Surface energetics and structure of the Ge wetting layer on Si(100)

Ge deposited on Si(100) initially forms heteroepitaxial layers, which grow to a critical thickness of ∼3 MLs before the appearance of three-dimensional strain relieving structures. Experimental observations reveal that the surface structure of this Ge wetting layer is a dimer vacancy line (DVL) superstructure of the unstrained Ge(100) dimer reconstruction. In the following, the results of first-principles calculations of the thickness dependence of the wetting layer surface excess energy for the c(4 × 2) and 4 × 6 DVL surface reconstructions are reported. These results predict a wetting layer critical thickness of ∼3 MLs, which is largely unaffected by the presence of dimer vacancy lines. The 4 × 6 DVL reconstruction is found to be thermodynamically stable with respect to the c(4 × 2) structure for wetting layers at least 2 ML thick. A strong correlation between the fraction of total surface induced deformation present in the substrate and the thickness dependence of wetting layer surface energy is also shown.