Highly ordered silicide ripple patterns induced by medium-energy ion irradiation

Abstract

We study the nanopatterning of silicon surfaces under near-normal 40-keV Ar + sputtering with simultaneous Fe oblique codeposition. The ion-beam incidence was kept at 15°, for which no pattern is produced in the absence of metal incorporation. Morphological and compositional analyses were performed by atomic force microscopy, in its morphological and electrical modes, Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, scanning Auger, as well as transmission and scanning electron microscopy. Initially, nanodot structures randomly emerge, which, with increasing ion fluence, become progressively aligned along the perpendicular direction to the Fe flux. With increasing fluence, they coalesce, leading to a ripple pattern. The pattern dynamics and characteristics are faster and enhanced, respectively, as the distance to the metal source decreases (i.e., as the metal content increases). For the highest metal flux, the ripples can become rather large (up to 18 μm) and straighter, with few defects, and a pattern wavelength close to 500 nm, while keeping the surface roughness close to 15 nm. Furthermore, for a fixed ion fluence, the pattern order is improved for higher metal flux. In contrast, the pattern order enhancement rate with ion fluence does not depend on the metal flux. Our experimental observations agree with the predictions and assumptions of the model by Bradley [R. M. Bradley, Phys. Rev. B 87, 205408 (2013)] Several compositional and morphological studies reveal that the ripple pattern is also a compositional one, in which the ripple peaks have a higher iron silicide content, in agreement with the model. Likewise, the ripple structures develop along the perpendicular direction to the Fe flux, and the pattern wavelength increases as the metal flux decreases with a behavior qualitatively consistent with the model predictions.

Coherent Atomic-Scale Ripples on Metallic Glasses Patterned by Low-Energy Ion Irradiation for Large-Area Surface Structuring

“…It is interesting that, from the very initial stage of the patterning with the emergence of random dots to the development of near-perfect ripple order, both the wavelength and amplitude are retained (see Figures and ). Our observation that the ripples maintain the atomic-scale amplitude throughout the pattern formation is prominent in comparison to previous studies where coarsening of the ripples was generally observed. − ,− ,− Therefore, we stress that our results indicate not only the stabilization of the ripples at prolonged exposure but more importantly their height is stable on glass surfaces from the very beginning of the structuring process. During irradiation, the only change is the ordering of the ripples, indicating a controllable and atomically precise surface patterning.…”

“…According to the widely accepted linear theory concerning ion-irradiation-induced surface rippling developed by Bradley and Harper (BH), it is the balance between curvature-dependent sputtering that roughens the surface and diffusion that smoothens the surface, leading to the buildup of a regular ripple pattern with a characteristic wavelength. In other generalized theories, nonlinear terms and additional smoothing mechanisms were included. ,,− In view of the numerous previous observations involving various materials, the produced ripple patterns have an amplitude of several nanometers or more, and it is rather unlikely that atomic-scale structuring can be achieved because the amplitude will always grow after the initial appearance of the pattern. − ,− ,− …”

Surface nanopatterning by ion beam irradiation: compositional effects

“…Finally, it is worth mentioning that similar behaviours were found when medium-energy ion beams were employed for the irradiations [108,120], specifically for a 40 keV Ar + ion beam. In both works, a plate set-up, similar to those used above, was employed.…”

“…In contrast, at θ = 15 • , an initial disordered dot pattern evolves towards parallel rows of dots. This pattern evolves under further irradiation into a quite ordered ripple pattern [108]. Noticeably, the roughness of the pattern is rather stable and much lower than that produced at θ = 60 • .…”

“…These two features, peak width and number of harmonics, are frequently used as a measure of pattern ordering. Furthermore, the pattern ordering can be also evaluated following different procedures for the ripple [106][107][108] and dot patterns, e.g. the latter by Voronoi tessellation [109,110].…”