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

Abstract

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 $c(8\ifmmode\times\else\texttimes\fi{}4)$ 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 $2\ifmmode\times\else\texttimes\fi{}1\ensuremath{-}\mathrm{Pb}$ 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 $2\ifmmode\times\else\texttimes\fi{}1$ phase. Detailed analyses of the Si $2p$ core levels for the well-ordered $2\ifmmode\times\else\texttimes\fi{}2,$ $c(8\ifmmode\times\else\texttimes\fi{}4),$ and $2\ifmmode\times\else\texttimes\fi{}1$ phases at 0.5, 0.75, and 1.0 ML, respectively, are given for the discussion of the Pb-induced Si surface reconstructions.