Calculation of the behaviour of Si ad-dimers on Si(001)

The modified embedded atom method (MEAM) is used to investigate the geometry and energetics of Si ad-dimers and adatoms on the (2 x 1) reconstructed Si(001) surface. Site stability and saddle point energies are predicted for a large number of possible mechanisms. The calculations predict the relative stability of parallel and normal ad-dimers to be 80 meV, in excellent agreement with 60+/-10 meV from experiment, the barrier to rotation of these dimers to be 0.66 eV compared with 0.68+/-0.01 eV, and the activation energy for diffusion parallel to the dimer rows to be 0.74 eV compared with 0.94+/-0.09 eV. These calculations are the first of any kind to predict quantitative agreement with experiment. We also predict substitutional Ni atoms near the Si(001)-(2 x 1) surface are bound to the second and first layer below the dimer layer by 0.65 and 0.47 eV relative to bulk substitutional Ni atoms. The effect of these Ni impurities on the ad-dimer stability is predicted. A parallel ad-dimer is repelled by a substitutional Ni impurity by 30 meV compared with the 20-35 meV observed in experiment for an unknown substrate defect.