Both the separate systems Pb/Si, Sn/Si show different chemical states of adsorbates in the identical T4 adsorption site on the Si(111)-7x7 surface. The surface atoms of the(√3x√3)R30° phase in the Pb/Si system are much more strongly displaced than the same Sn phase, forming the "one up two down" corrugation. The number of up and down atoms, that should correspond to low temperature 3x3 phase, is not, however, distinct. Co-adsorption of these elements was studied to understand the local atomic arrangement and electronic structure. The phase corrugation and the population of particular chemical states depending on the concentrations can offer a deeper insight into the local arrangement of the 2D alloy.
We studied the Pb,Sn/Si(111) system with photoemission at total coverage between 1/9 and 4/3 ML. An initial coverage of Sn (the more stable element) was kept constant and the amount of Pb (initial coverage ~1 ML) on this surface was varied by subsequent short annealing at ~400 °C. The analysis of Sn 4d, Pb 5d and Si 2p of photoemission data taken with synchrotron radiation shows the presence of Sn in three (chemical shift 0.4 and 0.6 eV), Pb in two (0.3 eV) and Si in three ( 0.37 and 0.47 eV) chemical states. At a Sn coverage of 1/3 ML, we found Pb adsorbed at two sites. One is the √3 position already occupied by Sn atoms. Tentatively, we called it the on-top position. The second chemical state is connected with a Pb atom in a hole surrounded by three atoms occupying the √3 position. The on-top position is more stable as Pb atoms in holes are first evaporated during the reduction of the Pb coverage. STM images show simple √3x√3 structures, but this technique does not distinguish between on-top and hole positions, and does not have chemical sensitivity. STS and photoemission valence band spectra exhibit similar features at the top of the valence band. They are dominated with one broad peak with maximum between -1 and -1.5 eV. The empty states spectra possess two maxima-first just above Ef and the second at 0.7 eV. In the first approximation, the electronic structure can be interpreted as an alloying effect. The stability of a Sn-Pb couple is discussed in term of DFT calculations.
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