The purpose of this thesis is to identify the origin and evolution of two types of extended defects frequently observed in physical vapor transport (PVT) grown hexagonal SiC bulk crystals. They are basal plane dislocations and threading edge dislocations.

PVT-grown 4H-SiC single crystals were investigated using high-resolution x-ray diffraction, defect selective etching, and transmission electron microscopy. The (0008) diffraction peak position shifted about 2.5° along the 3 inch crystal diameter indicating the bending of the basal plane. Transmission electron microscopy revealed that the bending is due to deformation-induced basal plane dislocations with at least a partial edge character. The basal plane dislocations had a non-zero net Burgers vector with the net extra half plane pointing toward the seed side of the investigated crystals, and thus caused the basal plane to bend concave toward the growth direction. Approximately parallel basal plane dislocation arrays were shown to correspond to basal plane slip bands. The bending of entire boules and the global distribution of the slip bands indicated that the basal plane slip system was activated by the macroscopic shear stress resulting from the thermal gradient imposed on the crystal during the PVT growth.

The fine structure of the 'star defect' in PVT-grown 4H-SiC single crystals was investigated using molten KOH etching, transmission x-ray topography, and transmission electron microscopy. The star defect is one of the macroscopic extended defects consisting of the defect center and symmetric dislocation arrays extending along six equivalent <11-20> directions. The <11-20> arms of the star defect were shown to consist of polygonized threading edge dislocation arrays, which correspond to prismatic slip bands. Transmission electron microscopy revealed that Burgers vectors of the dislocations are anti-parallel to corresponding arm directions, and the extra half planes of the dislocations extend toward the outside of the arm. The nature of the dislocations making up the <11-20> arms of the star defect indicated that the local compressive stress was induced during the PVT growth, and resulted in the activation of the prismatic slip system.