Simplification of selective imaging of dislocation loops: diffraction-selected on-zone STEM

Contrary to the conventional belief that, in transmission electron microscopy (TEM), selective and sharp imaging of dislocation loops can be realized only by accurate tilting of a specimen from the condition that the symmetrical axis of incident electron beam distribution is parallel to a zone axis of the TEM specimen (on-zone condition), we demonstrate that selective dark-field (DF) imaging of dislocation loops at on-zone condition is possible with a scanning TEM (STEM) mode that uses an objective lens aperture to select a diffraction disk of interest. Diffraction-selected on-zone STEM (DsoZ-STEM) has been applied to selective DF imaging of dislocation loops with a short axis length of <2 nm in a single-crystal aluminum irradiated by argon ions and an electron beam at room temperature. It was found that a Kikuchi line enhances the contrast among the dislocation loops and the matrix of DsoZ-STEM images. DsoZ-STEM obeyed g·b invisibility criterion and showed good agreement with a typical visibility change of a dislocation line and a loop in conventional DF images with a specific pair of ±g. In addition, dislocation loops always showed much higher brightness in the inner side compared to the outer side in DsoZ-STEM images, simplifying the distinction of dislocation loops with apparently the same long-axis direction but different b. Thus, DsoZ-STEM can simplify the selective DF imaging for the determination of the number and the character of dislocation loops.