  In this paper we use density-functional theory calculations to analyze both
the stability and diffusion of Cu adatoms near and on the H-passivated Si(001)
surface. Two different Cu sources are considered: depositing Cu from vacuum,
and contaminating Cu outdiffusing from bulk Si. Deposited Cu from vacuum
quickly moves subsurface to an interstitial site in the third Si layer (T2).
Once there, Cu adatoms enter a subsurface zigzag migration route between T2 and
another subsurface site, T2->HSL->T2, along the dimer row direction.
Contaminating Cu outdiffusing from bulk is found to be a fast diffuser along
both parallel and perpendicular directions to the dimer row when far from the
surface. It is attracted to the layers close to the surface and becomes trapped
at an interstitial site located at the sixth Si layer (T3). As the outdiffusing
Cu atoms get closer to the surface, a channeling zigzag diffusion along the
dimer row direction, similar to that one followed by deposited Cu from vacuum,
is favoured over diffusion along the perpendicular direction. These results are
consistent with previous experimental work done on similar systems and will
motivate further experiments on the interesting interaction between Cu and Si
surfaces.
