Mapping Atomic Structure at Epitaxial Interfaces
Roy Clarke
University of Michigan
Epitaxial heterostructures constitute a wide variety of modern microelectronics devices. In the limit of ever decreasing feature dimensions, now well into the nanoscale in some cases, the interfaces of such devices are crucial to their operation and performance. To date, direct, non-destructive characterization of the atomic-level structure of films and interfaces has not been readily available and this has hampered the design and optimization of heteroepitaxial devices. In this talk we describe recent work using an x-ray interference method which is specific to thin film materials with structurally coherent interfaces. The method, known as Coherent Bragg Rod Analysis (COBRA), is useful for probing such structures with sub-Å spatial resolution while also providing chemical composition information from a three-dimensional map of the electron density (see Figure 1). We illustrate our studies with recent results on GaSb-InAs heterostructures, of interest as infrared sources and detectors. We show that, with detailed knowledge of the interfaces gained from COBRA, it is now feasible to correlate specific molecular beam epitaxy growth conditions with desired electronic characteristics associated with the interface bonding. The proposed Cornell ERL source, with its very high brightness and relatively short pulse duration, offers enticing possibilities to extend such measurements to more complex materials and into the time domain. Our recent COBRA results on ferroelectric PbTiO3 –SrTiO3 heterostructures at the Advanced Photon Source, suggest that this will be a promising area for future studies at ultrabright x-ray sources such as the ERL.
