Submonolayers of Lead on Silicon (111) Surfaces: An X-Ray Standing Wave Analysis
Systematic studies of adsorbed atoms on silicon surfaces have greatly contributed to our present understanding of the structures and the mechanisms of interface formation. In the case of the group III metals Al, Ga and In deposition of 1/3 monolayer (ML) on the Si(111) 7×7 surface produces √3×√3 R30° (denoted by √3×√3 from now on) reconstruction. Although several models were proposed to explain this reconstruction recent theoretical studies indicate /l/ that the eclipsed (T4) model in which the metal atom is placed in the threefold-symmetric site above a second-layer Si atom has the lowest total energy if substrate relaxation is included and provides the best agreement with photoemission experiments. When group IV atoms are deposited on the Si(lll) 7×7 surface several different reconstructions (e.g., 5×5, 7×7, c(2×8), l×l, √3x√3) are found. For the 5×5 and 7×7 reconstructions the dimer-adatom stacking-fault model proposed by Takayanagi et al. is generally accepted whereas the c(2×8) structure is best explained by the adatom model. By analogy with the group III metals it might be expected that the √3×√3 structures produced by 1/3 ML Sn or Pb on Si(lll) could be explained by the threefold eclipsed model. In the present study we have applied the x-ray standing wave (XSW) technique to the Si(lll) √3×√3 -Pb system in order to determine the position of the Pb atoms relative to the bulk Si lattice.
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