Formation and evolution of sp² clusters in amorphous carbon networks as predicted by molecular dynamics annealing simulations

A comparative study of clustering of sp² -bonded atoms in the as-deposited and annealed tetrahedral amorphous carbon (ta-C) films is presented. The as-deposited ta-C with a grown-in compressive stress of ~10 GPa is modelled using amorphous networks generated by ion-beam film deposition simulations with a modified hydrocarbon potential of Brenner. The ta-C networks were annealed in the temperature range of 600-1200 K, using molecular-dynamics with the same interatomic potential. The size and type of the sp² clusters were analysed as a function of the annealing temperature as well as the simulation parameters for the stress and temperature control. An essential finding of this study is that at the density less than 3.0 g/cm³ the structure of ta-C can become unstable with respect to formation of large sp² clusters. The influence of the sp² clustering on the film stress is also discussed.