Maximum Achievable N Content in Atom-by-Atom Growth of Amorphous Si–C–N

The maximum achievable N content in atom-by-atom growth of Si–C–N films is examined by combining ab initio molecular dynamics simulations in a wide range of compositions and densities with experimental data. When and only when the simulation algorithm allows the formation and final presence of N2 molecules, the densities leading to the deepest local energy minima are in agreement with the experiment. The main attention is paid to unbonded N2 molecules, with the aim to predict and explain the maximum content of N bonded in the amorphous networks. There are significant differences resulting from different compositions, ranging from no N2 at the lowest energy density of a-Si3N4 (57 atom % of bonded N) to many N2 at the lowest energy density of a-C3N4 (42 atom % of bonded N). The theoretical prediction is in agreement with the experimental results of reactive magnetron sputtering at varied Si+C sputter target compositions and N2 partial pressures. A detailed analysis reveals that while there is a relationship between the N2 formation and the packing factor, which is valid in the whole compositional range investigated, the lowest-energy packing factor depends on the composition. The results are important for the explanation of experimentally reported maximum N contents, design of technologically important amorphous nitrides and pathways of their preparation, prediction of their stability, and identification of what may or may not be achieved in this field.