Scanning tunneling microscopy andab initiocalculations:c(4×8)reconstructions of Pb on Si and Ge(001)

Abstract

The $c(4\ifmmode\times\else\texttimes\fi{}8)$ reconstructions induced by Pb atoms on Si(001) and Ge(001) surfaces have been studied by scanning tunneling microscopy (STM) and first-principles total-energy calculations. Two different structures were found experimentally. In both cases, there is a coexistence of Pb and Si(Ge) dimers either in perpendicular and/or in parallel orientations. The $c(4\ifmmode\times\else\texttimes\fi{}8)\ensuremath{-}\ensuremath{\alpha}$ phase is similar to the structure previously reported for Pb on Si and Ge(001) and Sn on Si(001). It is formed by three rows of asymmetric Pb dimers forming chains along the [110] direction, and it corresponds to a coverage of 0.75 monolayer (ML). In the second structure, the $c(4\ifmmode\times\else\texttimes\fi{}8)\ensuremath{-}\mathrm{l}$ phase, the middle Pb dimers are replaced by Si(Ge) dimers resulting in a Pb coverage of 0.5 ML. The agreement between bias-dependent STM images and calculated local density of states for the two structures is excellent. Our calculations show that the $c(4\ifmmode\times\else\texttimes\fi{}8)\ensuremath{-}\ensuremath{\alpha}$ structure is generally the most stable configuration for Si and Ge surfaces covered by Pb. However, if there is a deficit of Pb atoms, the $c(4\ifmmode\times\else\texttimes\fi{}8)\ensuremath{-}\mathrm{l}$ structure becomes more stable for Pb on Si(001).