Self-organized Ge & Si nanostructures by heavy-ion irradiation

Heavy ions like Bi or Au of a few tens of keV deposit a high energy density into the collision cascade volume of due to (i) their high mass and (ii) their low projected range. At higher energies, this density becomes diluted as the cascade volume increases super-linearly with ion energy. Compared to monatomic ions, heavy polyatomic ions deposit an even higher energy density. This is sufficient to form a pool of a localized, almost classical melt in a semiconductor surface lasting up to half of a nanosecond.
Local melting and re-solidification by single polyatomic ion impacts is proven by molecular dynamics calculations. Well-ordered, self-organized dot patterns on Si and Ge surfaces have been found after heavy polyatomic ion irradiation, which can be attributed to impact-induced local transient melting. Similar patterns were found with monoatomic ions at elevated substrate temperatures, where the energy per substrate atom exceeds a critical value within a larger volume.
The kinetics of localized melt pools leads to a generic, Kuramoto-Sivashinsky-type (KS) partial differential equation for the surface evolution. Whereas so far the mechanisms of ion-induced surface pattern evolution are assumed to be surface-curvature-dependent ion erosion or ion-momentum-induced mass drift of surface atoms, for heavy polyatomic ions we have identified a completely different mechanism.
The local melting and quenching process is so far from equilibrium that particularities of phase diagrams like the Bi state in Si or Ge are frozen into the nanostructure of the re-solidified volume. This opens the possibility to study extremely fast solid-liquid phase transitions.
The authors thank the German Research Foundation for financial support.