The Preference of Silicon Carbide for Growth in the Metastable Cubic Form

A paradox is discussed concerning the growth of SiC polytypes from the vapor or the melt, based on recent ab initio quantum calculations of the relative energies of several polytypes in bulk. Why does the cubic (3C) structure grow in preference to all others, although the calculations indicate it is not the stable phase at any temperature? This can be explained from the calculations, with some further approximations, as due to the constrained equilibrium when adding one atomic double layer at a time to the growing crystal in the hexagonal direction without allowing rearrangement of the lower layers. The differing roles of donor and acceptor impurities are also discussed, with donors being found to favor the cubic structure.