Transmission electron microscopy studies of dislocations in physical-vapour-transport-grown silicon carbide

Micropipes (the hollow cores of axial superscrew dislocations with Burgers vectors that are multiples of a crystal's c lattice parameter) in hexagonal silicon carbide (SiC) semiconductor wafers were observed by transmission e lectron microscopy (TEM) in both plan-view and longitudinal geometries. Mic ropipes were seen to be facetted along primary and secondary prismatic face s of the crystal. This hexagonal facetting occurred over a full range of Bu rgers vector magnitude. The hexagonal cross-sections of the larger micropip es were elongated into slot shapes. Members of closely spaced groups of mic ropipes had cross-sectional shapes distorted by the strain fields of their neighbours. As the sample was tilted, Bragg contours resulting from the ext ensive strain fields of the superscrew dislocations swirled around the micr opipes. At a two-beam condition in a plan-view sample, twin loops of bright and dark contrast centred on a micropipe were explained to be similar in n ature to the twin lobes of bright and dark contrast characteristic of eleme ntary screw dislocations with line directions perpendicular to the surface of a thin foil. Other dislocations with both line directions and Burgers ve ctors confined to the basal plane of hexagonal SiC semiconductor wafers wer e observed by TEM, frequently in the immediate environs of micropipes or oc casionally in the bulk of the crystal. These dislocations were most often p airs of partials separated by narrow ribbons of stacking fault, although pe rfect dislocations were sometimes observed that split into the partials at points along their lengths. Observations on the TEM scale were related to t hose of similar basal plane dislocations seen in X-ray topographs. Because the dislocations terminated at micropipes' surfaces, and sometimes extensiv e networks of them looped outwards from the micropipes, it was suggested th at the micropipes were involved in the generation of basal plane dislocatio ns, acting as stress concentrators for basal plane slip during crystal grow th.