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 electron microscopy (TEM) in both plan-view and longitudinal geometries. Micropipes were seen to be facetted along primary and secondary prismatic faces of the crystal. This hexagonal facetting occurred over a full range of Burgers vector magnitude. The hexagonal cross-sections of the larger micropipes were elongated into slot shapes. Members of closely spaced groups of micropipes had cross-sectional shapes distorted by the strain fields of their neighbours. As the sample was tilted, Bragg contours resulting from the extensive strain fields of the superscrew dislocations swirled around the micropipes. 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 nature to the twin lobes of bright and dark contrast characteristic of elementary screw dislocations with line directions perpendicular to the surface of a thin foil. Other dislocations with both line directions and Burgers vectors confined to the basal plane of hexagonal SiC semiconductor wafers were observed by TEM, frequently in the immediate environs of micropipes or occasionally in the bulk of the crystal. These dislocations were most often pairs of partials separated by narrow ribbons of stacking fault, although perfect dislocations were sometimes observed that split into the partials at points along their lengths. Observations on the TEM scale were related to those of similar basal plane dislocations seen in X-ray topographs. Because the dislocations terminated at micropipes’ surfaces, and sometimes extensive networks of them looped outwards from the micropipes, it was suggested that the micropipes were involved in the generation of basal plane dislocations, acting as stress concentrators for basal plane slip during crystal growth.