Sub-angstrom imaging of dislocation core structures: how well are experiments comparable with theory?

Abstract

During the past 50 years Transmission Electron Microscopy (TEM) has evolved from an imaging tool to a quantitative method that approaches the ultimate goal of understanding the atomic structure of materials atom by atom in three dimensions both experimentally and theoretically. Today's TEM abilities are tested in the special case of a Ga terminated 30 degree partial dislocation in GaAs:Be where it is shown that a combination of highresolution phase contrast imaging, Scanning TEM, and local Electron Energy Loss Spectroscopy allows for a complete analysis of dislocation cores and associated stacking faults. We find that it is already possible to locate atom column positions with picometer precision in directly interpretable images of the projected crystal structure and that chemically different elements can already be identified together with their local electronic structure. In terms of theory, the experimental results can be quantitatively compared with ab initio electronic structure total energy calculations. By combining elasticity theory methods with atomic theory an equivalent crystal volume can be addressed. Therefore, it is already feasible to merge experiments and theory on a picometer length scale. While current experiments require the utilization of different, specialized instruments it is foreseeable that the rapid improvement of electron optical elements will soon generate a next generation of microscopes with the ability to image and analyze single atoms in one instrument with deep sub Ångstrom spatial resolution and an energy resolution better than 100 meV.

Progress in Aberration-Corrected High-Resolution Transmission Electron Microscopy of Crystalline Solids

“…The observed alteration of atomic dumbbell lengths in the vicinity of stacking fault ribbons of about 10 pm is in fair agreement with recent measurements focusing on intrinsic stacking faults ribbons in heavily beryllium doped GaAs [28]. This analysis revealed an average expansion of dumbbell lengths up to 158 pm in the faulted lattice plane which was explained by the segregation of beryllium dopant atoms and the subsequent formation of antisite defects inside of the plane of the faulted ribbon.…”

Aberration correction: zooming out to overview

“…Significant structural variations along the dislocation line can arise from a number of effects, but may not be strongly visible in the usual images. Surface relaxations such as the Eshelby twist at a screw dislocation, although easily noticed in diffraction contrast imaging (Tunstall et al 1964), are much less perceptible in HREM images (Sigle 1999;Kisielowski et al 2006). Minimum energy configurations for partial dislocations do not in general lie parallel to one another or to the thin film normal (Hazzledine et al 1975).…”

Threading Dislocations Piercing the Free Surface of an Anisotropic Hexagonal Crystal: Review of Theoretical Approaches

“…Each dislocation line is oriented parallel to 3 and is associated with a finite core. According to some high-resolution transmission electron microscopy observations and numerical simulations, for example, [23][24][25], a TD core can have a variety of atomic structures. These latter have certainly an influence on the elastic properties very near the theoretical dislocation line [25].…”