The process steps critical for the production of low defect, large diameter conducting and semi-insulating SiC substrates include optimization of the vapor stoichiometry, temperature field, thermal history - in particular during the growth initiation - and impurity control. Nucleation control, optimization of the thermal conditions throughout the growth run and impurity reduction were essential steps to reduce the density of structural defects such as micropipes, built-in stress and low angle grain boundaries in 3-inch and 4-inch diameter SiC wafers. The impurity control aids the production of large area semi-insulating material.