A self-consistent semi-empirical molecular orbital method was used to determine whether the adsorption properties of K atoms, and the formation of K adsorbate chains or clusters in the low-coverage regime, could be affected by the nature of the semiconductor surface (that is, perfect or stepped). It was found to be possible to determine the microscopic structures of monatomic and diatomic K molecules on perfect and stepped (110) GaAs surfaces. The results for K adsorption on the perfect GaAs(110) surface were consistent with scanning tunnelling microscopic observations of Na on (110) GaAs; with the stable site for K being the bridge site which encompassed one Ga and two As surface atoms. The equilibrium geometry for diatomic K involved the second K atom occupying the next-nearest neighbor bridge site; thus strongly supporting the formation of an open linear structure parallel to the zig-zag surface atomic chains. The calculated K-K distance in this equilibrium configuration was 0.802nm. This was similar to the Na-Na distance (0.8nm) which was deduced from scanning tunnelling microscopy experiments. The results for the stepped (110) GaAs surface suggested that a step was unlikely to assist the clustering of K atoms. However, the formation of the linear adsorbate chain appeared to be influenced more by the orientation of the steps. On the perfect surface, the K adsorbates were bound more strongly at steps than at bridge sites.

G.S.Khoo, C.K.Ong: Journal of Physics - Condensed Matter, 1993, 5[36], 6507-14