Anomalies in silicon carbide polytypes

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 36 H a , 36 H b and 90 R are described in detail. The structures 36 H a and 36 H b were found in a single crystal piece and have identical lattices with a = b = 3.078 Å and c = 90.65 Å. Both structures belong to the space group P 3 m . The polytype 90 R belongs to the space group R 3 m with hexagonal unit cell dimensions a = b = 3.078 Å, c = 226.6 Å. The detailed atomic structure of type 90 R has been worked out and has a Zhdanov symbol [(23) 4 3322] 3 . It is shown that the polytypes 36 H a and 36 H b are based on the 6 H phase while type 90 R is based on the 15 R phase. The creation of such polytypes requires a screw dislocation with a Burgers vector which is an integral multiple of the c spacing 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.