Mechanisms for NH_3 Decomposition on Si(100)-(2X1) Surface: a Quantum Chemical Cluster Model Study

摘要

In this paper, we present a detailed mechanism for the complete decomposition of NH_3 to NH_x(a) (x = 0-2). Our calculations show that the initial decomposition of NH_3 to NH_2(a) and H(a) is facile, with a transition-state energy 7.4 kcal mol~(-1) below the vacuum level. Further decomposition to N(a) or recombination-desorption to NH_3(g) is hindered by a large barrier of approx46 kcal mol~(-1). There are two plausible NH_2 decomposition pathways: 1) NH_2(a) insertion into the surface Si-Si dimmer bond, and 2) NH_2(a) insertion into the Si-Si backbond. We find that pathway (1) leads to the formation of a surface Si=N unit, similar to a terminal Si=N_t pair in silicon nitride, Si_3N_4, while pathway (2) leads to the formation of a near-planar, subsurface Si_3N nit, in analogy to a central nitrogen atom (N_c) bounded to three silicon atoms in the Si_3N_4 environment. Based on these results, a plausible microscopic mechanism for the nitridation of the Si(100)-(2X1) surface by NH_3 is proposed.