First-principles study of the self-organization mechanism of NH$_{3}$ on Si(001)

We have investigated the self-organization of NH$_{3}$ molecules on the Si(001) surface using the first-principles pseudopotential calculations. In order to find out the adsorption pathways and understand the mechanism of self-organization, we have calculated the potential energy surfaces of an incoming NH$_{3}$ molecule with one pre-adsorbed NH$_{3}$ molecule. Based on the results, we propose a kinetic process model of NH$_{3}$ self-organization: (i) the incoming molecules are attracted towards the pre-adsorbed molecules due to the H-bonding interaction. (ii) By forming the H-bond with the pre-adsorbed molecule, an incoming molecule can achieve physisorption states. (iii) Subsequently, the physisorbed NH$_{3}$ molecule is attracted to adjacent ``down'' Si atoms to complete the molecular adsorption process. (iv) Finally, the adsorbed NH$_{3}$ dissociates into NH$_{2}$ and H fragments. The resultant self-organized pattern is in accordance with recent STM experiments. However, it is in stark contrast with the energetically favored pattern that is characterized by H-bond formation between the dissociated fragments. This indicates that the self-organization of NH$_{3}$ on Si(001) is governed by the kinetics rather than the energetics.