Low-Energy Ion Damage Overrides Initial Atomic Structural Disorder in Silicon Nano-Patterning

Poster presented at the MRS Spring Meeting & Exhibit, held on April 6-10th, 2015, in San Francisco (United States). Symposium QQ: Plasma-Based Materials Science and Engineering.Ion beam sputtering (IBS) nanopatterning of Si has attracted a large interest both experimentally (due to its technological relevance) and theoretically (as a paradigmatic monoelemental system) [1]. In monocrystalline targets, the stress induced in the damaged layer and the formation of an amorphous/crystalline interface seem to play a key role in the pattern formation. However, it has been shown that the pattern dynamics may be also affected by the initial target conditions, such as the different roughness [2]. Recently, it has been also suggested that the thickness of a pre-existing amorphous layer underneath the surface [3] might influence the pattern amplitude. To deepen into this issue, we analyze here the influence of the initial atomic disorder of the target on the formation and further dynamics of the IBS pattern. In the current work we study ripple nanopatterns produced by low-energy IBS on four different Si systems: bulk single-crystal, amorphous thin film grown by DC magnetron sputtering, implanted with 200 keV Ar+ ions, and self-implanted with 150 keV Si+ ions. Both implanted samples were designed to have an equivalent amorphized layer of ~200 nm, using a relatively low fluence of 5middot;1015 ions/cm2 to avoid microstructural modification (e.g., gas bubbles). The IBS nanopatterning was carried out from 2 min up to 120 min using a 500 eV Ar+ beam from a CSC Kaufman-type ion gun impinging at 650 with respect to the target normal. Surface topography was imaged ex-situ by atomic force microscopy. Despite the different sample preparation, results show a similar morphological evolution of both the ripple wavelength and the roughness in the four cases, with a marked pattern already developed at 20 min. Molecular Dynamics simulations show that the low-energy IBS overrides the initial atomic arrangement in all the cases, generating a shallow damaged Si layer with significant differences, in terms of local order and density, with the as-prepared amorphous systems. Thus, our results evidence the formation of an effective damaged surface layer also in non-crystalline targets, supporting current viscous flow models [4]