zbl-potential in ion-bombardement-simulation

I am not going to check your commands in detail, but the overall idea looks sound. I encourage you to do some careful checking. For example, calculate the potential energy of a Ga atom as it approaches an Si surface, both for an interstitial site and directly over an Si atom. Also, recognize that while ZBL is a good model for collision, it is not a good model for low energy interactions involving either Ga-Si or Ga-Ga.