AVS 45th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS1-TuA

Paper SS1-TuA6
Binding Energies of Hydrogen on Flat and Vicinal Silicon Surfaces

Tuesday, November 3, 1998, 3:40 pm, Room 308

Session: Semiconductor Surface Chemistry
Presenter: M.B. Raschke, Max-Planck-Institut für Quantenoptik, Germany
Authors: M.B. Raschke, Max-Planck-Institut für Quantenoptik, Germany
U. Höfer, Max-Planck-Institut für Quantenoptik, Germany
Correspondent: Click to Email
Despite the importance of hydrogen interaction with silicon in many technological applications and its role as a model system for chemisorption on semiconductor surfaces, the binding energy of hydrogen on silicon surfaces is not well known. In a novel experimental approach, a bakeable UHV-quartz apparatus was used to establish thermal equilibrium between molecular hydrogen and well-defined Si(111) and Si(001) surfaces at gas pressures of 10@super -5@ 1 mbar and temperatures of 770 - 980 K. Under these conditions, continuous dissociative adsorption and recombinative desorption of H@sub 2@ lead to an equilibrium hydrogen coverage on the surfaces which is measured sensitively using optical second-harmonic generation (SHG). The resulting adsorption isotherms allow for the accurate determination of the hydrogen binding energies and their dependence on surface coverage. In the case of H/Si(111)7x7, the binding energy increases from 2.9 eV to 3.1 eV in the investigated coverage range between 0.05 ML and 0.3 ML. The Si-H bond is thus found to be considerably weaker than hydrogen bonding in silanes (3.7-3.9 eV). The values are compatible with a large barrier (0.9 eV) for phonon-assisted sticking of H_2/Si.@footnote 1@ They are in good agreement with theoretically predicted Si-H bond strengths at the adatoms and restatoms of this surface.@footnote 2@ Exploiting substantial differences in reactivity between the step and terrace sites towards molecular hydrogen, equilibrium and non-equilibrium hydrogen coverages on vicinal Si(001) could also be investigated. From measurements for various surface temperatures on samples with different angles of miscut it is inferred that the steps are thermodynamically favored by approximately 0.2 eV. @FootnoteText@ @footnote 1@P. Bratu and U. Höfer, Phys. Rev. Lett. 74 (1995) 1625. @footnote 2@H. Lim, K. Cho, I. Park, J. D. Joannopoulos, and E. Kaxiras, Phys. Rev. B 52 (1995) 17231