The ability to design the composition and microstructure of electronic ceramics for emerging technological applications requires sophisticated characterization techniques that can provide quantitative information about local structure and chemistry. Such structure quantification is particularly important to the fundamental understanding of properties in many important non-linear dielectrics, where chemical heterogeneities associated with dopants or intrinsic lattice defects give rise to local inhomogeneities in charge, strain and polarization. Such local deviations from the global average structure and symmetry are often linked to enhancements in macroscopic dielectric, ferroelectric and electromechanical properties. This seminar discusses the use of scanning transmission electron microscopy (STEM) to quantify short- and medium-range lattice disorder in electronic oxides, including antiferroelectrics, ferroelectrics and relaxor ferroelectrics. It will highlight recent work on novel ferroelectrics (e.g. compositions based on AlN, ZnO and HfO2) that exhibit robust polarization reversal and CMOS-compatible processing, providing new opportunities for integrated ferroelectrics. The ability to quantify local structure on a sublattice basis and in real space provides unique insight into the polarization of these materials.
Acknowledgements
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 under Award Number DE-SC0021118.

For neuromorphic computing that mimics the human brain, digital memory needs to adopt synaptic plasticity. One of the most promising and technologically advanced approaches for memristive devices is the resistive random-access memory (ReRAM). Recent experimental results of our group show resistive switching in single crystalline SrTiO₃ films with Sr deficiency up to 16% grown by liquid-delivery spin metal–organic vapor phase epitaxy on Nb-doped SrTiO₃ substrates. These samples show forming-less resistive switching with on/off ratios as high as 10³ without a forming step. Our results suggest that the resistive switching phenomenon in off-stoichiometric films can be attributed to trap-assisted tunneling through Ti antisite defects, which induce a switchable polarization. Crucial parameters such as on/off ratio and retention time depend on the extent of off-stoichiometry. To validate this model, we apply advanced scanning transmission electron microscopy (STEM) techniques. These techniques include quantitative annular dark field (ADF) STEM experiments, based on density functional theory (DFT) derived simulations of SrTiO₃ supercells, and 4D-STEM experiments to measure the field distribution and ferroelectric domains in the films. Atomic-scale chemical analyses using in parallel energy dispersive X-ray spectroscopy and electron energy-loss spectroscopy (ELNES) provide evidence for Ti on Sr-sites with a Ti³+ charge state. Our results show that approximately 50% of the Sr-vacancy sites are occupied by Ti and that these antisite defects are responsible for inducing ferroelectric polarization. Differential phase contrast measurements reveal the polarization of these domains. In situ 4D-STEM studies applying an external voltage evidence the switching of nanopolar domains in the film induced by the Ti antisite defects.

Novel domain structures (domain walls, vortices, and skyrmions) in ferroelectrics may have potential uses in next generation devices. Studying their properties and behaviour is crucial to developing future applications. Previously piezoresponse force microscope (PFM) and transmission electron microscope (TEM), have been applied to characterize the structures. Scanning transmission electron microscope (STEM) provides good ways to characterize domain structures by studying local atomic structures. Local polarization may be mapped by calculating the atomic column displacement through high resolution STEM (HR-STEM) images. However, high-quality samples and good TEM conditions are needed and only small range polarization in certain projections can be calculated. Four-dimensional STEM (4D-STEM) provides the ability to directly measure long-range and short-range internal electric fields over a large field of view. By using fast response pixelated electron detectors, a 2D convergent beam electron diffraction (CBED) pattern is captured for each pixel of a 2D raster scan across the sample, thus forming a 4D dataset, and so called 4D-STEM. Plenty of information may then be extracted from the same dataset through different computational methods. When the electron beam is scanned across the sample, electric fields in the sample plane will tilt the beam to one direction, resulting in a shift of the CBED patterns. These shifts may be measured by calculating centre of mass (CoM) of the CBED patterns in 4D-STEM. In this work we combine 4D-STEM with several techniques to study different types of domain structures in flux-grown PbTiO₃ single crystals. TEM specimens are prepared from bulk single crystals using focused ion beam (FIB). 4D-STEM were conducted by using a double aberration-corrected JEOL ARM and a Merlin pixelated direct electron detector from Quantum Detectors. Analysis of multidimensional datasets was done using open-source python toolsets. Long range strain and electric field mapping are calculated and compared with other characterization methods including HRSTEM. This work not only validates the application of 4D-STEM in characterizing ferroelectrics, but also provides a deeper understanding of domain structures in ferroelectric single crystals.