Atomic scale simulation of the stress relief in tetrahedral amorphous carbon

Tetrahedral amorphous carbon (ta-C) is an amorphous form of carbon with a high content of sp3 bonded atoms, which can be deposited as thin films by energetic carbon ions or neutral atoms. As-grown ta-C films possess a high level of intrinsic compressive stresses (~10 GPa) inhibiting the use of this very promising material. The stress is due to a specific growth mechanism (subplantation), resulting from a competition between the densification of subsurface layers by incoming ions and the thermally activated diffusion of subplanted atoms to the surface. In agreement with experiment, we demonstrate using atomic scale simulation that low-temperature annealing can induce a considerable stress reduction in as-grown ta-C with minor
changes in its atomic structure and density. Simulating annealing by means of empirical molecular-dynamics with the interatomic Brenner potential for carbon and realistic boundary conditions, the dependence of the residual stress on the annealing temperature was investigated. It is found that a complete stress relief in ta-C is not accompanied by a change in the short-range order. The average atomic coordination remains nearly constant up to 1400 K. The stress relaxation mechanism discussed involves only structural optimization within the sp3 bonded constituent of ta-C and does not require clustering of sp2 bonded atoms.