An atomic force microscopy study of super-dislocation/micropipe complexes on the 6H-SiC(0001) growth surface

We have used atomic force microscopy (AFM) to study the (0 0 0 I) growth surface of a 6H-SiC single crystal at the points where micropipes emerge on the growth surface. All of the micropipes examined are origins of spiral steps, indicating that dislocations intersect the surface at these points. The dislocations observed at the surface/micropipe intersections have Burgers vectors of at least 4b(0), where b(0) is the Burgers vector of a unit screw dislocation aligned along the c-axis (b(0) = 15.19 Angstrom). Single and double unit dislocations were also observed, but they are not associated with micropipes. Micron-scale deposits of a heterogeneous phase were observed in the vicinity of the micropipes. The curvature of growth steps around these heterogeneities indicates that they impeded step motion while the crystal was growing. Based on our observations, we propose a model for the formation of super-dislocation/micropipe complexes that involves the coalescence of unit screw dislocations that are forced towards one another as large steps grow around heterogeneous material on the surface.