An extensive study of threading dislocations in 4H SiC crystals was carried out by etching in molten KOH. In contrast to the well-defined hexagonal pits formed on lightly-doped 4H epilayers, the etching of bulk 4H SiC crystals heavily doped with nitrogen produced rounded etch-pits with their sizes varying over a wide range. Neither the shape nor the size of the etch pits in the bulk n+4H crystals could be used to distinguish between threading edge and treading screw dislocations. Data on the density of threading screw dislocations were obtained by counting etch pits on the carbon face of the wafers. Sequential steps of material removal, which included polishing followed by KOH etching, were used to track threading dislocations along the growth direction. It was found that a threading dislocation could produce etch pits of different sizes at different depths in the wafer. Mobility of the front of threading dislocations during growth was assessed by measuring changes in the position of the dislocation etch pits upon sequential material removal. The statistical distribution of such displacements in the wafer plane was found to be log-normal. On average, the growth distance of 8μm corresponded to a change in the etch-pit position of about 2μm. This showed that the front of threading dislocations exhibited significant mobility during SiC sublimation growth; resulting in tilted or curved dislocation lines in the grown crystal.

Etching Study of Dislocations in Heavily Nitrogen Doped SiC Crystals. P.Wu: Journal of Crystal Growth, 2010, 312[8], 1193-8