Core-level photoelectron study of Si(1 1 1) √7x√3-(Pb, Sn) surface

The Sn 4d and Pb 5d core-level photoelectron spectra have been studied in order to clarify their bonding properties and atomic arrangement on a Si(1 1 1) √7x√3-(Pb, Sn) surface, which is formed by the coadsorption of 0.4 ML Pb and 0.4 ML Sn and shows two kinds of bright spots in the scanning tunneling microscopic (STM) images: (A) those aligned zigzag on the T1 site and (B) those on the T1 and H3 sites along the [1 1 -2] direction. The Pb 5d spectrum shows a single spin-orbit-split feature with weak tailing towards the high binding energy side, while the Sn 4d spectrum exhibits shoulder structures at the high binding energy side of the main peaks. This definitely indicates at least two different Sn-Si bonds or inequivalent Sn adsorbing sites and single bond or site for Pb. Thus the spots A at the T1 site and those B at the T1 and H3 sites in the STM images are ascribed to Pb and Sn adatoms, respectively. The formation process of this surface will be also discussed in terms of the core-level spectra