On the thickness periodicity of atomic-resolution images of dislocation cores

Experimental conditions needed to obtain atomic-resolution images of dislocation cores are discussed. Experimental and computer-simulated images are used to show that, to a limited but useful resolution, these images are periodic functions of the specimen thickness as a consequence of dynamical scattering. Useful images may therefore be obtained from some specimens of sufficient thickness to allow surface relaxation effects to be neglected. End-on projections of stair-rod dislocations and dislocation dipoles are suggested for further study.