Hexagonal voids which were observed in sublimation-grown boules were examined by using optical microscopy, atomic force microscopy, scanning electron microscopy, KOH etching and synchrotron white-beam X-ray topography. The voids formed at imperfections in the attachment layer between the seed and crucible cap. They were platelet-like in shape, with lateral sizes of between 50 and 750μm and thicknesses along the c-axis of between 5 and 25μm. Growth steps were observed on the void facets that were closest to the seed, and evaporation steps were observed on the void facets closest to the growth surface. This indicated the occurrence of void movement during crystal growth. Atomic force microscopic images revealed that the growth steps nucleated at a void side-wall, flowed across the bottom of the void, and terminated in a trench-like depression. The KOH etching revealed dislocations which lined up along the trace of the void path, and often had higher densities which corresponded to the location of the trench. The X-ray topographs revealed a random distribution of screw dislocations in the volume above the void, and the absence of screw dislocations in the volume directly below the void. Hollow-core super-screw dislocations (so-called micropipes) were found at the corners of the void traces. Image forces which were associated with growth steps and void side-walls were used to explain the formation of micropipes.

Hexagonal Voids and the Formation of Micropipes during SiC Sublimation Growth. T.A.Kuhr, E.K.Sanchez, M.Skowronski, W.M.Vetter, M.Dudley: Journal of Applied Physics, 2001, 89[8], 4625-30