Structure and energetics of Si_nN_m clusters: Growth pathways in a heterogenous cluster system

We present a detailed study of the structures and energetics of Si_nN_m clusters with n + m≤6. We have determined the lowest-energy isomers of these clusters as a function of total cluster size and cluster stoichiometry. The properties of the low-energy isomers were calculated using an accurate, all-electron full-potential density-functional method at both the local density approximation (LDA) and the generalized gradient approximation (GGA) levels of theory. We found the most stable clusters by conducting an extensive phase space exploration for all the clusters containing up to 6 atoms, checking all bonding topologies and all possible atom type decorations. The search was done using a fast, but accurate, density-functional based tight-binding method. The calculations reveal several trends in the silicon-nitrogen binary cluster system. For N-rich clusters, linear or quasi-linear structures predominate, with strong multiple-bond character. Si-rich clusters favor planar or three-dimensional structures. Near the n = m stoichiometry the lowest energy isomers feature a strong alternation of Si and N atoms. Pairing of nitrogen atoms is unfavourable as is strong isolation of nitrogens. We use the results of the calculations to discuss possible growth pathways for the clusters.