Ultrascale epitaxial BaTiO₃ nanoislands on silicon and their topological polar texture

Nanoscale ferroelectrics exhibit topological polar textures, which hold great promise for advanced nanoelectronics. However, challenges remain in reliably creating and controlling these textures on silicon and with compounds that do not contain volatile elements such as Pb or Bi. In this study, we investigate the electric field manipulation of center-type domains in epitaxial BaTiO₃ nanoislands embedded in an epitaxial BaTiO₃ thin film grown by molecular beam epitaxy on silicon. The nanoislands were characterized by atomic force microscopy, scanning electron microscopy, scanning transmission electron microscopy (STEM), and piezoresponse force microscopy (PFM). Their density can be tuned by changing the Sr flux during the silicon passivation process. The excess Sr atoms act as nucleation sites for the nanoislands, as observed by TEM. The nanoislands have a conical shape, with a remarkably small size of 30 to 60 nm at the top. We will discuss the polarization domain pattern based on Ti-atomic displacement mapping, piezoresponse force microscopy, polarization vector reconstruction and phase-field modelling. We show that the nanoislands possess a center down-convergent polarization pattern, with a swirling lateral component. It can be reversibly switched to a center up-divergent pattern by application of an electrical field. Chirality emerges from the whirling polarization configuration. Finally, insights gained from phase-field modeling on the dynamical process of polarization switching in these nanoislands will be presented. Our study highlights the potential for controllable topological polar states in ferroelectrics integrated on silicon.