Theoretical Investigation of the Reactivity of Halogenated and Hydrogenated Si(100)-2x1 Surfaces toward NH3"

F. A. SORIA; E. M. PATRITO; P. PAREDES-OLIVERA

Niza, Francia

Congreso; The 61st Annual Meeting of the International Society of Electrochemistry; 2010

Theoretical Investigation of the Reactivity of Halogenated and Hydrogenated Si(100)-2x1 Surfaces toward NH3.   F. A. Soriaa, E. M. Patritob, P. Paredes-Oliveraa   aDepartamento de Matemática y Física and bDepartamento de Fisicoquímica – INFIQC. Facultad de Ciencias Químicas. Universidad Nacional de Córdoba. Ciudad Universitaria. (5000) Córdoba. Argentina.   E-mail: patricia@fcq.unc.edu.ar The adsorption and reaction of ammonium with silicon surfaces is of great importance because surface Si-NH2 groups are precursors for the synthesis of thin films of silicon nitride. Traditionally, the synthesis of Si-NH2 groups has been performed under UHV conditions on Si(100)-2x1 [1]. However, the development of procedures [2] to chlorinate the 100 surface of silicon, allowed the manipulation of this surface in chemistry laboratories, out of expensive UHV chambers. When the Cl-Si(100)-2x1 surface is exposed to NH3, a partial amination occurs [3]. However, ammonium does not react on the hydrogenated surface, H-Si(100)-2x1, despite the fact that the Si-H bond is weaker than the Si-Cl bond. In this work we performed a comparative investigation of the reaction pathways between ammonium and the hydrogenated and halogenated (Cl and Br) by means of density functional theory calculations performed on silicon slabs exposing the 100 face with the 2x1 reconstruction. The reaction mechanism involves different number of elementary steps on both types of surface. On the hydrogenated surface there are two elementary steps whereas on the chlorinated surface four elementary steps have been identified. The activation energy barriers explain the different reactivities. On the hydrogenated surface the activation barrier is 38.7 kcal/mol whereas on the chlorinated surface, the highest activation barrier is 14 kcal/mol. The reaction of ammonium on the brominated Si(100)-2x1 surface follows a similar mechanism to that on the chlorinated surface. However, the activation energy barriers are higher than on the chlorinated surface due to steric effects. The highest barrier on this surface is 22.7 kcal/mol.  This shows that the reactivity of the Si(100)-2x1 surface may be tuned depending on the type of halogenating procedure.   [1] H. Yu, L. J. Webb, R. S. Ries, S. D. Solares, W. A. Goddard, J. R. Heath, N. S. Lewis, J. Phys. Chem. B 109 (2005) 671. [2] K. T. Queeney,  Y. J. Chabal, K. Raghavachari, Phys. Rev. Lett., 86 (2001) 1046. [3] C. C. Finstad, A. G.  Thorsness, A. Muscat, Surf. Sci., 600 (2006) 3363.