THE STRUCTURAL EVOLUTION OF LELY SEEDS DURING THE INITIAL STAGES OF SiC SUBLIMATION GROWTH

The as-grown surfaces of 6H SiC seed crystals grown by the Lely method have been characterized by visible light microscopy and atomic force microscopy. The surfaces were then used to seed the sublimation growth of SiC at 2300'C, in 650 torr of Ar, with a gradient of 8 °C/cm. Repeated observations of the seed surface structure after predetermined intervals in the growth reactor demonstrated that transport starts at less than 2160 'C and that voids are nucleated by heterogeneous material on the original seed surface. Near these voids, the concentration of unit screw dislocations is increased and these defects migrate during growth. Screw dislocations of opposite sign act as step sources and build mesa structures around the heterogeneous material. After only 10 min at the growth temperature, micropipe/super screw dislocation complexes form in the centers of the mesas, possibly by the migration of unit dislocations to the voids.