"NH3 dissociation on Si (111)-7x7 from density functional theory"

R. M. FERULLO, S.A. FUENTE, G.F. CABEZA, N.J. CASTELLANI

San Carlos de Bariloche-Argentina

Conferencia; FyQS I Encuentro de Física y Química de Superficies.; 2004

Centro Atómico Bariloche e Instituto Balseiro

                The study of NH3 interaction on Si(111)-7x7 surface is of interest in the development of silicon nitride films. NH3 molecule adsorbs dissociatively over a restatom-adatom pair site yielding to NH2 and H fragments. Despite the large number of experimental species are bonded. In this work the interaction of an ammonia molecule with a Si(111)-7x7 surface model was represented using a Si22H22 cluster. The terminal Si atoms were saturated with hydrogens to eliminate spurious effects due to the dangling bonds. At the Si surface only the position of the restatom (R) and the adatom (A) was optimized. The rest of the geometric structure was kept fixed following the coordinates reported by Tong et. al. [1]. The geometry of the adsorbed NH3 was fully optimized. The molecular orbitals were expanded using the 6-31G basis set and polarization functions were added to those atoms directly involved in the geometrical optimization. The results indicate that the NH3 molecule prefers to adsorb on R. Indeed, the calculated adsorption energies on R and on A are -1.36 and -0.98 eV, respectively. The distances between a Si active site and its nearest neighbours decrease around 2% when the adsorption occurs on R and around 9% when it occurs on A. The NH3 molecule dissociates surpassing a barrier or 0.75 and 0.28 eV from R and from A, respectively. The final state with NH2 and H species adsorbed on R-A pair is stabilized at nearly 3 eV with respect to the separated systems NH3 + Si.