Dislocation contrast from phonon-scattered electrons in weak-beam images

A theory for the contrast from phonon-scattered electrons is presented. In this theory single-phonon scattering is taken into account. The influence of the strain field of the crystal defect on the electrons which have been scattered by phonons is considered dynamically. The dislocation contrast in the thermal diffuse intensity is calculated under weak-beam conditions at 100 kV and 1MV. It is shown that the thermal diffuse and elastic intensities are comparable in magnitude for foil thicknesses of the order of the mean free path for electron-phonon scattering. The thermal diffuse intensity has a minimum close to the position of the dislocation. The dependence of the visibility in weak-beam images on the foil thickness, the accelerating voltage, the aperture radius and the temperature is discussed. The diffuse intensity from contaminating carbon layers has been estimated.