Diffusion by bond hopping of hydrogen atoms on the Si(111)-7×7 surface

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Abstract
Site-specific adsorption and diffusion of atomic hydrogen on the Si(111)-7×7 surface at elevated temperatures are studied by scanning tunneling microscopy. Hydrogen atoms are found to adsorb preferentially on rest-atom sites rather than adatom sites with a binding-energy difference of ∼0.2 eV. The adsorption causes a reverse charge transfer from rest atoms back to adatoms. Above ∼280 °C, atomic hopping between two rest-atom sites within a half-cell can occur which is mediated by an adatom site. Above ∼330 °C, H atoms start to hop across the cell boundary via two adatom sites, or they can diffuse across the surface. The activation barrier for hopping from a rest-atom site to a corner adatom site is ∼50 meV lower than that to an edge adatom site. Thus, in cross boundary jumps, they hop preferentially via two corner adatom sites. From Arrhenius plots, the hopping barriers within the cell and across the cell boundary are determined. The hopping paths, relative binding energies, and site selectivity of hydrogen atoms on the Si(111)-7×7 surface agree in general with theoretical results, but our result are both site and path specific. The dynamic behavior of two to three H atoms inside a half-cell is also investigated.