Atomic-scale characterization of interfaces and defects in electronic heterostructures

The significant advances in epitaxial thin film growth techniques have fostered wide applications of heterostructured systems in microelectronics. However, the defects formed at the interfaces of heterostructures or in the films can severely affect the properties and performance of the designed microelectronic devices. Microstructural characterizations of defects and interfaces have always been a subject of great scientific interest for understanding and improving the physical properties of devices. Among various microstructural characterization methods, scanning transmission electron microscopy (STEM) is well known to possess the ability to form a high angle annular dark field (HAADF) Z-contrast image, in which intensity is proportional to the square of the mean atomic number of the probed volume and is directly interpretable. Moreover, the instrumental and technique development in the STEM field has fostered the conjunction of electron energy loss spectroscopy (EELS) and Z-contrast imaging and their widespread use for nearly atomic level chemical analysis at interfaces and isolated defects. In this thesis, I discuss the practical aspects of the experimental STEM techniques and demonstrate the application of these techniques to the study of interfaces and defects in multiple heterostructured systems: ferroelectric PbTiO3-Pt electrode, superconductor YBCO-SrTiO3, diluted magnetic semiconductor Co:TiO2-LaAlO3, and II-VI semiconductor HgTe-CdTe superlattices.; Valuable experimental results have been acquired on these heterostructured systems. For example, an amorphous Ti-rich interfacial layer containing nano-scale precipitates was identified on the PbTiO3-Pt interface, which provides direct experimental evidence to support the recently proposed passive layer model for degradation mechanisms in ferroelectrics. We first observed the coexistence of 124 and 125 YBCO defect phases in YBCO thin films prepared by pulsed laser depositions. Crystallographic shear defect structures were first observed in the anatase (3%Co)-TiO2 thin film. Strikingly, annealing experiments of HgTe-CdTe superlattices for 12 hrs at 140°C and 185°C appear to sharpen the HgTe-CdTe interfaces rather than blurring them. The corresponding tentative models and mechanisms are built to explain these phenomena. This thesis highlighted that successful characterization and analysis based on interfaces and defects in advanced materials would provide the basis for predictive material design with more desirable physical properties and higher performance.