ORIGIN OF BUCKLING-DIMER-ROW FORMATION OF SI(001) SURFACES

Structural transformations of dimer rows on 2x1-reconstructed Si(001) surfaces have been investigated with ab initio quantum theoretical cal culations. It has been revealed that the adsorption of Li or H atom in duces the buckling of the dimer at the adsorbed site, and this bucklin g propagates along the dimer-row to produce a zigzag dimer row structu re. There appear extra filled- and empty-state band structures near th e Fermi level, which consist of occupied and unoccupied molecular orbi tals localized on the upper- and lower-side Si atoms of buckled dimers , respectively. Scanning tunneling microscopy images obtained in a zig zag dimer row on Si(001) surfaces have been demonstrated to reflect th e spatial distributions of electrons in these extra bands. The origin of the buckling propagation is explained by electron distribution on t he surface, and dimer-row structures at low or room temperature have a lso been discussed.