X-ray topographic dislocation contrast visible in reflections orthogonal to the Burgers vectors of axial screw dislocations in hexagonal silicon carbide

Contrast is associated with micropipes in X-ray topographs of SiC crystals obtained with prismatic reflections, representing an apparent violation of the g . b = 0 invisibility criterion. This is explained as a population of basal-plane dislocations with Burgers vectors of the set b = 1/3[11(2) over bar0] that occur in a high density within a few micrometers of the micropi pes, below the resolution of Xray topography. These basal-plane dislocation s could be observed under an electron microscope. The presence of the surfa ces of the micropipes influences the dislocation images in the topographs t aken with prismatic reflections, often resulting in a band of light contras t along the axes of the micropipes.