Ion induced patterns on Ge surfaces

Low energy ion irradiations of surfaces can induce the formation of periodic patterns with periodici-ties in the range of a few tenths to a few hundreds of nanometers. These patterns have been used as templates for growing thin films with interesting anisotropic properties resulting from the modu-lation of their interface and surface.
At off-normal incidence ripple patterns oriented perpendicular to the ion beam direction are ob-served after prolonged ion irradiation. At normal incidence or for incidence angles smaller than 55° smoothing dominates on Si and Ge surfaces. However, if more than one atomic species is present on the irradiated surface, e.g. due the ion beam itself or co-deposited atoms on elemental materials or for compounds, additional instabilities may exist leading to periodic patterns also at normal ion incidence. These patterns are isotropic and can either appear as dot or hole patterns exhibiting short range hexagonal order.
We studied the formation of hexagonally arranged hole patterns on Ge(001) surfaces induced by irradiation with a scanned focused Ga+ ion beam (FIB). Hole patterns with characteristic length of 50 nm are observed in a narrow energy range of 5 - 7 keV (Fig. 1 a,b). These patterns are inde-pendent 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 energies. No dif-ferences were found demonstrating that FIB irradiations with a large overlap of the scanned beam are identical to conventional broad beam irradiations.
We studied also ion induced pattern formation on Ge surfaces with 1 keV Ar+ at higher temperature. In contrast to irradiations at low temperature we found pattern formation even at normal ion incidence. Similar to the case of ion irradiated crystalline metal surfaces on the crystalline Ge sur-face a new instability appears at higher temperature due to the Ehrlich-Schwoebel barrier. Here, we observe regular checkerboard or hole patterns with the symmetry of the patterns reflecting the crystal structure of the irradiated surface (see Fig. 1c,d).