High-resolution core-level photoemission study of dense Pb overlayers on Si(111)

Structure and bonding configuration of dense Pb overlayers on the Si(111) surface have been studied by low-energy-electron diffraction and high-resolution photoelectron spectroscopy using synchrotron radiation. Several representative phases in its devil's staircase phase diagram have been systematically investigated by varying the Pb coverage at 200-300 K. Pb5d photoelectron spectra indicate that there exist two distinct bonding configurations of Pb, which are interpreted as the hollow and on-top (T1) sites of the structure models proposed earlier. In case of surface Si atoms, mainly two different bonding environments are revealed by surface Si2p components for the low-density √7 x √3 phase. These can be assigned to T1 and modified on-top (T1′) sites surrounding hollow-site adatoms. As the coverage increases, the minority site T1 converts to T1′ making the topmost Si layer have a unique bonding configuration. This behavior is also consistent with the structure models. The temperature-dependent study reveals that the √7 x √3 phase undergoes a reversible phase transition into a 1 x 1 phase. This phase transition induces no significant change in Pb core levels but a marginal increase in the Si2p component for the T1′ sites. We suggest a plausible scenario of the phase transition based on the structure model with 1.2 monolayer Pb and the active diffusion of hollow-site adatoms.