Anomalies in silicon carbide polytypes

Abstract

Frank’s dislocation theory of the origin of polytypism received direct experimental support from the observation of a correlation between the step height of growth spirals on silicon carbide polytypes and the heights of their X-ray unit cells (Verma 1952, 1957). A detailed X-ray diffraction and microscopic investigation of silicon carbide structures has revealed anomalies that cannot be explained on the dislocation theory. Three new unusual polytypes 36Ha, 36Hband 90Rare described in detail. The structures 36Haand 36Hbwere found in a single crystal piece and have identical lattices witha = b= 3.078 Å andc= 90.65 Å. Both structures belong to the space groupP3m. The polytype 90Rbelongs to the space groupR3mwith hexagonal unit cell dimensionsa = b= 3.078 Å,c= 226.6 Å. The detailed atomic structure of type 90Rhas been worked out and has a Zhdanov symbol [(23)43322]3. It is shown that the polytypes 36Haand 36Hbare based on the 6Hphase while type 90Ris based on the 15Rphase. The creation of such polytypes requires a screw dislocation with a Burgers vector which is an integral multiple of thecspacing of the basic structure, and is therefore not understood on Frank’s theory. A surface examination of the faces of these crystals does not reveal any growth spirals, showing that they have not grown by the dislocation mechanism. The growth of the different polytypes of silicon carbide is discussed and it appears that screw dislocations determine the surface structure but not the contents of the unit cell and therefore the cause of polytypism needs to be reconsidered.