Nanoscale topographic pattern formation on Kr+-bombarded germanium surfaces

The nanoscale pattern formation of Ge surfaces uniformly irradiated by Kr+ ions was studied in a low-contamination environment at ion energies of 250 and 500 eV and at angles of 0° through 80°. The authors present a phase diagram of domains of pattern formation occurring as these two control parameters are varied. The results are insensitive to ion energy over the range covered by the experiments. Flat surfaces are stable from normal incidence up to an incidence angle of θ = 55° from normal. At higher angles, the surface is linearly unstable to the formation of parallel-mode ripples, in which the wave vector is parallel to the projection of the ion beam on the surface. For θ ≥ 75° the authors observe perpendicular-mode ripples, in which the wave vector is perpendicular to the ion beam. This behavior is qualitatively similar to those of Madi et al. for Ar+-irradiated Si but is inconsistent with those of Ziberi et al. for Kr+-irradiated Ge. The existence of a window of stability is qualitatively inconsistent with a theory based on sputter erosion [R. M. Bradley and J. M. Harper, J. Vac. Sci. Technol. A 6, 2390 (1988)] and qualitatively consistent with a model of ion impact-induced mass redistribution [G. Carter and V. Vishnyakov, Phys. Rev. B 54, 17647 (1996)] as well as a crater function theory incorporating both effects [S. A. Norris et al., Nat. Commun. 2, 276 (2011)]. The critical transition angle between stable and rippled surfaces occurs 10°–15° above the value of 45° predicted by the mass redistribution model.