 18 $\mu$C/cm$^2$ - 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 our understanding of Carpy-Galy phases in thin films but also lays a foundation for their application in advanced ferroelectric device architectures.