Ballistic versus electronic processes in ion-induced nanostructuring of ionic surfaces

High-resolution noncontact atomic force microscopy in UHV has been used for characterization of KBr(001) surface morphology development due to an oblique incidence of low-energy ion beams (4 keV He(+) and Ar(+) at 75 degrees). We have found several features of the process directly related to the ionic nature of halide surfaces, such as formation of two-dimensional (2D) pits and rectangular 2D epitaxial adislands on the initially atomically flat terraces. At low bombardment fluence the evolution of the 2D pits proceeds along main surface crystallographic directions. Such behavior is typical for the electron stimulated desorption (ESD) process, well-known from electron and photon irradiation experiments. No epitaxial adislands formation by ESD or by ion impact has been reported so far. For prolonged ion bombardment surface topography transforms into a regular network of grooves and rims (a ripple structure) oriented parallel to the incident beam. Such a structure has been observed for many other materials (metals and semiconductors), with a major distinction, however, that for KBr the ripples are composed of small nanosize crystallites with a persistent long-range order of the (001) surface. We have demonstrated that the fluence threshold for a transition from a random network of 2D pits and adislands into a well-oriented crystalline nanoripple structure is directly related to the balance between the electronic and ballistic stopping of the impinging ions. The theoretical interpretation of our observations is based on an atomistic approach to the ion-solid interaction and supplementary molecular-dynamics computer simulations of a single-ion impact on flat and atomic step covered surfaces. In particular, the computer simulations demonstrate that the sputtering yield for ions impinging against the ascending step edges on the irradiated surface are much greater than the ones obtained for the descending step bombardment.