2D polarization map from HRSTEM images treated with AbStrain/Relative Displacement

Decembre 10, 2024

Two researchers from the MEM group have demonstrated the formation of trapezoidal funnel-shaped nano-islands in thin films of the ferroelectric oxide BaTiO₃ epitaxial on Silicon. They revealed their singular, downward-convergent polar state, which can be seen as a swirling vortex of polarization flux and can be reversibly electrically switched, a promising result for future topological nanoelectronics applications.

A fascinating aspect of nanoscale ferroelectric materials is the emergence of topological polar textures, which include various complex and stable polarization configurations. The manipulation of such topological textures through external stimuli like electric fields holds promise for advanced nanoelectronics applications but integration of polar texture on silicon, the ubiquitous electronic technology platform, remains challenging.

In the context of the ANR FEAT project, our colleagues from the Helmholtz-Zentrum für Materialien und Energie group in Berlin succeeded to grow epitaxial BaTiO3 films on Si using a SrTiO3 template modifying the classical passivation step of the Si surface by introducing an excess of Sr in order to disrupt the Sr (2×1) surface with Sr aggregates.

High angle annular dark field (HAADF) high resolution STEM images of these films revealed the presence of trapezoidal nanoislands in place of these Sr-enriched zones. By correcting these images for distortions using the “AbStrain” method, the rotation of unit cells within the nano-islands was highlighted (colored mapping). At the same time, the direction of displacement of Ti atoms relative to the position of the Ba(Sr) cell barycentre, linked to polarization, was obtained after isolating Sr/Ba and Ti atoms using the “Relative displacement” approach (arrows superimposed on the coloured mapping). The particular “down-convergent” polarization configuration was thus identified.

Complementary investigations by piezoresponse force microscopy (PFM) demonstrated that such center down-convergent polarization domains can be reversibly switched by an electric field to center up-divergent domains. PFM data reconstruction and phase field modelling gave also insight into the 3D patterns of the polarization confirming the presence of a whirling component of the polarization around the nanoisland axis, which confers chirality.

The ability to create and electrically manipulate chiral whirling polar textures in BaTiO3 nanostructures grown monolithically on silicon holds promise for applications in future topological nanoelectronics.

Cross-sectional (110)^Si STEM image with two nanoislands in a BaTiO₃ film and colored map of the unit cells rotation where superimposed arrows indicate the direction/intensity of the polarization

Contacts:
Sylvie Schamm-Chardon | sylvie.schammchardon[at]cemes.fr
Nikolay Cherkashin | nikolay.cherkashin[at]cemes.fr

Publications:
Switchable topological polar states in epitaxial BaTiO3 nanoislands on silicon
I. Olaniyan, I. Tikhonov, V. Hevelke, S. Wiesner, L. Zhang, A. Razumnaya, N. Cherkashin, S. Schamm-Chardon, I. Lukyanchuk, D.-J. Kim, and C. Dubourdieu
Nature Communications 15 (2024) 10047
DOI : https://doi.org/10.1038/s41467-024-54285-z

Quantitative mapping of strain and displacement fields over HR-TEM and HR-STEM images of crystals with reference to a virtual lattice
N. Cherkashin, A. Louiset, A. Chmielewski, D.J. Kim, C. Dubourdieu, and S. Schamm-Chardon
Ultramicroscopy 253 (2023) 113778
DOI : https://doi.org/10.1016/j.ultramic.2023.113778