The evolution of 3 x 3, 6 x 6, √3 x √3R30° and 6√3 x 6√3R30° superstructuves on 6H-SiC (0001) surfaces studied by reflection high energy electron diffraction

The technique of reflection high energy electron diffraction (RHEED) has been applied to study the evolution of various superstructures on 6H-SiC (0 0 0 1) as a function of annealing temperature. Between the evolution of the stable 3 x 3 and root3 x root 3R30 degrees phases on a silicon-enriched 6H-SiC (0 0 0 1), a mixed phase 3 x 3/2 x 2 reconstruction followed by a well-defined 6 x 6 reconstruction was observed by RHEED for the first time. The 6 x 6 reconstruction is distinct from the pseudo-periodic 6 x 6 structure suggested previously for graphite moire pattern on 6H-SiC (0 0 0 1) [Surf. Sci. 48 (1975) 463; Surf. Sci. 256 (1991) 354]. The mechanisms for the formation of these superstructures in the sequence of 3 x 3, 6 x 6, root3 x root 3R30 degrees and 6 root3 x 6 root 3R30 degrees between 800 degreesC to 1200 degreesC were discussed. The 6 x 6 structure is proposed to evolve directly from the 3 x 3 following the missing of consecutive Si clusters in the twisted silicon adlayer model. Annealing the 6 x 6 reconstructed surface to 1000 degreesC gives rise to a root3 x root 3R30 degrees reconstruction From here, the segregation of carbon domains occurs readily and these form an incommensurate 6 root3 x 6 root 3R30 epilayer at 1200 degreesC. At the early stages of the annealing, the 6 root3 x 6 root 3R30 RHEED pattern consists of a series of cluster satellite streaks superimposed on 1 x I SiC. Further annealing results in the appearance of graphite streaks with its basis vectors rotated 30 degrees to SiC. Prolonged annealing of the graphitized surface results in the growth of single crystalline graphite multilayers on the 6H-SiC substrate. (C) 2001 Published by Elsevier Science B.V.