Geometric models and electronic structure of Pb overlayers on Ge(111): a first-principles study*

Lead deposition on Ge(111) shows several low dimensional phases [1]: the (√3×√3)R30^o Pb structure forms at coverages of 1/3 monolayer (ML) (α phase) and 4/3 ML (β phase), while at 1 ML the overlayer is (1 × 1). In all cases the Ge(111) surface is unreconstructed. The geometric and electronic structure of the phases are not yet fully understood. We present results from our density functional theory based calculations with and without inclusion of spin-orbit coupling (SOC) to shed light onto atomic and electronic structures of these phases and the nature of their binding to the substrate. We find that Pb atoms adsorb at the T₄ sites at a height of 2.94 Å, at the T₄ and H₃ sites at height of 2.84 Å, and the off-centered T₁ sites at height of 2.84, from the underlying Ge surface, on the α, β and 1 ML phases of the structure, respectively, and at the T₁ site at a height of 2.76 Å directly above the nearest Ge on the Pb(1 × 1) phase. We find that these phase are metallic. Our results reinforce the importance of SOC in reproducing geometric structures of the Pb over-layers, in good agreement with experimental observations.
[1] Y. Sato et al., Surf. Sci. 603, 2300 (2009).