Comparison of pattern formation by Focused Ion Beam and Broad Beam Sputtering

Under specific conditions low energy ion beam sputtering of surfaces leads to self-organized periodic patterns. These can be ripple patterns with periodicities in the nanometer range for oblique ion incidence or hexagonal dot patterns on compound materials for normal ion incidence. Recently, periodic nanohole patterns were observed on Ge surfaces when sputtered at normal incidence using a 5 keV Ga+ focused ion beam (FIB) [1]. These patterns resemble the dot patterns on GaSb mirrored with respect to the surface plane, suggesting a similar formation mechanism.
In this work we studied the pattern formation using FIB and compared the results with patterns produced by sputtering with a broad beam. In case of the focused beam a variation of the incidence angle as well as a variation in the energy of the incoming Ga+ ion on a Ge substrate was done. Using a broad Ga+ beam leads to the same pattern formation on Ge independent of scanning. Additionally GaSb dot patterns were produced using a broad beam of Ar+ and the size dependence of the energy of the incoming ion was studied. A comparison of both patterns shows no flux dependence in case of Ga+ sputtered on Ge (neither for FIB nor for broad beam), whereas the size and also the shape of the GaSb dots changes with flux.
[1] Q. Wei, X. Zhou, B. Joshi, Y. Chen, K. Li, Q. Wei, K. Sun, and L. Wang, Adv. Mater. 21, 2865 (2009).