Anisotropic diffusion of hydrogen atoms on the Si(100)-2×1 surface

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Abstract
This paper presents first-principles total-energy calculations of hydrogen-adatom diffusion on a Si(100)-2×1 reconstructed surface. The transition states for hydrogen-atom-diffusion pathways were established by mapping out the potential energy of a hydrogen atom jumping between the dangling bonds of a Si(100)-2×1 surface modeled by embedded finite silicon clusters. The diffusion barriers are high (2–3 eV) and wide (∼3–4 Å), suggesting that H-atom diffusion on Si(100) proceeds via mostly a classical hopping mechanism instead of tunneling. Furthermore, diffusion of hydrogen atoms is predicted to be anisotropic, being preferentially directed parallel to the silicon-dimer rows, with an activation energy of 2.0 eV. Higher activation energies of 2.5 and 2.7 eV are predicted for diffusion perpendicular to dimer rows, for the cases of hydrogen atoms hopping from one dangling bond to a neighboring dangling bond on the same dimer and on an adjacent dimer, respectively. The mechanism for H-atom diffusion along dimer rows is markedly different from that previously proposed for Si-adatom diffusion on Si(100): H atoms are predicted to diffuse along edges of the dimer rows rather than down the middle.