UC Irvine

Due to their intrinsic polarization, ferroelectric materials have a variety of applications in electronic, optical, and electromechanical devices. In addition, modern thin film growth techniques such as molecular beam epitaxy and pulsed laser deposition have made it possible to grow heterostructures with atomically sharp interfaces leading to the discovery of many new phenomena and functional properties stemming from the coupling between the ferroelectric and the substrate material. In the study of these thin films, transmission electron microscopy has been a powerful technique for high resolution structural and chemical characterization of these heterostructures. With the development of fast pixelated detectors, four-dimensional scanning transmission electron microscopy (4D STEM) has led to the emergence of new techniques for directly interrogating the electronic properties of ferroelectrics with atomic resolution. This opens new opportunities for studying both the mechanisms for how macroscopic properties can emerge from atomic scale electronic phenomena and the fundamental physics of ferroelectricity. In this work, new analysis techniques based on 4D STEM electric field and charge density imaging are developed and applied to reveal the electronic properties of ferroelectric heterostructures.

First, we systematically study the effect of sample thickness and electron probe defocus on the electric field measured in 4D STEM and demonstrate that the technique can be extended to samples up to 20 nm thickness. Next, new quantitative analysis techniques for 4D STEM charge density images are developed. These techniques are applied to a BiFeO3-SrTiO3 (ferroelectric-insulator) heterostructure, which reveals the local electronic properties of the interface. Through a combination of methods, we showed that there is charge accumulation at the interface due to an asynchronous change in the structural polarization and the charge distribution across the interface. Applying 4D STEM electric field imaging to PbTiO3-SrTiO3 superlattice, we found that asymmetric features in atomic scale images originate from the charge distribution in perovskite ferroelectrics and while nanometer-resolution images reveal the total electric field of the material system. Finally, an in-depth study of how Bader charge analysis, adapted for use with 4D STEM charge density images, can be leveraged to understand the bonding properties of PbTiO3. These results demonstrate the potential of 4D STEM for investigating the electronic properties of materials and provide new insights into the behavior of ferroelectric heterostructures.