Ion beam sputtering (IBS) of targets amorphizing under low to medium energy irradiation has attracted much interest as a technique to produce high quality surface nanopatterns [1]. In spite of the large effort devoted for the last 20 years to elucidating this instance of surface self-organization, recent experiments [2] are challenging previous notions, including the physical nature of the morphological instability as due to the curvature dependence of the sputtering yield [3]. Thus, while classically pattern formation is expected e.g. for any ion incidence angle and energy, experiments on silicon show featureless surfaces for angles up to an energy independent value [2,4], above which patterns are found. The typical wavelength diverges at this morphological transition, while it remains finite at a different transition occurring at relatively low energies and small angles [2]. We report on a hydrodynamic description of IBS nanopatterning that is able to account for these complex morphological properties within an unified framework. Our model addresses dynamics at the surface and in the bulk of the amorphous layer generated by the beam on top of the target. Agreement with experimental findings suggests a novel view on the main mechanisms at play in these systems, as the competition between irradiation-induced flow of a highly viscous, incompressible layer, and sputtering phenomena.

[1] See R. Cuerno, L. Vázquez, R. Gago, M. Castro, J. Phys. Condens. Matter 21 (2009) 220301, and other papers in the same issue.

[2] C. S. Madi, B. Davidovitch, H. B. George, S. A. Norris, M. P. Brenner, M. J. Aziz, Phys. Rev. Lett. 101 (2008) 246102.

[3] R. M. Bradley, J. M. Harper, J. Vac. Sci. Technol. A 6 (1988) 2390.

[4] G. Carter, V. Vishnyakov, Phys. Rev. B 54 (1996) 17647.