Atomistic simulation of ion-beam-induced defect production within 100 – 1000 ps after ion impact

The formation and evolution of radiation damage by ion bombardment consists of three stages: (i) atomic displacements during ballistic processes, (ii) formation of a (meta)stable defect structure after fast relaxation, and (iii) long-term thermally activated defect rearrangement, migration, recombination and reduction. The first two stages occur on very short time scales and are hardly accessible by available experimental methods. Atomistic computer simulations can contribute to a better understanding of these processes and of their microscopic mechanisms.
In this talk, a combined simulation method is employed to study a relatively simple case, the defect production by a single ion impact in silicon. Processes in the collision cascade with energy transfers above about 100 eV are treated by computer simulations based on the binary collision approximation (BCA). Classical molecular dynamics (MD) calculations are applied to consider processes in certain parts of the cascade which start with energy transfers less than about 100 eV. Detailed investigations are performed to study the temporal evolution of the defect structure until the beginning of the thermally activated phase, and to determine the damage morphology obtained after the fast relaxation. The influence of nuclear energy deposition and target temperature is discussed. The combination of BCA and MD methods allows the effective calculation of the total number and the depth distribution of different defect species (isolated vacancies and self-intersitials as well as more complex defects) formed on average per incident ion after the fast relaxation processes are finished. The procedure yields more realistic initial conditions for the simulation of post-implantation annealing than hitherto used.