Atomic-level structure and composition at interfaces

Abstract

The latest generation of high-resolution electron microscopes (HREMs) have structural resolution limits on the order of atomic dimensions, thereby opening up the possibilities for direct observation of atomic (re)arrangements at crystal defects and discontinuities such as interfaces. With recent advances in growth methods, novel types of optical, X-ray and electronic devices which are based upon nanometer-scale multilayers (MLS) have been developed. This short review considers the prospects and limitations of the HREM technique for elucidating the structure and composition of interfaces in these layered materials, including such details as the interface roughness, the extent of interpenetration (i.e. compositional gradients) and the coherent/incoherent nature of the interface. The complementary nature of information available from other TEM methods is also briefly discussed.The common (and essential) requirement for HREM characterization of layered structures is that they must be aligned with their interfaces(s) edge-on with respect to the incident beam direction. It is therefore useful if the multilayers are deposited onto a well-defined surface of a crystalline substrate,typically Si or GaAs, which can then provide a basis for orientation purposes. The development of routine methods for preparing cross-sectional MLS specimens, either by ion-milling or cleavage , has also done much to stimulate recent HREM observations.