ta-C deposition simulations: Film properties and time-resolved dynamics of film formation

Ion beam deposition of carbon films was studied by molecular-dynamics simulations. Using an analytic hydrocarbon potential of Brenner with an increased C-C interaction cutoff value, 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 ranging from 100 to 900 K. The bulk properties of the computed tetrahedral amorphous carbon (ta-C) films as well as structure and roughness of their sp² -rich surface layers agree qualitatively with experiment. At low ion energies and low substrate temperatures, the sp³ fraction in the films increases with ion energy, resulting in a highly sp³ -bonded ta-C with a high compressive stress for E_ion >30 eV. This trend remains also at room temperature, however with lower sp³ content and stress.
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 . The calculated transition temperature T_c is a bit too low (T_c ~100 ^oC for E_ion =40 eV). For the ion energies E_ion ≤ 80 eV, the incidence atom is predicted to come to rest in the sp² -rich surface layer. A time-resolved analysis of the film formation shows that atom subplantation leads generally to a highly tetrahedral structure, but above T_c the kinetic energy of the atoms is sufficiently large to overcome the barrier in cohesive energy between ta-C and the more stable graphite-like films.