Computer simulations of damage due to passage of a heavy fast ion through diamond

We present tight-binding molecular dynamics simulations of the structural modifications that result from the ``thermal spike'' that occurs during the passage of a heavy fast ion through a thin diamond or amorphous carbon layer, and the subsequent regrowth upon cooling. The thermal spike and cooling down are simulated by locally heating and then quenching a small region of carbon, surrounded either by diamond or by a mostly sp³ bonded amorphous carbon network. For the case of the thermal spike in diamond we find that if the ``temperature'' (kinetic energy of the atoms) at the center of the thermal spike is high enough, an amorphous carbon region containing a large fraction of threefold coordinated C atoms (sp² bonded) remains within the diamond network after cooling. The structure of this amorphous layer depends very strongly on the ``temperature'' of heating and on the dimensions of the thermal spike. Scaling is found between curves of the dependence of the percentage of sp² bonded atoms in the region of the thermal spike on the heating ``temperature'' for different volumes. When the thermal spike occurs in an initially amorphous sample the structure of the damaged region after cooling exhibits the above dependencies and is found to be a function of the structure of the original amorphous carbon layers.