Transition from Pits to Mounds in Ion Induced Patterning of Germanium

Low energy ion irradiation drives surfaces out of equilibrium by continuous creation of displacements in the sub-surface region. At room temperature the accumulation of displacements leads to the amorphization of the irradiated surfaces and self-organized ripple pattern perpendicular to the ion beam direction are formed for incidence angles higher than 50° [1]. At normal incidence irradiation smoothing dominates and no pattern are observed for low energy ion irradiation. At higher temperatures, point defects created by the displacements in the ion collision cascade can diffuse longer distances, thus vacancies and interstitial recombine more effectively or diffuse to the surface. Finally, at temperatures higher than the recrystallization temperature, all defects in the sub-surface region are annealed before an ion creates new defects and the surface remains crystalline. The average density of surface vacancies and ad-atoms on the surface is, however, much higher than the corresponding densities in thermal equilibrium resulting in a much higher entropy.
In this regime, ion irradiation creates an excess of vacancies on the crystalline surface due to sputtering and the surfaces morphology is determined primarily by their kinetics. The diffusion of vacancies is biased by the Ehrlich-Schwoebel barrier, i.e. an additional barrier for crossing terrace steps, similar to the diffusion barrier of ad-atoms known from growth by molecular beam epitaxy. Consequently, ion sputtering leads to the erosion of 3D structures in a “reverse epitaxy” process. The resulting patterns are arrays of inverse pyramids growing into the Ge surface [1,2]. The morphology of these patterns is given by the crystal symmetry of the surface. Hence, checkerboard patterns appear on the Ge (001) surface Here, we show that the inverse pyramid pattern on Ge(001) surface, which is observed for normal incidence ion irradiation at higher temperatures, turns into a pyramidal mound pattern at incidence angles between 50° and 70° with respect to the surface normal, and finally, into ripple patterns above 80° incidence. All irradiations were performed at 350° C with 1 keV Ar+ at a fluence of 1x1018 cm-2 from a Kaufman ion source.
The observed transition from pit to mound patterns in reverse epitaxy can be understood by assuming a transition from vacancy dominated pattern formation to ad-atom dominated pattern formation. Therefore, at incidence angles above 50° the pattern resemble mound patterns observed in growth. Furthermore, the transition to ripples patterns at higher incidence angles is ascribed to a shadowing instability at these grazing incidence angles.

[1] A. Keller and S. Facsko, Materials 2010, Vol. 3, Pages 4811-4841 3, 4811 (2010).
[2] X. Ou, A. Keller, M. Helm, J. Fassbender, and S. Facsko, Phys. Rev. Lett. 111, 016101 (2013).
[3] X. Ou and S. Facsko, Nucl. Instr. Meth. B 341, 13 (2014).