The purposes of this dissertation were to identify plastic deformation as a major dislocation formation mechanism during the physical vapor transport (PVT) growth of bulk SiC crystals, and to identify thermoelastic stress as the major cause of the deformation.; KOH etching and optical microscopy, transmission electron microscopy, and synchrotron white beam x-ray topography were used to directly observe deformation related dislocation structures in commercial SiC wafers.{09}Slip bands of threading edge and basal plane dislocations were identified indicating that plastic deformation does occur during the bulk growth.; The threading dislocation slip bands were aligned along the ⟨112¯0⟩ directions on the Si-face of KOH-etched wafers. The dislocations were shown to lie along the c-axis with Burgers vectors of the a/3⟨112¯0⟩ type. The threading edge dislocations also formed low angle grain boundaries aligned along the ⟨11¯00⟩ directions. These arrays were interpreted to form by polygonization of the dislocations introduced by plastic deformation.; The basal plane slip bands were aligned perpendicular to the off-cut direction on the Si-face of KOH-etched off-cut wafers. The length of the slip bands was consistent with the thermoelastic stress due to temperature distribution in growing crystals being the cause of the deformation. It was shown that gliding basal plane dislocations interact with grown-in polygonized domain walls and pile up against them, creating mixed-tilt domain boundaries.; The thermoelastic stress in a growing 6H-SiC crystal was calculated for a typical PVT process using a two dimensional finite element model. Based on the stress distribution, possible plastic deformation of the crystal was postulated.{09}The measured deformation in as-grown crystals was consistent with the postulated deformation indicating that the thermoelastic stress is an important dislocation formation mechanism during the PVT growth.; We have observed conversion of basal plane dislocations in off-axis 4H-SiC substrates into threading edge dislocations in the homo-epilayers. The conversion was interpreted as the result of an image force in the epilayers between flowing growth steps and basal plane dislocations. It is argued that this mechanism can cause an increase of the threading edge dislocation density in bulk crystals, and can lead to an apparent structural quality improvement of epilayers.
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