Relaxation kinetics in amorphous carbon films: An insight from atomic scale simulation

Using molecular-dynamics (MD) techniques and a hydrocarbon potential of Brenner with an increased C-C interaction range, we investigate kinetics of relaxation processes in tetrahedral amorphous carbon films (ta-C) with high sp³ fractions. The structural models for as-grown non-equilibrium ta-C networks were generated using MD film deposition simulation with ion energy of 80 eV. The results of MD simulations of low temperature annealing show that both film stress and energy behave with annealing time t as a+bexp(-t/t₁ ). This exponential dependency is interpreted in terms of structural transformations in the amorphous carbon networks. It is also shown that the relaxation times for the energy and stress do not coincide, indicating that some atomic rearrangements in the networks may reduce their energy without changing the stress. At the annealing temperature of 1000 K, the relaxation constant t₁ for energy was found to be in the range of 0.5-0.7 ns, depending on the simulation parameters. It considerably exceeds an estimate of 10 ps for the thermalization stage of the subplantation model for ta-C film formation from energetic C ions. A decrease in the ta-C energy during the first 10 ps of the simulated annealing is much larger than that predicted by the exponential law with the relaxation constant t₁ .