Persistence of Ferroelectricity Close to Unit-Cell Thickness in Structurally Disordered Aurivillius Phases

Multiferroics intertwine ferroelectric and ferromagnetic properties, allowing for novel ways of manipulating data and storing information. To optimize the unique Bi6TixFeyMnzO18 (B6TFMO), multiferroic, ultrathin (<7 nm) epitaxial films were synthesized by direct liquid injection chemical vapor deposition (DLI-CVD). Epitaxial growth is, however, confounded by the volatility of bismuth, particularly when utilizing a postgrowth anneal at 850 °C. This results in microstructural defects, intergrowths of differing Aurivillius phases, and formation of impurities. Improved single-step DLI-CVD processes were subsequently developed at 710 and 700 °C, enabling lowering of crystallization temperature by 150 °C and significantly enhancing film quality and sample purity. Ferroelectricity is confirmed in 5 nm (1 unit-cell thick) B6TFMO films, with tensile epitaxial strain enhancing the piezoresponse. In-plane ferroelectric switching is demonstrated at 1.5 unit-cell thickness. The persistence of stable ferroelectricity near unit-cell thickness in B6TFMO, both in-plane and out-of-plane, is significant and initiates possibilities for miniaturizing novel multiferroic-based devices.