Time-resolved dynamics of ta-C film deposition as predicted by molecular-dynamics simulations

The temporal pattern for ion beam deposition of amorphous carbon films is presented on the basis of molecular-dynamics simulations. An analytic interatomic potential of Brenner was adopted, but with an increased C-C interaction range. Deposition of films with a thickness of up to 10 nm was simulated for ion energies E_ion =10-80 eV and for substrate temperatures T_s =100-900 K. The approach used describes quite accurately the properties (including sp² clustering) of highly tetrahedral amorphous carbon (ta-C) films, overestimating, however, density of graphitic films, since the potential does not account for the long-range repulsion between non-bonded pi-orbitals.

A time-resolved analysis of atomic trajectories from the film deposition simulations revealed a short-term temperature-dependent relaxation stage (t~70-1000 fs), where the film formation is considerably influenced byT_s . During this stage, depending on T_s , the carbon atoms at metastable highly coordinated sites can relax into either three- or fourfold coordinated positions. In agreement with experiment the molecular-dynamics simulations predict a sharp (within the range of about 50 K) transition from ta-C to graphitic carbon as T_s exceeds a critical temperature T_c . The behaviour of the sp³ content, density and the film stress near the transition temperature is discussed. For super-critical substrate temperatures (T_s > T_c ), the kinetic energy of the atoms is high enough to overcome the barrier in cohesive energy between a diamond-like and graphite-like film region. As a result, the relaxation processes lead finally to the energetically more favoured graphitic amorphous network. The diamond-like network remains stable in the case of deposition at sub-critical substrate temperatures (T_s <T_c ).