Novel Nanopattern on Ge after Heavy Bi Cluster Bombardment

Ge surfaces have been irradiated with 30 keV Bi+ and 60 keV Bi++ monomers, 30 keV Bi2+ dimers, 60 keV Bi3++ trimers as well as 30 keV Bi4+. The ions are obtained from a Bi-liquid metal ion source in a mass separating 30 kV focused ion beam system. The surface pattern found after dimer and trimer irradiation differ drastically from the well-known porous or sponge-like nanostructures formed on Ge by ion irradiation with monomers: The surface remains crystalline as proven by Raman measurements, and the dots and ripples heights were in the order of their wavelengths. Under monomer irradiation a porous surface layer was obtained. The structure formation was investigated in the fluence range from1015 to 1017 ions/cm2 in dependence on the angle of incidence and the energy per atom of the different projectile ions. The high mass of the cluster ions leads to a patterning mechanism different from the Bradley-Harper mechanism, which becomes strikingly apparent by the crystalline Ge surface. An identified threshold of this new patterning helped to understand the mechanism: The ion-impact-induced deposition of energy per volume (as estimated by SRIM) must exceed a value which coincides with the energy needed for melting. Thus, Bi segregation during melt pool re-solidification and the 5% volume difference between molten and solid Ge can cause the observed Bi separation and Ge patterning, respectively. A consistent, qualitative model will be discussed.