The binary collision approximation (BCA) is a Monte Carlo method widely used to provide the statistical distribution of defects generated when a solid is bombarded by energetic particles. It assumes that an energetic particle only collides with the closest target atom in its neighborhood, losing its kinetic energy by elastic collisions with atoms and inelastic interactions with electrons. Some target atoms may receive enough energy to produce additional collisions. By simulating a large number of collisions the depth distribution of defects and particles can be obtained.

We use the BCA technique to get the coordinates of generated interstitials, vacancies and the introduced ions or particles, which it is used as the starting point to simulate the dynamics of the system by means of a kinetic Monte Carlo simulator. We have developed an improved BCA model which considers not only the ballistic mechanism but also the thermal spike regime for damage generation based on results obtained from fundamental simulation techniques.

Damage generated by a B_{18} cluster implant in Si as simulated by classical molecular dynamics (left) and our improved BCA model (right). A better description of damage is obtained by considering the additional Si interstitials produced by the thermal spike process (I. Santos *et al.*, J. Appl. Phys. 105, 083530 (2009) - Improved atomistic damage generation model for binary collision simulations).