An optical and X-ray topographic study of giant screw dislocations in silicon carbide

A 6H SiC crystal plate exhibiting three large growth spirals on its (0001) face was examined. One spiral maintained a simple step structure over an area 0.8 mm in diameter, enabling its step height to be measured interferometrically as 31.7 nm corresponding to 21 repeats of the 6H structure. X-ray topography confirmed the presence of a giant screw dislocation with Burgers vector parallel to [0001] at the centre of each spiral and showed that these dislocations had hollow cores running their lengths. Diameters of the hollow cores within the crystal were measured non-destructively by X-ray section topographs. The values found (~10 μm) were an order of magnitude greater than the diameters of the microscopically visible holes at the surface outcrops of the dislocations. A possible reason for this discrepancy is discussed. Attempts were made to measure the magnitude of the giant screw dislocation Burgers vector by X-ray diffraction methods, firstly from the size of the diffraction-contrast image of the dislocation and secondly from the tilt of the lattice planes close to the dislocation. Both methods were subject to much uncertainty, the former underestimating and the latter overestimating the Burgers vector magnitude by factors of more than two relative to the magnitude inferred from step-height measurements