By exploiting the behavior of HBO2 molecules, which initially reacted preferentially with center adatoms in unfaulted half-units of the 7 x 7 structure, the stability of corner holes in the (111) structure was studied by means of scanning tunnelling microscopy. The 7 x 7 structure began to break down in areas below step edges. Along the boundaries between the 7 x 7 structure, and disordered areas which included the 3 x 3 phase, the structure of the corner hole in the 7 x 7 structure was maintained. A similar structure was observed on quenched (111) surfaces. The results indicated that corner holes played an important role in the formation of the 7 x 7 structure. Structures in which a dimer-adatom stacking fault structure seemed to be formed from a corner hole were observed on quenched (111) surfaces.

K.Miyake, M.Ishida, K.Hata, H.Shigekawa: Physical Review B, 1997, 55[8], 5360-3