Sebastian Calderon1,John Hayden2,Steven Baksa2,Samantha Jaszewski3,Jon Ihlefeld3,Ismaila Dabo2,Jon-Paul Maria2,Elizabeth Dickey1
Carnegie Mellon University1,The Pennsylvania State University2,University of Virginia3
Sebastian Calderon1,John Hayden2,Steven Baksa2,Samantha Jaszewski3,Jon Ihlefeld3,Ismaila Dabo2,Jon-Paul Maria2,Elizabeth Dickey1
Carnegie Mellon University1,The Pennsylvania State University2,University of Virginia3
The discovery of ferroelectricity in HfO<sub>2</sub> [1] and wurtzite-based ternary materials such as AlScN [2] and AlBN [3] has attracted tremendous attention due to their compatibility with CMOS processes and their potential to be integrated in non-volatile memory devices. The origin of the ferroelectricity and the fundamental switching mechanisms in these materials is still under intensive investigation. Understanding the mechanisms that control and induce switchable states in these materials is an important goal to find solutions to the high coercive fields and avoid breakdown failures. We employ scanning transmission electron microscopy to observe the polarization of the films at atomic scale, by differential phase contrast, which allows simultaneous observation of light and heavy atomic columns. An internal electric field capable of switching these materials is attained by exposing them to long periods to the electron beam, provoking a positive charge accumulation due to the emission of secondary and auger electrons that are not compensated due to the insulating properties of these materials. STEM-DPC experimental images are acquired in a probe-corrected ThermoFisher Titan-Themis at 200kV, using a four-segment annular detector (DF4), a probe convergence angle of 18 mrads and a camera length that results in acceptance angles between 11 and 43 mrads. The experimental data are complemented with STEM simulations using Dr. Probe V1.9 software package [4]. Simulations are carried out using the frozen-phonon configuration method at 200 keV and 1 µm of spherical aberration. Our results show conclusively the pathway for the polarization inversion at the atomic scale in both wurtzite [5] and fluorite ferroelectrics, offering unprecedented insights into the mechanisms for ferroelectric switching. DFT calculations also support our finding, demonstrating the pathways for the polarization inversion.<br/><br/><b>Acknowledgements</b><br/>This material is based upon work supported by the Center for 3D Ferroelectric Microelectronics (3DFeM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Energy Frontier Research Centers program under Award Number DE-SC0021118. The authors acknowledge use of the Materials Characterization Facility at Carnegie Mellon University supported by grant MCF-677785.<br/><br/>[1] T. Böscke, J. Müller, D. Bräuhaus, U. Schröder, U. Böttger, Ferroelectricity in hafnium oxide thin films. Appl. Phys. Lett. 99, 102903 (2011).<br/>[2] S. Fichtner, N. Wolff, F. Lofink, L. Kienle, B. Wagner, AlScN: A III-V semiconductor based ferroelectric, J. Appl. Phys. 125, 114103 (2019).<br/>[3] J. Hayden, M.D. Hossain, Y. Xiong, K. Ferri, W. Zhu, M.V. Imperatore, N. Giebink, S. Trolier-McKinstry, I. Dabo, J.-P. Maria, Ferroelectricity in boron-substituted aluminum nitride thin films, Phys Rev Mater. 5 (2021) 044412. https://doi.org/10.1103/PhysRevMaterials.5.044412.<br/>[4] J. Barthel, Dr. Probe: A software for high-resolution STEM image simulation, Ultramicroscopy. 193 (2018) 1–11. https://doi.org/https://doi.org/10.1016/j.ultramic.2018.06.003.<br/>[5] Sebastian Calderon V, John Hayden, Steven M. Baksa, William Tzou, Susan Trolier-McKinstry,Ismaila Dabo, Jon-Paul Maria, Elizabeth C. Dickey, Atomic-scale polarization switching in wurtzite ferroelectrics, Science 380, 1034–1038 (2023)