Defect production mechanisms in metals and covalent semiconductors

We discuss how defect production mechanisms in displacement cascades vary according to the nature of the irradiated material. Our discussion is based on Molecular Dynamics (MD) simulation studies and reveal very different mechanisms for metals and covalent semiconductors. For metals we show how melting of the cascade core, in combination with long replacement collision sequences along low index crystallographic directions leads to the production of large number of defects in clusters and a small (≈ 10%) fraction of isolated interstitials well separated from the cascade region. In silicon, we show how the energy deposition process leads to the production of local amorphous regions and very few isolated Frenkel pairs. Because replacement collision sequences are extremely short in the open diamond lattice, very few or no isolated interstitials result. We argue that these observations provide a basis to understand the very large difference in freely migrating defect production efficiency in metals and silicon. The results provide an underlying cause for the extremely high bulk recombination efficiency observed in ion implanted and annealed silicon and provide a physical basis for the "+1" interstitial model in ion implanted silicon.