Large domains of single or double layer Graphene are produced on 6H-SiC(0001) because Si desorption is better controlled by changing the annealing rate to reach the graphitization temperature, ~1300 C. Because the retraction step speed is not uniform, the carbon that remains forms a second layer of Graphene, and a single-layer of Graphene on the rest of the terraces. The bi-layer Graphene domains appear as "fingers" extending from triple-step edges.
M. Hupalo, E. H. Conrad, and M. C. Tringides, Phys. Rev. B 80, 041401 (2009). "Growth mechanism for epitaxial graphene on vicinal 6H-SiC(0001) surfaces: A scanning tunneling microscopy study"