ABINITION CALCULATIONS OF SI, AS, S, SE, AND CL ADSORPTION ON SI(001) SURFACES

Optimal adsorption geometries and respective surface band structures for monolayers of group-IV to group-VII adatoms on semi-infinite Si(001) substrates have been calculated from first principles using a self-consistent total-energy scheme. The calculations are based on the local-density approximation and employ nonlocal, norm-conserving pseudopotentials together with Gaussian orbital basis sets. The semi-infinite geometry of the substrate is properly taken into account by employing our scattering theoretical method together with one-particle Green's functions. Chemical trends in the adsorption of Si, As, S, Se, and Cl monolayers on Si(001) are discussed. Our calculational scheme treats all these adlayers on equal footing. The clean Si(001)-(2 X 1) surface shows an asymmetric dimer reconstruction which is 0. 14 eV per dimer lower in energy than the respective optimal symmetric dimer configuration. An As adlayer gives rise to symmetric As dimer chains at the chemisorbed surface which run orthogonal to the former Si dimer chains at the clean surface. 'rhe substrate atoms in this As:Si(001)-(1 X 2) system reside close to the lattice sites of the Si bulk crystal Adsorption of a monolayer of group-VI adatoms is found to restore the substrate surface in its ideal, truncated bulk configuration and to passivate the surface. S or Se adlayer atoms are adsorbed in bridge positions above the surface in sites very close to the lattice positions of the continued bulk lattice and form S:Si(001)-(1 X 1) and Se:Si(001)-(1 X 1). A Cl adlayer, finally, gives rise to CI:Si(001)-(2 X 1). The adatoms adsorb on top of the dangling bonds of a (2 X l)-reconstructed substrate surface showing symmetric Si dimers. The results of our structure optimizations are in excellent agreement with a whole body of experimental data. The calculated electronic structure for Si(001)-(2 X 1) and for As:Si (001)-(1 X 2) shows very good agreement with angle-resolved photoemission spectroscopy (ARPES) data. Well-ordered S:Si(001)-(1 X 1) and Se:Si(001)-(1 X 1) surfaces have not been fabricated successfully, so far. Our calculated electronic surface band structure for Cl:Si(001)-(2 X 1) does not yet easily allow for an obvious interpretation of available ARPES data.