Structure and missing-dimers probability distribution of the (2xn) Bi-induced Si(001) surface

Bismuth adsorbed on Si(001) induces successive (2×n) phases after annealing, with n ranging from 12 to 5. The structure consists of dimer rows, with missing-dimer defects forming and ordering in the perpendicular direction. We investigated by grazing-incidence X-ray diffraction the n=6.45 surface, which results from missing-dimer line ordering each 6 or 7 units. The structural refinement is based either on a (2×6) or a (2×7) cell. The Bi dimers are lying at 1.86A° on top of Si, with a dimer bond length of 3.11A° . They are displaced along the row towards the missing dimer, while the Si atoms are at bulk positions. A photoemission study of the Bi 5d and Si 2p core levels confirms the model of symmetric Bi dimers, after breaking Si dimers. The diffracted intensity is calculated using the phase-matrix method. The positions and widths of the peaks are analysed in terms of the dimer blocks probability distribution.