Atomic and electronic structure of the diamond (100) surface: Reconstructions and rearrangements at high hydrogen coverage

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Abstract
The atomic and electronic structure of the diamond (100) surface has been investigated theoretically via a semiempirical tight-binding model for a range of hydrogen coverages. Model parameters for C-C interactions have been taken from the work of Goodwin [J. Phys. Condens. Matter 3, 3869 (1993)], while new parameter sets have been determined for C-H and H-H interactions. The model gives results for the clean and monohydrogenated surfaces in good agreement with previous studies, but different features have been identified for higher H coverages. When the H coverage is sufficiently high, the substrate lattice is found to distort in order to reduce steric repulsions between dihydride units. As an important example, we obtain two structures for the dihydrogenated surface that are significantly more stable than those proposed previously. For H coverages intermediate between the monohydrogenated and dihydrogenated surfaces, stable geometries consisting of monohydrogenated dimer units and dihydride units are found. In contrast, geometries that include isolated monohydride units, such as have been previously investigated, are found to be thermodynamically and kinetically unstable. Tight-binding molecular dynamics is used to illustrate a mechanism for the rapid removal of isolated monohydride units. The electronic structures of the surfaces are described via the total and partial electronic densities of state, which are obtained directly from the tight-binding Hamiltonian. For the monohydrogenated surface and higher coverages, the stable geometries are found to yield no states in the bulk band gap.