Imaging of crystalline specimens and their defects

Abstract

A crystal can be imaged with the primary beam (bright field) or with a Bragg reflection (dark field). The local intensity depends on the thickness, resulting in thickness (or edge) contours, and on the tilt of the lattice planes, resulting in bend contours, which can be described by the dynamical theory of electron diffraction. In certain cases, the intensity of a Bragg reflection depends so sensitively on specimen thickness that atomic surface steps can be observed. The most important application of diffraction (Bragg) contrast is the imaging of lattice defects such as dislocations, stacking faults, phase boundaries, precipitates, and defect clusters. The contrast depends on the Bragg reflection excited and its excitation error, the type of the fault, and its depth inside the foil. The Burgers vector of a dislocation or the displacement vector of a boundary can thus be determined quantitatively. The resolution of the order of 10 nm when a strongly excited Bragg reflection is used can be reduced to the order of one nanometer by the weak-beam technique, which allows us to measure the width of dissociated dislocations, for example. Different types of contrast for precipitates are associated with coherent and incoherent precipitates, which can hence be distinguished. Electron spectroscopic imaging can remove the inelastically scattered electrons in the background of a diffraction pattern and increase the contrast and resolution of defect images. With crystalline specimens, the interference of the primary and a Bragg reflected wave in the final image creates images of lattice planes. When the objective aperture is large and includes a large number of Bragg reflections, the exit distribution of electrons can be imaged. Irradiation along zone axes produces a projection-like image of the crystal lattice with a resolution of 0.1-0.2 nm. For reliable interpretation, such images must be compared with a computer simulation that takes into account the thickness, the potential coefficients, and the wave aberration. The high resolution of the crystal-structure image can be exploited to investigate lattice defects and interfaces. By using electrons scattered through large angles, contrast increasing with atomic number can be superposed on the crystal-structure image. Atomic surface steps and surface-reconstruction structures can be investigated by special methods, notably by reflection electron microscopy (REM). © 2008 Springer Science+Business Media, LLC All rights reserved.