CALCULATION OF THE BEHAVIOR OF SI AD-DIMERS ON SI(001)

The modified embedded atom method (MEAM) is used to investigate the ge ometry and energetics of Si ad-dimers and adatoms on the (2 x 1) recon structed 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 dime r 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 exper iment. 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 laye r by 0.65 and 0.47 eV relative to bulk substitutional Ni atoms. The ef fect of these Ni impurities on the ad-dimer stability is predicted. A parallel ad-dimer is repelled by a substitutional Ni impurity by 30 me V compared with the 20-35 meV observed in experiment for an unknown su bstrate defect.