Using standard molecular-dynamics methods, we have simulated silicon molecular-beam epitaxy on a (100) substrate, subject to a second beam of low-energy argon atoms. We find that the presence of the ion beam has an important effect on the structure of the deposits formed: good crystalline material forms at significantly lower temperatures, where otherwise a thermal beam yields an amorphous deposit. This is because the ion beam provides local excitations, which allow the deposited atoms to find their proper crystal lattice sites.