Atomic structures of 6H-SiC(0001) and (0001) surfaces

We have studied the reconstructions of the 6H-SiC (0001) and (000 (1) over bar) surfaces after annealing at 850-950 degrees C under a Si flux using scanning tunneling microscopy (STM). On both the Si-terminated (0001) and C-terminated (000 (1) over bar) surfaces, we observed a (3 x 3) reconstruction after annealing at 850 degrees C, which changed to a (root 3 x root 3) reconstruction after further annealing at 950 degrees C. We propose a model for the (3 x 3) surface with a 4/9 ML adatom-coverage. The (root 3 x root 3) surface has a 1/3 ML adatom-coverage. These observations are consistent with previous LEED results. We also observed a new (9 x 9) reconstruction on the (0001) surface after further annealing the (3 x 3) surface at: 900 degrees C under a Si flux. When the (0001) surface was flashed to 1150 degrees C without a Si flux, a graphitized surface with a (6 x 6) reconstruction was formed. The empty-state and filled-state images showed a contrast reversal. When Si was deposited in the (6 x 6) surface, the polarity of the contrast reversal was reversed, confirming the validity of our previous electronic structure calculations for this surface.