ORDERING OF VACANCIES ON SI(001)

Missing dimer vacancies are always present on the clean Si(001) surfac e. The vacancy density can be increased by ion bombardment (Xe+,Ar+), etching (O-2,Br-2,I-2, etc.) or Ni contamination. The equilibrium shap e at low vacancy concentrations (<0.2-0.3 monolayers) of these vacancy islands is elongated in a direction perpendicular to the dimer rows o f the upper terrace, whereas for higher vacancy concentrations the equ ilibrium shape is rotated by 90 degrees. The absence of dimerisation w ithin the narrow vacancy islands at low vacancy concentration alone is not sufficient to explain this shape transformation, therefore it is suggested that the specific rebonding of these structures also plays a role. A more detailed analysis of the annealing behaviour, width, spa cing between and depth of the elongated low-vacancy concentration vaca ncy islands reveals that the Ni-induced vacancy islands differ signifi cantly from the etching-induced vacancy islands.