Ion impact crater asymmetry determines surface ripple orientation

Ion bombardment causes surface instabilities on a range of materials including metals, semiconductors, and insulators. However, the proposed mechanisms for these instabilities have yet to explain the rich range of nanometer-scale patterns that are observed experimentally. Here we show that smoothing balanced by impact angle dependent mass redistribution explains the atomistic origin of ripple formation and orientation, particularly angle dependent transitions between different orientations. A competition between the mass accumulated on the surface and the hole created on the surface determines the orientation of ripples. Results are consistent with experimental observations for a range of ions, ion energies, and targets. (C) 2011 American Institute of Physics. [doi:10.1063/1.3650469]