On the spiral growth of polytype structures in SiC from a faulted matrix I. Polytypes based on the 6H structure

Abstract

The different intrinsic and extrinsic fault configurations that can occur in the close-packed 6H structure, ABCACB, are considered and their stacking fault energies calculated to determine the relative probability of their occurrence. Two-fault configurations are found to be most probable. Polytype structures that can result by spiral growth around single screw dislocations of different Burgers vector created in a 6H matrix containing the most probable fault configurations have been deduced. It is shown that polytype structures can result from screw dislocations of integral as well as non-integral Burgers vectors and a much larger number of structure series are actually possible than those deduced earlier from a perfect 6H matrix. A screw dislocation step of a given number of layers can give rise to several different polytype structures depending on the position of the fault in the exposed ledge. Of these the most probable structures have been determined by calculating their stacking fault energies. The most probable series of structures predicted theoretically from such a faulted matrix model for the growth of polytypes are compared with the structures observed so far. There is an excellent agreement between the theoretically predicted and experimentally observed structures. The model explains the observed limitations on the Zhdanov number of the SiC polytypes and can be used in structure determination for predicting the most probable structures for a new polytype.