Photoemission study of two dimensional phase transitions on the Pb/Si(1 1 1) surface

We have studied the low coverage, reconstructed phases of the Pb/Si(1 1 1) adsorption system by photoelectron spectroscopy. With decreasing coverage from 1.3 ML, the top of the valence band shows a transition from a metallic surface to a semiconducting surface, which is stable below a coverage of 0.2 ML. The Pb 5d core level was fitted by one symmetric peak for coverage from 0.1 to 0.2 ML without any observable shift. This state corresponds to Pb adatoms in a two dimensional surface alloy with Si where both atoms are located in the T4 position, and the surface reconstruction is (√3×√3)R30°. The peak energy and profile are independent of the relative Pb and Si adatom concentrations in the surface. For higher coverages we observe a varying width of the Pb 5d core level due to the appearance of an extra state shifted by about 0.4 eV; the results are consistent with information from STM images. Core level shifts of the Si 2p level are also reported.