Process conditions for stable single polytype growth of 4H-SiC boules via a seeded sublimation technique have been developed. Reproducible results can be obtained in a narrow temperature interval around 2350 degrees C and on the C-face of 4H-SiC seeds. Evidence is presented that during the initial stage of growth, morphological instabilities may occur resulting in structural defects. A solution is proposed based on the experimental findings, i.e. the first regions of growth ought to be carried out at a low supersaturation (growth rate similar to 100 mu m/h) until a proper growth front has developed.