First-principles Molecular-dynamics Simulations of Ammonia Adsorption on Si(001) Surface.

Abstract

Adsorption processes of NH3 on Si (001) -2×1 surface have been investigated by the first-principles molecular-dynamics simulations. The total energy calculation shows that NH3 adsorbs molecularly on the cave site with the adsorption energy of 5.89eV and the relaxation of surface Si dimers is crucial in determining the adsorption eitergy. The optimized configuration of the adsorbed NH3 is found at 1.86Å above the topmost Si surface with the N-H bond length of 1.32Å and the H-N-H bond angle of 92.0°.The dissociative adsorption process of NH3 is also studied. The decomposition process of NH3 to form NH2 and H has the activation barrier of at most 0.4 eV. After the decomposition, however, both NH2 and H adsorb to the Si surface dangling bonds without activation barrier. The dissociative adsorption is more stable than the molecular adsorption of NH3; the total energy of the former is 7.37eV, while that of the latter is 5.89eV.