The character of micropipes in silicon carbide crystals

There has been some question as whether micropipes in SiC crystals, giant dislocations that possess hollow cores and lie roughly along the hexagonal c-direction, are screw dislocations with Burgers vectors of nc, where n is an integer, or are dislocations of mixed character, with Burgers vector nc?+?ma, where m is equal to n or possibly different from n. X-ray topographs of the basal faces of PVT-grown 4H-SiC wafers containing micropipes taken in both back-reflection and grazing-incidence geometries are compared with computer simulations of micropipe images. The deviation from circular symmetry of micropipe images in such topographs may be explained as well by the micropipe's tilt from the c-direction as by an nc?+?ma nature. The contrast of micropipes in electron micrographs comprises of a pair of light and dark lobes consisting of a detailed system of subsidiary fringes. These are comparable with micropipe image computer simulations for Burgers vector nc. When an na edge component is introduced to the simulation, a small feature, a few hundred nm wide, appears, which is absent from the micrographs.