Hexagonal voids and the formation of micropipes during SiC sublimation growth

Hexagonal voids observed in sublimation grown SiC boules were examined using optical microscopy, atomic force microscopy (AFM), scanning electron microscopy, KOH etching, and synchrotron white-beam x-ray topography. Voids formed at imperfections in the attachment layer between the seed and crucible cap. They are platelet-like in shape with lateral sizes between 50 and 750 μm and thickness along the c axis between 5 and 25 μm. Growth steps were observed on the void facets closest to the seed and evaporation steps were observed on void facets closest to the growth surface, providing evidence for void movement during crystal growth. AFM images revealed that growth steps nucleate at a void sidewall, flow across the bottom of the void, and terminate in a trench-like depression. KOH etching of waters between the void and seed revealed dislocations lining up along the trace of the void path, often with higher densities corresponding to the location of the trench. X-ray topographs showed a random distribution of screw dislocations in the crystal volume above the void, and an absence of screw dislocations in the volume directly below the void. Hollow-core superscrew dislocations, called micropipes, were found at the corners of the void trace. Image forces associated with growth steps and void sidewalls are used to explain the formation of micropipes.