Study on the atomic and electronic structures of oxide heterointerfaces

This experimental project aims to study, understand and control the atomic and electronic structures at the oxide heterointerfaces and their effects on the properties. Novel phenomena and functionalities at artificial oxide heterointerfaces have been attracting extensive scientific attention in both materials science and fundamental condensed matter physics. The interplay between degrees of freedom at interfaces of complex oxides could lead to exotic and unexpected states of matter. Considering the critical interfacial science in nanoscale governs the applications of devices, the objective of this proposal is to study the interfacial characteristics of oxide heteroepitaxial structures. Although recent progress has been achieved, however, the oxide heterostructures are always loaded with defects. As defects are increasingly recognized as being the cause of many properties in oxide heterostructures, in this proposal a specific focus will be on characterizing the chemical non-stoichiometric and defect occurred at the heterostructure interface, i.e. intermixing and oxygen vacancy at the interface, and their effects on the atomic and electronic structure of interfaces and the electrical and magnetic properties. Up to now, chemical intermixing and oxygen vacancy have not been fully understood, particularly in the ferroelectricferromagnetic perovskite oxide heterostructure interfaces, and the atomic mechanism of these effects on the properties remains unclear. The understanding of these effects is an essential explanation on the properties of oxide heterostructures display, and it is also important for clarifying the fundamental physical mechanisms pertain to complex systems. Understanding how to control these is key to taking oxide heterostructure from scientific curiosity to real technologies. To explore these effects in detail, one particular oxide heterointerfaces will be concentrated: ferroelectric ferromagnetic perovskite oxide interface, i.e. LSMO/PZT. The approach is to intentionally introduce the chemical intermixing and oxygen vacancy into the interface, carry out the magnetic and electrical properties, and further to characterize the atomic and electronic structures of oxide heterointerface. To achieve the above goals, the experimental methods to be used include modern advanced TEM techniques, i.e. CS-corrected TEM and quantitative atom measurements, and the advanced cross- sectional scanning tunneling microscopy (XSTM). The project will benefit from such unique combination of the techniques, and will result in a complete picture of the structure and property relationship in oxide heterostructure systems, including: (i) insight into the interfacial phenomena in oxide heterostructure; (ii) insight in the chemical intermixing and oxygen vacancy concentration effects, and (iii) insight in the functionalities of oxide heteroepitaxial structures and fundamental physical mechanisms in oxide systems. The conclusion drawn here is of general significance for other oxide heterostructure interfaces, and further provides technological guidelines for designing advanced oxide heterostructure devices applied in novel storage media and spintronics

Artificial heterostructure interfaces of perovskite oxides attract scientific attention in fundamental physics and possible applications. Advanced analytical transmission electron microscopy is one of the few experimental techniques that allow understanding fundamental processes and effects on an atomic scale. The structure discontinuity at heterostructure interfaces usually creates novel physical phenomena. In this project, we start with two heterostructures. We found that hybrid oxide superconductor/ ferromagnet/ superconductor (SFS) structures exhibit supercurrent phenomenon when the intermediate ferromagnet layer reduces to several unit-cells (i.e., 1- 2 nm) owing to a strong coupling effect at interfaces. Furthermore, we found that the interface mismatch strains on oxide heterostructures could significantly affect the oxygen vacancy formation and oxygen transportation. Both phenomena are of significance in applications. This project also explored the heterostructure interface-induced phenomenon in the Ca-doped Bismuth Ferrite films. (i) Segregation processes of dopants towards non-surface interfaces in perovskite-oxide heterostructures are scarcely studied. We first reported a Ca dopant's significant segregation in Bismuth Ferrite towards a compressively strained interface. DFT calculations validated the experimental results. The supposed mechanisms are strain relief and chemical interactions of the Ca-rich layer with the heterostructure. Understanding such segregation effects is critical since they can lead to the failure of a device. (ii) In higher Ca-doped Bismuth Ferrite thin films, the relation of stripes with dark contrast in HAADF images and regular distances to each other is established. We first show that dark stripes with irregular distances at lower dopant ratios are caused by agglomerated oxygen vacancies but that they additionally also are negatively charged domain walls. This interlinkage could not be observed for regular arrangements and is highly interesting since charged defects are an influential factor for the conductivity in the field of domain wall nanoelectronics. (iii) The secondary phase (bismuth oxide) can be used to trigger the super-tetragonal Bismuth Ferrite phase without the limited choice of a few substrates, which cause highly compressive strain. The phase space in this material combination is relatively unexplored. In this project, a significantly different Ca Solubility in the primary and secondary phases is found. This is crucial knowledge for precisely controlling the dopant level in the presence of both phases and needs to be considered for device application designs. Additionally, it represents a valuable experimental data point for assessing the ternary system in the thermodynamic database. Some results obtained in the present project were published in 5 peer-reviewed high-ranking journals (ACS Nano, ACS Applied Materials & Interfaces, Communications Materials, ACS Applied Electronic Materials, etc.). The results were presented at numerous national and international conferences.