  We analyze point defect bulk and surface diffusion near the crystal-vacuum
interface and show that bulk diffusion is altered by atomic reactions at the
surface-vacuum boundary. A new atomic mechanism for point defect generation and
recombination is presented. In this mechanism, atomic steps are considered as
sources and sinks not only for adsorbed atoms and surface vacancies but also
for bulk native point defects -- vacancies and self-interstitials. We
demonstrate that bulk diffusion is coupled with the diffusion of adatoms and
surface vacancies at the surface via the atomic processes in the intermediate
subsurface layer, containing adsorbed self-interstitials and bulk vacancies.
The results show the existence of the fundamental relation between equilibrium
concentrations of the point defects in bulk and at the surface of the crystal.
