Ion irradiation of Ge: from sponge-like structure to periodic pattern formation

Ion irradiation of materials can lead to swelling and the formation of sponge like structures. Especially Ge is susceptible to ion induced swelling for ions with energies in the range of 10 keV to 100 MeV. At lower ion energy, however, surface nanopattern can be produced by ion irradiation. These structures show periodicities in the range of a few tenths to hundreds of nanometers and are promising templates for producing nanostructured thin films. Periodic ripple patterns are observed frequently for ion irradiation at incidence angles greater than 55° to the surface normal. At normal incidence dot or hole patterns with hexagonal symmetry are observed only under special irradiation conditions.
We studied the formation of hexagonally arranged hole patterns on Ge(001) surfaces induced by irradiation with a scanned focused Ga+ ion beam (FIB) at normal incidence. Hole patterns with characteristic length of about 50 nm are observed in a narrow energy range of 4 - 6 keV (Fig. 1a). These patterns are independent of ion flux in a range of several orders of magnitude. In addition, the patterns induced by FIB irradiations were compared to broad beam Ga+ irradiations at the same ion energy. No differences were found demonstrating that FIB irradiations with a large overlap of the scanned beam are identical to conventional broad beam irradiations. Using heavy ions, like Bi dimers and trimers, in a mass separated FIB regular hexagonally ordered dot patterns are formed instead at normal incidence (Fig. 1b).
Furthermore, ion induced pattern formation on Ge surfaces with 1 keV Ar+ at normal incidence and higher temperature was studied. Similar to the case of ion irradiated crystalline metal surfaces on the crystalline Ge surface a new instability appears at higher temperature due to the Ehrlich-Schwoebel barrier. In this case, we observe regular checkerboard or hole patterns with the symmetry of the patterns reflecting the crystal structure of the irradiated surface (see Fig. 1c).