Mechanisms of surface pattern formation under irradiation with heavy ions

The driving force for surface pattern formation under ion irradiation has been under discussion for many years. Bradley and Harper suggested that curvature dependent sputtering is the source for the surface instability. Later on, Carter and Vishnyakov concluded that the transfer of the ion momentum to atoms causes a mass drift which smoothes the surface but destabilizes it at large off-normal impact angles. Thus, no pattern formation is expected for normal incidence on elemental semiconductors. However, very recently we found that normal incidence irradiation of Ge with ultraheavy ions (Bi_3^++ , Bi_2^+, 10...20 keV/atom) leads to very pronounced, hexagonally ordered dot pattern [1]. This pattern form if the energy density deposited close to the surface in a single ion impact exceeds a threshold, which can be achieved by ultraheavy ions or by substrate heating [2]. A model of pattern formation based on transient melt pool formation with local surface minimization will be presented. [1] L. Bischoff, K.-H. Heinig, B. Schmidt, S. Facsko, W. Pilz, NIMB 272 (2012) 198; [2] R. Böttger, L. Bischoff, K.-H. Heinig, W. Pilz, B. Schmidt, JVST B30 (2012) 06FF12.