THE USE OF SYNCHROTRON EDGE TOPOGRAPHY TO STUDY POLYTYPE NEAREST-NEIGHBOR RELATIONSHIPS IN SIC

A brief review of the phenomenon of polytypism is presented and its pr olific abundance in silicon carbide discussed. An attempt has been mad e to emphasise modern developments in understanding this unique behavi our. The properties of synchrotron radiation are shown to be ideally s uited to studies of polytypes in various materials and in particular t he coalescence of polytypes in SiC. It is shown that with complex mult ipolytypic crystals the technique of edge topography allows the spatia l extent of disorder to be determined and, from the superposition of L aue type reflections, neighbourhood relationships between polytypes ca n be deduced. Fine features (<5 mu m) have now been observed with the advent of second generation synchrotrons, the resolution available ena bling the regions between adjoining polytypes to be examined more clos ely. It is shown that long period polytypes and one dimensionally diso rdered layers, often found in association with regions of high defect density, are common features at polytype boundaries. An idealised conf iguration termed a ''polytype sandwich'' is presented as a model for B C grown by the modified Lely technique. The frequency of common sandwi ch edge profiles are classified and some general trends of polytype '' neighbourism'' are summarised.