ATOMIC ARRANGEMENT OF ALKALI ADATOMS ON THE SI(001)-2X1 SURFACE

Adsorption sites of Na and Li on the Si(001)-2 x 1 surface at the saturation coverage are determined theoretically by using the simulated annealing method proposed by Williams and Soler. For Na, the most stable configuration is the combination of the hollow site along the dimer chain and the trough site on top of the third layer Si atom in contradiction to an earlier calculation. The substrate relaxation is a crucial factor for stabilizing this configuration. On the other hand Li atoms are adsorbed at low symmetry positions. For both of Na and Li, we observe a significant movement of the substrate atoms towards the ideal surface configuration.