Understanding the growth of Pb on Ge(111): a first principles study

The growth of single crystal metal islands on semiconductor surfaces is an important technological process. In our joint experimental and theoretical examination of the growth of Pb on Ge(111), Low Energy Electron Microscopy (LEEM) measurements reveal an initial appearance of a Pb wetting layer until a critical coverage when nanocrystalline Pb islands abruptly form. Density functional theory based calculations find this critical coverage to be 1.33 ML, beyond which Pb atoms prefer to occupy the second layer. Ab initio thermodynamic calculations confirm that for Pb coverages below 1.33 ML the chemical potential of Pb (μ_Pb) in the wetting layer is less than that in bulk Pb, suggesting that Pb atoms prefer to be in the wetting layer, while at coverages higher than 1.33 ML the order is reversed implying that Pb atoms prefer to form bulk crystal, i.e. cluster. More interestingly, we find that at above 1.33 ML coverage, the μ_Pb in bilayer on Ge(111) is higher than that in bulk Pb, alluding that Pb atoms prefers to form a 3-d island, not a bilayer, in agreement with the LEEM observations that 3-d islands emerge suddenly out of the compressed wetting layer.