Imaging and manipulating heterostructure interfaces

Epitaxial heterostructures constitute a wide variety of modern microelectronics devices. In the limit of ever decreasing feature dimensions, now entering the nanoscale in some cases, the interfaces of such devices are crucial to their operation and performance. In general the properties of the interfaces will differ significantly from those of the bulk structure of either the substrate or the heteroepitaxial film. To date, direct, non-destructive characterizations of the atomic-level structure of films and interfaces have not been readily available and this has hampered the design and optimization of heteroepitaxial devices. We describe here a novel x-ray interference method which is useful for imaging such structures with sub-Ångstrom spatial resolution while also providing chemical composition information from a map of the electron density. We illustrate the method, known as Coherent Bragg Rod Analysis (COBRA), with recent results on GaSb-InAs heterostructures of interest as infrared sources and detectors. We show that, with detailed knowledge of the interfaces from COBRA, it is now feasible to correlate specific molecular beam epitaxy growth conditions with desired electronic characteristics associated with the interface bonding. The COBRA method is quite general and only requires an epitaxial relationship between the substrate and the nanostructure that is deposited on it.