Si(111) 7©7 reconstruction: Strain in the adatom model

Abstract

The adatom model of the Si(111) 7©7 structure obtained by scanning tunneling microscopy, which is an experiment in real space, is extended to take into account the displacement of atoms. The 12 first-layer adatoms in the unit cell have an ${\mathrm{sp}}^3$-electron configuration; the 12 second-layer dangling-bond-site atoms have ${\mathrm{sp}}^2$; the second-layer dangling-bond-site atom at the center of the half unit cell has ${\mathrm{sp}}^{2.5}$. The adatoms shift upward by almost 50% of the layer distance. The dangling-bond-site atoms do not shift, and other second-layer atoms bonding with adatoms shift in the direction of the adatoms. The third-layer atoms shift by more than 25% of the layer distance. Changes in bond length are small and bond angles of surface-layer atoms change more than 10°; the strain energy is released by changing mainly bond angles. The atomic structure obtained gives quite revised spectra of ion scattering from the adatom model. The spectra are now close to those of the "pyramidal" model, which was previously asserted to be the best model explaining the result of another experiment in real space, ion scattering. Furthermore, a mechanism of the phase transition from the 7©7 structure to others by a coherent motion of adatoms is presented.