Basic processes in deposition and annealing of ta-C films as predicted by molecular-dynamics simulations


Basic processes in deposition and annealing of ta-C films as predicted by molecular-dynamics simulations

Jäger, H. U.; Belov, A. Y.

The results of classical molecular-dynamics simulations of amorphous carbon films are reviewed. 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 Eion=10-80 eV and for a wide range of substrate temperatures. The thermodynamically favoured structural transformations in these films during post-deposition annealing at low temperatures (less than ~600 °C) were carefully examined. The approach used describes quite accurately the properties 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.

The film deposition simulations revealed a short-term temperature-dependent relaxation stage (t~70-1000 fs), where the film formation is considerably influenced by substrate temperature Ts. During this stage, depending on Ts, the carbon atoms coming to rest at metastable highly coordinated sites can relax to 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 Ts exceeds a critical temperature Tc. The as-deposited ta-C films contain many small clusters (mostly pairs) of sp2 bonded atoms, but some extended sp2 networks are also found in the films. The ring statistics within these networks will be presented.

During low-temperature annealing the potential energy and stress of the as-deposited ta-C films are released with only minor changes in the short-range order and density. It is demonstrated that as a result of annealing the volumes of sp3 atoms decrease, which is consistent with experiment. The change of potential energy with annealing time can be fit by an exponential function, which enables to analyse the relaxation kinetics in ta-C, in particular, to obtain the dependence of the relaxation time on annealing temperature.

  • Invited lecture (Conferences)
    5th Specialist Meeting on Amorphous Carbon,ITC-irst, Sept. 9-10, 2004, Povo-Trento, Italy

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Publ.-Id: 6066