By using ab initio total-energy calculations, a study was made of the adsorption and diffusion of Ag atoms on a dimer-reconstructed Si(001) surface. For a single Ag adsorption, the 2-fold-coordinated cave site above the fourth Si layer atom was found to be the most stable; in agreement with previous work. Inspection of the electronic structures at the cave site revealed that the Ag-Si bonds originated from low-lying 4d electrons and were covalent. The calculations also revealed another stable adsorption at the pedestal site that was slightly higher by 0.03eV in energy than the cave site. Further potential-energy-surface calculations showed that the surface diffusion of a single Ag adatom was unexpectedly highly isotropic and that the energy barrier was 0.5eV. When more Ag adatoms were adsorbed on the Si surface, the Ag adatoms were expected to form dimers. Actually, an energy gain of 0.36eV/dimer was obtained through the dimerization. The diffusion of an Ag dimer was also investigated. Surprisingly, a very rapid surface-dimer diffusion was found; with an energy barrier of 0.48eV. This was slightly lower than that of the single Ag adatom. In contrast to the diffusion of the single Ag adatom, the dimer diffusion was anisotropic and preferably occurred along the valley between Si dimer rows by concerted flip-flop processes.

Ab initio Study of Adsorption and Diffusion of Ag Atoms on a Si(001) Surface. K.Kong, H.W.Yeom, D.Ahn, H.Yi, B.D.Yu: Physical Review B, 2003, 67[23], 235328 (7pp)