THE SIC(0001)6-ROOT-X6-ROOT-3 RECONSTRUCTION STUDIED WITH STM AND LEED

We have used scanning tunneling microscopy (STM) to study the 6 root 3 x 6 root 3 reconstruction obtained by heat treatment of 6H-SiC(0001) samples at temperatures above 1100 degrees C. For surfaces showing a w ell-developed 6 root 3 x 6 root 3 low-energy electron diffraction (LEE D) pattern, we observe with STM two pseudo-periodic reconstructions wi th approximate periodicities of 6 x 6 and 5 x 5, respectively, in addi tion to root 3 x root 3-reconstructed regions. The fraction of the sur face exhibiting the root 3 x root 3 reconstruction in STM images and t he intensity of the root 3 x root 3 spots in the 6 root 3 x 6 root 3 L EED pattern decrease with increasing annealing temperature and time. T he 5 x 5 reconstruction is observed on a small fraction of the surface (less than or similar to 10%) and the 6 x 6 reconstruction becomes do minating upon annealing at temperatures above 1200 degrees C. For the 6 x 6 reconstruction, a Fourier analysis of the STM images reveals an underlying incommensurate 2.1 x 2.1-R30 degrees lattice with long-rang e order, The features defining the 6 x 6 periodicity have well-defined positions with respect to this lattice. A comparison between the Four ier transforms of the STM images and the 6 root 3 x 6 root 3 LEED patt ern shows that the LEED pattern can be fully explained by scattering f rom surfaces with a mixture of the root 3 x root 3 5 x 5 and 6 x 6 rec onstructions. For surfaces heated above 1250 degrees C, we observe a p artial graphitization which results in a modification of the 6 x 6 str ucture observed in STM.