Nanoscale Design of Polarization Using Lattice Chemistry Engineering in Layered Ferroelectrics

Nanoscale electrostatic control of oxide interfaces enables physical phenomena and exotic functionalities beyond the realm of the bulk material, including superconductivity, multiferroicity, and topological properties promising for future nonvolatile memory applications. Here, by exploiting spontaneously forming charged interfaces in layered materials, we control the electrostatic boundary conditions in ferroelectric oxide heterostructures. We directly access the polarization dynamics of the layered ferroelectric model system Aurivillius Bi₅FeTi₃O₁₅ (BFTO) films during growth using in-situ optical second harmonic generation (ISHG). We identify the characteristic Aurivillius antipolar ordering of the dipoles along the growth direction, which leads to an oscillating intensity of the ISHG signal during the layer-by-layer deposition. In combination with reflection high-energy electron diffraction monitoring, we show how the polarization orientation of the films consistently changes from out-of-plane during the growth of perovskite blocks, to fully in-plane upon the completion of the unit cell with the fluorite-like (Bi₂O₂)²⁺ planes. Finally, we incorporate various functional perovskite units into the Aurivillius layered-crystal structure using the direct access to structure-dependent polarization dynamics during growth. Our work thus expands the limits of engineering the properties of layered oxide films to the sub-unit-cell-scale for the development of energy-efficient oxide electronics.