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.