ELECTRON MICROSCOPY INVESTIGATION OF DEFECTS IN SiC. P. Pirouz, Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7204, U.S.A.

The applications of transmission electron microscopy (TEM) to investigate the structure of defects in various polytypes of SiC are discussed. The cubic 3C-SiC polytype is usually grown as an epitaxial film on (001) silicon or (0001) 6H-SiC substrates by chemical vapor deposition (CVD). In the SiC/Si system, a number of defects arise in the 3C-SiC film because of the large film/substrate lattice and thermal mismatches, as well because of demi-steps on the substrate surface. These include misfit and threading dislocations, a high density of stacking faults and twin bands, as well as inversion domain boundaries (IDBs). In the case of growth on the 6H-SiC substrate, depending on the vicinal angle of the substrate, [[phi]], the film grows homoepitaxially when [[phi]]>0, and heteroepitaxially, as the cubic 3C phase, when [[phi]]=0. In addition, a large number of planar faults known as double positioning boundaries (DPBs) occur in the film. When a thin film is grown homoepitaxially on a 4H-SiC wafer, yet a new kind of defect occurs which has a triangular-shaped. Mechanism of formation of the various defects will be briefly discussed.

In bulk grown 6H-SiC and 4H-SiC, a variety of other defects occur which are mostly growth related. A predominant defect in these crystals are "micropipes" which run parallel to the [0001] growth direction of the boules. Deformation-induced defects in different SiC polytypes are mostly line defects which are dissociated into two partial dislocations bounding a wide ribbon of stacking fault. Geometrically, the core of a partial dislocation in these materials consists of either all carbon or all silicon atoms. Using the technique of large-angle convergent beam electron diffraction (LACBED), we have recently distinguished the core nature of partial dislocations produced by deformation. It turns out that the partial which has a core of silicon atoms (the Si(g) dislocation), has a higher mobility than the partial that has a core of carbon atoms (the C(g) dislocation). It is thought that polytypic transformations in SiC occur because of the large difference in the mobility of these two types of partial dislocations. This mechanism is briefly discussed and some experiments in support of it are presented.