Curled polarization nanodomains in (BaTiO₃/SrTiO₃) epitaxial superlattices on silicon

In nanoscale ferroelectrics, the polarization pattern depends on the chemical, mechanical and electrical boundary conditions and is the result of a delicate balance to minimize electrostatic energy costs associated with the depolarization effects. In recent years, new exotic polar textures featuring curled polarization patterns (vortices, skyrmions, merons, etc…) have been unraveled in ultrathin films, superlattices or nanostructures. The development of topological nanoelectronics on chips that would take advantage of such exotic textures requires their integration on silicon.

Here, we investigated the crystalline structure, defects, and polarization pattern in (BaTiO₃/SrTiO₃)_n superlattices epitaxially grown on silicon substrates. The superlattices were synthesized by molecular beam epitaxy on 4 nm SrTiO₃-buffered Si (001) substrate and were characterized by X-ray diffraction, scanning transmission electron microscopy (STEM), and atom probe tomography (APT). We will focus the discussion on the properties of a (5 nm BaTiO₃ / 2 nm SrTiO₃)₁₀ heterostructure. Out-of-plane X-ray diffraction and reciprocal space maps show that the BaTiO₃ and SrTiO₃ layers are coherently grown and evidence the superlattice periodicity. From STEM high angle annular dark field images, the atomic displacements of Ti atoms relative to the center of the unit cell (as defined by the Ba atoms) were determined. From the resulting dipole maps, nanodomains of curled polarization were observed in the BaTiO₃ layers. The maps obtained at different locations of the superlattices will be discussed and compared to those determined for an epitaxial trilayer 2 nm SrTiO₃ /5 nm BaTiO₃ /2 nm SrTiO₃ grown in the same processing conditions on Si(001). The changes in tetragonality throughout the thickness of the superlattices will be discussed. Finally, chemical information gained from the APT analyses performed on needle-shaped specimens will be presented. The evidence of curled polarization nanodomains in (BaTiO₃/SrTiO₃)_n gives promises for a future integration into devices of topological polar nanodomains.