Monolayer Growth and Structure of Ga on Si(111).

The vertical position of Ga on Si(111) for coverages larger 1/3 ML has been quantitatively established using the X-ray standing wave technique. First principles total energy minimization calculations with the local density functional approach have been carried out for selected geometries of Ga on Si(111). The minimum energy configuration is found for Ga in a substitutional geometry, exchanging for the outermost double layer of Si atoms and the calculated position is in good agreement with the standing wave results. The LEED pattern, showing an incommensurate superstructure in the investigated range of coverage is associated with three-dimensional Ga islands which thus even at submonolayer coverage in coexistence with the threefold coordinated, substitutioned Ga.