Core-level photoemission study of the Pb overlayers on Si(001)

High-resolution core-level photoemission spectroscopy using synchrotron radiation and low-energy electron diffraction have been applied to study the intriguing structures of the Pb overlayers on Si(001) up to a coverage of 1 monolayer (ML). At a coverage of less than 0.6 ML, two distinct components are observed in Pb 5d core levels. This suggests that the Pb chains formed at this coverage range, as observed in the previous scanning tunneling microscopy studies, are composed of buckled Pb dimers consistent with the recent structure model. The Pb 5d core-level shift due to this buckling is measured to be 0.35 eV. The spectral line shape of Pb 5d at an intermediate coverage around 0.75 ML, exhibiting the c(8x4) phase in the low-energy electron diffraction, differs significantly from that of the lower coverages in contradiction to the available structure models with asymmetric buckled dimers. In contrast, the 2x1-Pb surface developed at a full monolayer coverage exhibits only single component in Pb 5d. This result is compatible only with the close-packed symmetric dimer model. The spectral line shape of Pb 5d suggests a gradual change from a semiconducting to a metallic surface at 0.75-1.0 ML, which agrees with the previous valence-band photoemission study indicating the metallic nature of the 2x1 phase. Detailed analyses of the Si 2p core levels for the well-ordered 2x2, c(8x4), and 2x1 phases at 0.5, 0.75, and 1.0 ML, respectively, are given for the discussion of the Pb-induced Si surface reconstructions.