First-principles studies of structures and stabilities of Pb/Si(111)

Various structures of Pb/Si(111) from 1/6 ML to 4/3 ML are investigated using first-principles calculations. Our calculations for the 1/6-ML mosaic phase and 1/3-ML beta-phase yield good agreement with the experimental results. The higher coverage range 1/3 < theta < 4/3 ML has been controversial experimentally, especially close to 4/3 ML with several different phases being observed (root3 x root7, hexagonal incommensurate, striped incommensurate, etc.). We compare the unit cell energies of different structures to deduce the energetically favorable ones. Structures consisting of very flat Pb overlayers have lower energy than missing-top-layer structures or structures with stacking faults. This is in very good agreement with experiments which measure high Pb mobility on the a phase.