The eŠect of nitrogen and aluminum as doped impurities on the stability of SiC polytypes (Cor Si face 4H and 6H substrates) formed by physical vapor transport (PVT) was investigated. The stability of the polytypes was analyzed using classical thermodynamic nucleation theory with numerical results obtained from a global model including heat, mass and species transfer in a PVT furnace. The results reveal that the formation of 4H SiC was more stable than that of 6H SiC when a grown crystal was doped with nitrogen using C face 4Hand 6H SiC as seed crystals. In contrast, formation of 6H SiC was favored over 4H SiC when Si face 4Hand 6H SiC seed crystals were used. Meanwhile, the formation of 4H SiC was more stable than that of 6H SiC when aluminum was the dopant and Cand Si faces of 6H SiC were used as seed crystals. 6H SiC was preferred to grow rather than 4H SiC in the cases of C and Si faces of 4H SiC as seed crystals.