DFT study of NH3 dissociation on Si(111)-7x7

SILVIA A. FUENTE, RICARDO M. FERULLO, NORBERTO J. CASTELLANI.

Bariloche

Conferencia; 13th International Conference on Solid Films and Surfaces; 2006

CNEA,; Instituto Balseiro

The study of NH3 interaction with Si(111)-7x7 surface is of interest in the development of silicon nitride films. NH3 molecule adsorbs dissociatively over a restatom (R)-adatom (A) pair site yielding NH2 and H fragments. In this work the adsorption of ammonia on Si(111)-7x7 surface was investigated within the density functional theory (DFT) using the hybrid B3LYP exchange-correlation functional as implemented in the software package Gaussian03. The silicon surface was represented by a Si22H22 cluster. The terminal Si atoms were saturated with hydrogens to eliminate spurious effects due to peripheral dangling bonds. At the Si surface, only the positions of R and A atoms were optimized. The rest of the geometric structure was kept fixed following the coordinates reported by Tong et al. [1]. The geometry of the ammonia molecules 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 calculations indicate that at the bare Si surface the triplet state is the most stable electronic configuration, corresponding to a state in which two electrons with the same spin are occupying A and R dangling bonds (DBs). Thus, the A-R pair in the free Si(111)-7x7 surface should be best regarded as a di-radical pair, in agreement with other DFT calculations [2]. NH3 adsorbs on R and A with adsorption energies of -0.82 and -0.43 eV, respectively, calculated as Eads = E(NH3/Si) – E(Si, triplet state) – E(NH3). The ammonia adsorption on a Si site induces an electron transfer from the DB of this site to the DB of the neighbour Si. Thus, the latter site (now with two paired electrons) is unable to adsorb a second NH3 molecule. However, if the second molecule interacts with a NH3 preadsorbed on R (A), the former couples strongly with the preadsorbed one (by means of an H-bond) realising -0.86 eV (-1.13 eV). The dissociative process was also studied considering one and two ammonia molecules. For the dissociative adsorption of a lonely NH3 molecule a dissociation barrier of 0.75 eV (0.28 eV) was calculated,  starting from the situation in which NH3 is adsorbed on R (A) and producing NH2 on R (A) and H on A (R). When two molecules are adsorbed, the NH3-NH3 interaction yields a weakening of a N-H bond of ammonia molecule adsorbed directly on silicon. As a consequence, the dissociation barrier decreases to 0.04 eV (0.005 eV).