Simulation of relaxation processes in amorphous carbon films

The atomic mechanisms of relaxation processes in carbon films during ion beam deposition and post-deposition annealing are studied using the method of molecular-dynamics with a modified hydrocarbon potential of Brenner. Deposition of films was simulated for ion energies E_ion =10-80 eV and for substrate temperatures T_s =100-900 K. Using a time-resolved analysis of atomic trajectories from the film deposition simulations, a short-term temperature-dependent relaxation stage (t~70-1000 fs), where the film formation is influenced by T_s , was identified. During this stage, depending on T_s , the carbon atoms at metastable four- or fivefold coordinated sites can relax into either three- or fourfold positions, giving rise to graphitic or tetrahedral (ta-C) amorphous carbon films, respectively. In agreement with experiment the simulations predict a sharp transition from ta-C to graphitic carbon as T_s exceeds a critical temperature T_c . Simulating post-deposition annealing, low-temperature structural relaxation of as-deposited ta-C was observed as evidenced by a reduction of potential energy and grown-in stress. The corresponding amorphous network rearrangements consist both in the sp³ -to-sp² conversion and in the
sp² -to-sp³ one. A nearly complete stress relief in ta-C with the sp³ content of ~80% and grown-in compressive stress of 11 GPa was simulated at an annealing temperature of ~1000 K.