Compositional mapping using large-angle electron scattering

Abstract

High-angle electron scattering represents a relatively small fraction of the total elastic scattering, typically 10%, but is still orders of magnitude larger than the intensity of secondary excitations such as X rays or core loss inelastically scattered electrons. It is possible, therefore, to form an image from thin samples at high resolution with acceptable signal to noise ratio in a reasonable collection time. The STEM high-angle annular detector introduced by Howie detects only the scattering which occurs very close to the atom sites so that the scattering cross section a approaches the Z2 dependence of unscreened Rutherford scattering. Furthermore, in all but the very thinnest crystals the detected signal is dominated by thermal diffuse scattering, which is generated incoherently by each atom in proportion to the electron intensity close to its site. Due to the large angular range of the detector, dynamical effects in the outgoing thermal diffuse scattering (Kikuchi lines) are averaged out and have no effect on the total intensity detected.