Patterns of polar vortices discovered in lead-free superlattices on silicon
A team at HZB has, for the first time, reported polar vortices in (BaTiO₃/SrTiO₃)₁₀ superlattices on silicon. These results expand the range of materials and substrates on which polar topological textures can occur and open up new options for incorporating complex ferroelectric states into silicon-based technologies.
Researchers at Helmholtz-Zentrum Berlin (HZB) have discovered polar vortices in lead-free superlattices grown on silicon. This marks the first time such textures have been observed in materials without lead, on silicon substrates. Polar textures, like vortices, are typically seen in lead-containing superlattices on oxide substrates, but this new finding expands the range of materials and substrates where polar topological textures can occur.
The study, published in Nature Communications, involved creating alternating layers of barium and strontium titanates on silicon through a process called molecular beam epitaxy. Using advanced microscopy and X-ray source analysis, the team observed regular stripes of polar vortices in the layer system. The silicon substrate provided the necessary mechanical strain to enable the formation of these polar vortices.
When the samples were heated, the polar texture vanished due to a structural phase transition. Upon cooling to room temperature, the polar texture reformed. The researchers used phase-field modeling to explain the alignment of vortex tubes in the BaTiO₃ layers, which was influenced by electrostatic forces and mechanical strain. This discovery could lead to the development of novel silicon-based technologies with complex ferroelectric states, replacing toxic lead with silicon.
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