Geometrical reconstructions and electronic relaxations of silicon surfaces. I. An electron density topological study of H-covered and clean Si(111)(1x1) surfaces

The relaxations of the first three interlayer distances in the H-covered Si (111)(1 x 1) surface were calculated using a fully periodic Hartree-Fock ap proach and a finely tuned slab model. All computed relaxations fall well wi thin the error bounds of the experiment, provided the relevant geometrical parameters and the basis set of the first layer Si atoms (Si1) are both opt imized. The quantum theory of atoms in molecules is applied on the wave fun ctions of Si bulk and of H-covered or clean Si(111)(1 x 1) slabs so as to s hed light on how the electronic perturbation caused by H adsorption and sur face formation propagates and dampens through the first Si atoms layers. In the H-covered surface, the large charge transfer from Si1 to H induces a n oticeable asymmetry in and strengthening of the surface Si1-Si2 back bonds, whereas in the clean slab the same bonds are found to be weakened compared to the bulk in agreement with the well-known tendency of this system to ev olve in favor of other reconstructions. The negatively charged hydrogen lay er in the Si(111)(1 x 1)-H slab is almost entirely counterbalanced by the f irst two silicon layers with the Si1 atoms bearing more than 94 percent of the compensating positive charge. The hydrogen and Si1 atoms in the H-cover ed surface polarize in such a way as to oppose the electric field created b y charge transfer into the surface double layer. The effect of H-coverage i s to reverse the outwards polarization of Si1 atoms present in the clean sy stem and to enhance its magnitude. Due to the surface electric field, the a tomic energies in both slabs are not found to converge towards bulk values even for the atoms of the innermost layers, although the other calculated l ocal and integrated properties exhibit an almost perfect convergence beyond the first two or three atomic layers. In the H-covered slab, the Si1 atoms have their interatomic surface completely isolated from the outside throug h their interaction with H atoms, while Si2 are found to be the only surfac e silicon atoms in agreement with the experimental observation that passiva nt substitution or oxidation are mediated by Si2 and never occur directly a t Si1 atoms. (C) 2000 American Institute of Physics. [S0021-9606(00)70202-9 ].