Ferroelectrics at the nanoscale

• Applicants: Professor Dr. Matias Bargheer; Professorin Dr.-Ing. Catherine Dubourdieu
• Subject Area: Experimental Condensed Matter Physics ; Synthesis and Properties of Functional Materials
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 431399790

A key functionality of ferroelectrics is the ability to switch their polarization between two stable orientations by application and reversal of an electrical field. As the size shrinks, polarization might become unstable, non-switchable or extremely low due to the absence or to the asymmetry in screening surface charges and asymmetry in their redistribution upon switching. Recently, it was shown that the ferroelectric polarization can be switched in epitaxial BaTiO3 films as thin as 1.6 nm, epitaxially grown on silicon. In this project, we address the ferroelectricity of nanostructures on a semiconductor, with not only ultrathin thickness but also submicrometer lateral dimensions. We will take up several technological challenges related to the fabrication of complex oxide nanostructures on planar semiconductor Si and on reversely tapered Si nanocones. Atomic layer deposition, reactive ion etching and helium ion beam milling will be used to prepare nanostructures of various shapes and size (2-100 nm thick, 50-800 nm lateral). Since the surface contribution to various physical quantities becomes as large as or even larger than the bulk one in nanostructures, the chemical screening will compete with domain formation in lowering the depolarization energy. Piezoresponse force microscopy/spectroscopy will be used to image domains and explore the intricate contributions from electrochemical and ferroelectric states which may result even in a ferro-ionic mixed state. Raman spectroscopy and geometrical phase analysis of aberration-corrected TEM images will be used in comparison with X-ray diffraction to study the strain in the nanostructures. To unravel the effects of different boundary conditions and nanostructure shapes on the distribution of the polarization (flux closure, vortex...), we will develop advanced TEM methodologies in order to map the polarization direction and amplitude at the nanoscale. We will address the question of the ultimate switching time of the polarization of ferroelectric nanostructures by conducting femtosecond pump-probe spectroscopies and ultrafast X-ray diffraction experiments. In addition, domain formation and switching will be studied in situ under an applied electric field in a dedicated TEM, which is at the frontier of the current technical development and has never been demonstrated for ferroelectrics on semiconductors. To conclude, we will address fundamental questions at the frontier of the current knowledge in the ferroelectricity at the nanoscale on silicon, with expected breakthrough discoveries and perspectives of integrating ferroelectrics on semiconductors fornanoelectronics applications.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partner: Professorin Dr.-Ing. Sylvie Schamm-Chardon, Ph.D.