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

Depending on temperature, thermodynamically favoured structural relaxations in as-deposited tetrahedral amorphous carbon (ta-C) films can either essentially retain the content of sp³ bonded atoms or result in a transition to a graphitic phase with dominating sp² bonding. The structural relaxation occurs at low temperatures (less than ~ 600 °C) and is accompanied by reduction of the grown-in film stress. The most recent NMR spectroscopy investigation [1] shows that the structural changes in highly tetrahedral a-C films due to low-temperature annealing don't reduce to simple sp³-to-sp² or sp³-to-sp² conversions and have more complicated nature than it was previously believed. We present results of long-term simulated annealing studies of the relaxation processes in ta-C using molecular dynamics with a modified Brenner potential for C-C interactions and the structural models of as-deposited (unrelaxed) ta-C generated by an ion-beam film deposition simulation [2]. It is shown that 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 [1]. It is also found that the change of potential energy with annealing time can be fit by an exponential function, which enables to analyze the relaxation kinetics in as-deposited ta-C, in particular, to obtain the dependence of the relaxation time on annealing temperature.
[1] T.M. Alam, T.A. Friedmann, P.A. Schultz, D. Sebastiani, Phys. Rev. B 67, 245309 (2003)
[2] H.U. Jäger, A.Yu. Belov, Phys. Rev. B 68, 024201 (2003)