Influence of stacking faults on the growth of polytype structures, I. Cadmium iodide Poly-types

Abstract

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 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 CdI₂ 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.