4″, 6″ and 8″ Cz crystals were grown with different heat shields which protect the growing crystals against radiation emitted from the melt surface and hot graphite parts, thereby determining their thermal environment. It was found that the critical pull rate, at which the oxidation induced stacking fault (OSF) ring vanishes in the wafer centre, varies with the crystal diameter and the type of heat shield. A calculation of the axial temperature gradient at the solid/liquid interface for each crystal diameter/heat shield combination revealed that the critical pull rate is proportional to this axial temperature gradient G, which, in turn, is a function of the crystal diameter and heat shield. Thus, the critical pull rate is entirely determined by the axial gradient G. The OSF ring appears in the wafer centre when the equation V G = C crit = 1.3 × 10 7t-3 cm 2 min −1 K −1 holds ( V is the pull rate). For V G > C crit flow pattern/D-defects are observed, whereas V G < C crit describes the condition for the growth of large pit defects.