The diffusion of Ge into sub-surface layers of (100) was studied by means of Auger electron diffraction measurements. The latter revealed Ge in the fourth layer, after sub-monolayer growth at as low as 500C. Density functional theory predictions of equilibrium Ge sub-surface distributions were consistent with the measurements. A surprisingly low energy pathway was identified, which resulted from a low interstitial formation energy in the third and fourth layers. The migration path involved the hopping of an adatom on top of a dimer row, and then displacement of the adatom down between 2 surface dimers to a dumb-bell interstitial geometry perpendicular to the surface dimers. It here shared a third layer site with a lattice atom. One of these atoms pushed an atom out of the fourth layer, thus forming a dumb-bell parallel to the surface dimers; with an energy of 1.8eV relative to the adatom. One of these atoms could jump further down into the fifth layer, to form a tetrahedrally coordinated interstitial. This configuration was about 2.2eV above the adatom in energy. The barrier to the formation of the interstitial in the fourth layer was 2.2eV, with respect to the Si adatom. The overall barrier for Ge diffusion down to the fourth layer was estimated to be 2.1eV, after adding 0.3eV for Si/Ge exchange. Doping significantly affected the formation energy. Thus, the stability of the fifth-layer interstitial, as compared with the adatom, went from 2.6eV at a charge of -2 to 1.6eV at a charge of +2. The insertion of B, with the associated loss of an electron, had a similar effect, and stabilized the interstitial by 0.5eV as compared with pure Si.

Diffusion of Ge below the Si(100) Surface - Theory and Experiment B.P.Uberuaga, M.Leskovar, A.P.Smith, H.Jónsson, M.Olmstead: Physical Review Letters, 2000, 84[11], 2441-4