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The origin of polytype structures has been attributed to spiral growth round suitable screw dislocations created in a basic structure. In deducing the different polytype structures that can result from such a mechanism, the basic structure has always been assumed to be perfect. It is shown that it is necessary to consider the possibility that the parent matrix may contain stacking faults near its surface at the time of the origin of the screw dislocation ledge. This can drastically affect the structure of the resulting polytype. The most probable fault configurations that can occur in a parent 4H or 2H structure of CdI 2 have been deduced from a calculation of the stacking fault energies. Polytype structures originating from single screw dislocations created in a faulted 2H or 4H matrix containing one of the more probable fault configurations near the surface have been deduced. It is shown that all the observedpolytype structures of CdI 2 can result from such a mechanism and represent structures with minimum, or close to minimum, stacking fault energy. There is an excellent agreement between the structures deduced on the basis of the faulted matrix model and those actually observed. It has been possible to explain the origin of the observed structure series in CdI 2 and the limitation of the Zhdanov numbers to 1, 2 and 3. It is possible to predict theoretically the more probable structures for a polytype, which greatly simplifies its structure determination.

A number of polytype structures observed in vapour grown SiC crystals have a unit-cell which is an integral multiple of the unit-cell of the basic 6H, 15R or 4H structure. The growth of such anomalous structures cannot be understood in terms of spiral growth round a single screw dislocation in a basic matrix. However many of these polytype crystals display a single growth spiral on their (0001) face indicating that they have resulted from spiral growth round a single screw dislocation. It is shown that this anomaly can be resolved if the basic matrix is assumed to contain stacking faults near the surface at the time of the origin of the screw dislocation ledge. This possibility, overlooked in the earlier deduction of polytype structures, must be taken into consideration since vapour grown SiC crystals frequently contain a high concentration of random stacking faults, producing continuous streaks on their X-ray diffraction photographs. The most probable fault configurations that can occur in 6H, 15R and 4H structures of SiC have been deduced from a calculation of their stacking fault energy. These fault configurations are then considered to lie at different distances from the surface at the time of the origin of a screw dislocation ledge. Such a faulted ledge gives rise to polytype structures during subsequent spiral growth even if the screw dislocation has an integral Burgers vector. The most probable series of polytype structures that can result from such a faulted matrix model are deduced. It is shown that nearly all the polytype structures of SiC hitherto regarded as anomalous (such as 36H, 54H, 66H, 45R, 90R etc.) are among the expected structures and there is no need to postulate a complicated configuration of cooperating dislocations to account for their growth.

The origin of polytype structures has been attributed to spiral growth round suitable screw dislocations created in a basic structure. In deducing the different polytype structures that can result from such a mechanism, the basic structure has always been assumed to be perfect. It is shown that it is necessary to consider the possibility that the parent matrix may contain stacking faults near its surface at the time of the origin of the screw dislocation ledge. This can drastically affect the structure of the resulting polytype. The most probable fault configurations that can occur in a parent 4H or 2H structure of CdI2 have been deduced from a calculation of the stacking fault energies. Polytype structures originating from single screw dislocations created in a faulted 2H or 4H matrix containing one of the more probable fault configurations near the surface have been deduced. It is shown that all the observedpolytype structures of CdI2 can result from such a mechanism and represent structures wi...