Ab initio cluster calculations of the chemisorption of hydrogen on the Si(111)√3x√3R30°-B surface

First principles all-electron Hartree-Fock and density functional theory cluster calculations have been performed to investigate the chemisorption of atomic hydrogen on the Si(111)root 3 x root 3R30 degrees-B surface. Both the equilibrium geometries corresponding to n hydrogen atoms (1 less than or equal to n less than or equal to 5) chemisorbing on the B-S-5 and B-T-4 reconstructed structures, and the desorption energies for a silicon or boron adatom bonded to x hydrogen atoms (0 less than or equal to x less than or equal to 3), have been obtained. As successively more hydrogen is chemisorbed, a silicon or boron adatom is found to move from its original threefold site to an adjacent bridge Site, and then to a neighbouring on-top site. It is also found that boron will most likely occupy a subsurface substitutional S-5 site at low hydrogen Coverages (less than or equal to 0.67 ML), but appear as an adatom at an on-top site directly above one of the first-layer silicon atoms for hydrogen exposures higher than 0.67 ML. This boron segregation at high hydrogen exposures prevents the formation of SiH2 and SiH3 complexes and leads to the prediction that only SiH and BH2 will be observed on an Si(111):B hydrogenated surface. It also provides an explanation for the lack of Si surface etching at high hydrogen exposure. All of these results are in good agreement with the available experimental data on hydrogenated B/Si(111) surfaces. (C) 1997 Elsevier Science B.V.