Surface-relaxation contributions to axial screw dislocation contrast in synchrotron white-beam X-ray topographs of SiC

Micropipe images appeared in synchrotron white-beam X-ray topographs of thin basal-cut SiC wafers taken using prismatic reflections where g · b = 0. They consisted of white ovals inclined along the direction of the topographs' g vector that were terminated with dark spikes at either end. The thin wafers tended to curl; the appearance of a defect's image varied depending on the sign of the curvature relative to the side serving as the diffracted beam's exit surface. The micropipe images were computer simulated using the ray-tracing method. The calculation assumed that they arose from the surface-relaxation strain component of a closed-core screw dislocation perpendicular to the surface of a thin foil. The qualitative features of the micropipe images were reproduced in their simulations, but the magnitude of the lattice misorientations predicted by the model was not large enough to account for the size of the experimentally observed dislocation images.