Ion bombardment induced ripple topography on amorphous solids

Investigation of Ripple Formation on Surface of Silicon by Low-Energy Gallium Ion Bombardment

Regular wave patterns were created by a 2 kV gallium ion on Si(111) monocrystals at incidence angles between 60° and 80° with respect to the surface normal. The characteristic wavelength and surface roughness of the structured surfaces were determined to be between 35-75 nm and 0.5-2.5 nm. The local slope distribution of the created periodic structures was also studied. These topography results were compared with the predictions of the Bradley-Harper model. The amorphised surface layers were investigated by a spectroscopic ellipsometer. According to the results, the amorphised thicknesses were changed in the range of 8 nm to 4 nm as a function of ion incidence angles. The reflectance of the structured surfaces was simulated using ellipsometric results and measured with a reflectometer. Based on the spectra, a controlled modification of reflectance within 45% and 50% can be achieved on Si(111) at 460 nm wavelength. According to the measured results, the characteristic sizes (periodicity and amplitude) and optical property of silicon can be fine-tuned by low-energy focused ion irradiation at the given interval of incidence angles.

Spatially extended dislocations produced by the dispersive Swift-Hohenberg equation

Motivated by previous results showing that the addition of a linear dispersive term to the two-dimensional Kuramoto-Sivashinsky equation has a dramatic effect on the pattern formation, we study the Swift-Hohenberg equation with an added linear dispersive term, the dispersive Swift-Hohenberg equation (DSHE). The DSHE produces stripe patterns with spatially extended defects that we call seams. A seam is defined to be a dislocation that is smeared out along a line segment that is obliquely oriented relative to an axis of reflectional symmetry. In contrast to the dispersive Kuramoto-Sivashinsky equation, the DSHE has a narrow band of unstable wavelengths close to an instability threshold. This allows for analytical progress to be made. We show that the amplitude equation for the DSHE close to threshold is a special case of the anisotropic complex Ginzburg-Landau equation (ACGLE) and that seams in the DSHE correspond to spiral waves in the ACGLE. Seam defects and the corresponding spiral waves tend to organize themselves into chains, and we obtain formulas for the velocity of the spiral wave cores and for the spacing between them. In the limit of strong dispersion, a perturbative analysis yields a relationship between the amplitude and wavelength of a stripe pattern and its propagation velocity. Numerical integrations of the ACGLE and the DSHE confirm these analytical results.

Swelling as a stabilizing mechanism in irradiated thin films: II. Effect of swelling rate

It has long been observed experimentally that energetic ion-beam irradiation of semiconductor surfaces may lead to spontaneous nanopattern formation. For most ion/target/energy combinations, the patterns appear when the angle of incidence exceeds a critical angle, and the models commonly employed to understand this phenomenon exhibit the same behavioral transition. However, under certain conditions, patterns do not appear for any angle of incidence, suggesting an important mismatch between experiment and theory. Previous work by our group (Swenson and Norris 2018J. Phys.: Condens. Matter30304003) proposed a model incorporating radiation-induced swelling, which is known to occur experimentally, and found that in the analytically-tractable limit of small swelling rates, this effect is stabilizing at all angles of incidence, which may explain the observed suppression of ripples. However, at that time, it was not clear how the proposed model would scale with increased swelling rate. In the present work, we generalize that analysis to the case of arbitrary swelling rates. Using a numerical approach, we find that the stabilization effect persists for arbitrarily large swelling rates, and maintains a stability profile largely similar to that of the small swelling case. Our findings strongly support the inclusion of a swelling mechanism in models of pattern formation under ion beam irradiation, and suggest that the simpler small-swelling limit is an adequate approximation for the full mechanism. They also highlight the need for more-and more detailed-experimental measurements of material stresses during pattern formation.

Nanoscale pattern formation on silicon surfaces bombarded with a krypton ion beam: experiments and simulations

The nanoscale patterns produced by bombardment of the (100) surface of silicon with a 2 keV Kr ion beam are investigated both experimentally and theoretically. In our experiments, we find that the patterns observed at high ion fluences depend sensitively on the angle of incidence Θ. For Θ values between 74° and 85°, we observe five decidedly different kinds of morphologies, including triangular nanostructures traversed by parallel-mode ripples, long parallel ridges decorated by short-wavelength ripples, and a remarkable mesh-like morphology. In contrast, only parallel-mode ripples are present for low ion fluences except for Θ = 85°. Our simulations show that triangular nanostructures that closely resemble those in our experiments emerge if a linearly dispersive term and a conserved Kuramoto-Sivashinsky nonlinearity are appended to the usual equation of motion. We find ridges traversed by ripples, on the other hand, in simulations of the Harrison-Pearson-Bradley equation (Harrisonet al2017Phys. Rev.E96032804). For Θ = 85°, the solid surface is apparently stable and simulations of an anisotropic Edwards-Wilkinson equation yield surfaces similar to those seen in our experiments. Explaining the other two kinds of patterns we find in our experiments remains a challenge for future theoretical work.

Formation of nanoripples on ZnO flat substrates and nanorods by gas cluster ion bombardment

In the present study Ar⁺ cluster ions accelerated by voltages in the range of 5-10 kV are used to irradiate single crystal ZnO substrates and nanorods to fabricate self-assembled surface nanoripple arrays. The ripple formation is observed when the incidence angle of the cluster beam is in the range of 30-70°. The influence of incidence angle, accelerating voltage, and fluence on the ripple formation is studied. Wavelength and height of the nanoripples increase with increasing accelerating voltage and fluence for both targets. The nanoripples formed on the flat substrates remind of aeolian sand ripples. The ripples formed at high ion fluences on the nanorod facets resemble well-ordered parallel steps or ribs. The more ordered ripple formation on nanorods can be associated with the confinement of the nanorod facets in comparison with the quasi-infinite surface of the flat substrates.

Effect of dispersion on the nanoscale patterns produced by ion sputtering

Our simulations show that dispersion can have a crucial effect on the patterns produced by oblique-incidence ion sputtering. It can lead to the formation of raised and depressed triangular regions traversed by parallel-mode ripples, and these bear a strong resemblance to nanostructures that are commonly observed in experiments. In addition, if dispersion and transverse smoothing are sufficiently strong, highly ordered ripples form. Finally, dispersion can cause the formation of protrusions and depressions that are elongated along the projected beam direction even when there is no transverse instability. This may explain why topographies of this kind form for high angles of ion incidence in cases in which ion-induced mass redistribution is believed to dominate curvature-dependent sputtering.

Designing Nanostructured Ti6Al4V Bioactive Interfaces with Directed Irradiation Synthesis toward Cell Stimulation to Promote Host-Tissue-Implant Integration

A new generation of biomaterials are evolving from being biologically inert toward bioactive surfaces, which can further interact with biological components at the nanoscale. Here, we present directed irradiation synthesis (DIS) as a novel technology to selectively apply plasma ions to bombard any type of biomaterial and tailor the nanofeatures needed for in vitro growth stimulation. In this work, we demonstrate for the first time, the influence of physiochemical cues (e.g., self-organized topography at nanoscale) of medical grade Ti₆Al₄V results in control of cell shape, adhesion, and proliferation of human aortic smooth muscle stem cells. The control of surface nanostructures was found to be correlated to ion-beam incidence angle linked to a surface diffusive regime during irradiation synthesis with argon ions at energies below 1 keV and a fluence of 2.5 × 10¹⁷ cm^-2. Cell viability and cytoskeleton morphology were evaluated at 24 h, observing an advance cell attachment state on post-DIS surfaces. These modified surfaces showed 84% of cell biocompatibility and an increase in cytoplasmatic protusions ensuring a higher cell adhesion state. Filopodia density was promoted by a 3-fold change for oblique incidence angle DIS treatment compared to controls (e.g., no patterning) and lamellipodia structures were increased more than a factor of 2, which are indicators of cell attachment stimulation due to DIS modification. In addition, the morphology of the nanofeatures were tailored, with high fidelity control of the main DIS parameters that control diffusive and erosive regimes of self-organization. We have correlated the morphology and the influence in cell behavior, where nanoripple formation is the most active morphology for cell stimulation.

Emergence and detailed structure of terraced surfaces produced by oblique-incidence ion sputtering

We study the nanoscale terraced topographies that arise when a solid surface is bombarded with a broad ion beam that has a relatively high angle of incidence θ. We find that the surface is not completely flat between the regions in which the surface slope changes rapidly with position: Instead, small-amplitude ripples propagate along the surface. Our analytical work on these ripples yields their propagation velocity as well as the scaling behavior of their amplitude. Our simulations establish that the surfaces exhibit interrupted coarsening, i.e., the characteristic width and height of the surface disturbance grow for a time but ultimately asymptote to finite values as the fully terraced state develops. In addition, as θ is reduced, the surface can undergo a transition from a terraced morphology that changes little with time as it propagates over the surface to an unterraced state that appears to exhibit spatiotemporal chaos. For different ranges of the parameters, our equation of motion produces unterraced topographies that are remarkably similar to those seen in various experiments, including pyramidal structures that are elongated along the projected beam direction and isolated lenticular depressions.

Theory of terraced topographies produced by oblique-incidence ion bombardment of solid surfaces

When a solid surface is bombarded with a broad ion beam at a relatively large angle of incidence, the surface often develops a terraced form. We introduce a model that includes an improved approximation to the sputter yield and that produces a terraced surface morphology at long times for a wide range of parameter values. Numerical integrations of our equation of motion reveal that the terraces coarsen as time passes, just as observed experimentally. We also show that the terrace propagation direction can reverse as the amplitude of the surface disturbance grows. This highlights the important role higher order nonlinearities play in determining the propagation velocity at high fluences.

Ion-Induced Nanoscale Ripple Patterns on Si Surfaces: Theory and Experiment

Nanopatterning of solid surfaces by low-energy ion bombardment has received considerable interest in recent years. This interest was partially motivated by promising applications of nanopatterned substrates in the production of functional surfaces. Especially nanoscale ripple patterns on Si surfaces have attracted attention both from a fundamental and an application related point of view. This paper summarizes the theoretical basics of ion-induced pattern formation and compares the predictions of various continuum models to experimental observations with special emphasis on the morphology development of Si surfaces during sub-keV ion sputtering.

Highly ordered nanopatterns on Ge and Si surfaces by ion beam sputtering

The bombardment of surfaces with low-energy ion beams leads to material erosion and can be accompanied by changes in the topography. Under certain conditions this surface erosion can result in well-ordered nanostructures. Here an overview of the pattern formation on Si and Ge surfaces under low-energy ion beam erosion at room temperature will be given. In particular, the formation of ripple and dot patterns, and the influence of different process parameters on their formation, ordering, shape and type will be discussed. Furthermore, the internal ion beam parameters inherent to broad beam ion sources are considered as an additional degree of freedom for controlling the pattern formation process. In this context: (i) formation of ripples at near-normal ion incidence, (ii) formation of dots at oblique ion incidence without sample rotation, (iii) transition between patterns, (iv) formation of ripples with different orientations and (v) long range ordered dot patterns will be presented and discussed.

Roughness evolution of ion sputtered rotating InP surfaces: pattern formation and scaling laws

The topography evolution of simultaneously rotated and Ar (+) ion sputtered InP surfaces was studied using scanning force microscopy. For certain sputter conditions, the formation of a highly regular hexagonal pattern of close-packed mounds was observed with a characteristic spatial wavelength which increases with sputter time t according to lambda approximately t(gamma) with gamma approximately 0.26. Based on the analysis of the dynamic scaling behavior of the surface roughness, the evolution of the surface topography will be discussed within the limits of existing models for surface erosion by ion sputtering.