Ab initiocluster-model study of the on-top chemisorption of F and Cl on Si(111) and Ge(111) surfaces

The interaction of atomic F and Cl with ${\mathrm{Si}}_4$ ${\mathrm{H}}_9$ and ${\mathrm{Ge}}_4$ ${\mathrm{H}}_9$ cluster models has been studied by using ab initio pseudopotentials and basis sets of increasing complexity. The results show that the effect of d orbitals is important in order to reproduce the experimental findings. However, the use of polarization functions in the atoms which are directly involved in the chemisorption bond leads to results which are very close to those obtained using extended basis sets. The local nature of the chemisorption bond is also interpreted by means of a Mulliken population analysis. For F-${\mathrm{Si}}_4$ ${\mathrm{H}}_9$ and Cl-${\mathrm{Si}}_4$ ${\mathrm{H}}_9$ the present results are in good agreement with previous ab initio all-electron calculations, and for the chemisorption of Cl on Si(111) and Ge(111) surfaces, good agreement is found with respect to the available experimental results as well as with previous slab calculations based on the local-density-functional formalism.