A density-functional-theory study of atomic nitrogen abstraction from Si(100)-(2 x 1) by a gaseous O(3P) atom

We have employed density-functional theory (DFT) to investigate the abstraction of a nitrogen atom from the Si(100)-(2 x 1) surface by a gas-phase O(3P) atom for different initial bonding configurations of nitrogen at the surface. For the N-Si(100) structures investigated, nitrogen abstraction by an O(3P) atom is predicted to be exothermic by at least 1.9 eV. Abstraction in a single elementary step is found only for the interaction of an O(3P) atom with nitrogen bound in a coordinatively saturated configuration, and an energy barrier of 0.20 eV is computed for this reaction. For nitrogen bound in coordinatively unsaturated configurations, abstraction is predicted to occur by precursor-mediated pathways in which the initial O-surface collision results in the formation of a N-O bond and the concomitant release of between 2.7 and 4.8 eV of energy into the surface, depending on the initial N-Si(100) structure. This initial step produces different surface structures containing an adsorbed NO species, which can then undergo a series of elementary steps leading to NO desorption. Since the barriers for these steps are found to be less than 1 eV in all cases, a significant excess of energy is available from initial N-O bond formation that could activate NO desorption within no more than a few vibrational periods after the initial gas-surface collision. Nitrogen abstraction by such a pathway is essentially an Eley-Rideal process since NO desorption occurs rapidly after the initial gas-surface collision, without the reactants thermally accommodating with the surface. These computational results indicate that nitrogen abstraction by gaseous O(3P) atoms should be facile, even at low surface temperatures, if nitrogen is bound to the Si(100) surface in coordinatively unsaturated configurations.