Structures of atomic steps on the 7x7 reconstructed Si(111) surface

We have carried out a systematic study of the structures of mono bilayer atomic steps on the 7X7 reconstructed Si(111) surface whose step edges are parallel to the unit-cell boundary. Atomic steps in all possible 14 step groups have been observed; thus a complete picture of the step structure is presented. The upper step edges of ten groups terminate at unit-cell boundaries, while the remaining four groups do not. The step structure formed is the result of two factors competing to minimize the surface free energy: the structural integrity of the upper step edge and the width of the transition region at the lower terrace. There is evidence that step formation is influenced by the annealing method used.