Nanoscale surface topography evolution on Ge surfaces irradiated by 1 keV Kr${}^{+}$ ions is examined in both directions perpendicular and parallel to the projection of the ion beam on the surface. Grazing incidence small angle x-ray scattering is used to measure in situ the evolution of surface morphology via the linear dispersion relation. A transition from smoothing (stability) to pattern-forming instability is observed at a critical ion incidence angle of approximately 62° with respect to the surface normal. The linear theory quadratic coefficients which determine the surface stability/instability are determined as a function of bombardment angle. The Ge surface evolution during Kr${}^{+}$ irradiation is qualitatively similar to that observed for Ar${}^{+}$ irradiation of Si. However, in contrast to the case of Si under Ar${}^{+}$ irradiation, the critical angle separating stability and instability for Ge under Kr${}^{+}$ irradiation cannot be quantitatively reproduced by the simple Carter-Vishnyakov mass redistribution model.

• Received 17 July 2012

DOI:https://doi.org/10.1103/PhysRevB.86.245412