STEM Optical Sectioning for Imaging Screw Displacements in Dislocation Core Structures

Abstract

Edge dislocations are characterised by columns of atoms in tension or in compression in a plane normal to the dislocation, above and below the slip plane. Such dislocations are conventionally viewed end-on at high resolution in transmission electron microscopes, because the tensile and compressive displacements are clearly visible. Screw dislocations are characterised by equivalent atoms in the columns parallel to the screw dislocations being displaced along the screw axis to form a helix. They cannot be observed in end-on images because the displacements are parallel to the viewing direction. In this paper we show that the helical displacements around a screw can be imaged with the dislocation lying in the plane of the transverse to the electron beam by optical sectioning in high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) imaging. This novel technique is illustrated by application to the measurement of the c-component in the dissociation reaction of a mixed [c+a] dislocation in GaN that has previously been observed end-on [1].