Ab initio HF/DFT studies of the chemisorption of hydrogen on the cluster simulated Si(111)-(√3x√3)R30°-Al and -Ga surfaces

The chemisorption of atomic hydrogen on the Si(111)-root 3 x root 3)R30 degrees-Al and -Ga surfaces has been studied by first-principles all-electron Hartree-Fock/density functional theor) (DFT) cluster calculations. It has been found that the hydrogen chemisorption behaviour on the Ga terminated cluster modelled surface is similar to that for the Al terminated cluster. Hydrogen atoms chemisorbing near an adatom site are found to cause the breaking of Si-Al (or Si-Ga) bonds and the formation of H-Si bonds. The aluminium (or gallium) adatoms move from their original threefold sites to the adjacent bridge sites (one hydrogen atom), and then to the neighbouring on-top sites (two hydrogen atoms). Calculations of the binding energies have shown that aluminium or gallium adatoms occupying the bridge and on-top sites may be able to migrate on the real surface. This suggests the possible formation of Al (or Ga) islands. together with a local (1 x 1)H surface reconstruction. further exposure to hydrogen does not cause any further structural transformations and supports the (root 3 x root 3)-(1 x 1) phase transition. (C) 1998 Elsevier Science B.V.