Ripple Morphology Research Articles

Morphology of ion sputtered Cu(0 0 1) surface: Transition from unidirectional roughening to bidirectional roughening

We have previously shown that under off normal ion sputtering, ripples parallel to the beam with a periodicity of hundreds of nanometers can be formed on a Cu(001) surface. This ripple formation behavior can be well explained by the Bradley and Harper (BH) instability mechanism. However, by lowering the temperature at a constant flux, we find that the resultant morphology transitions from a unidirectional modulation to a bidirectional modulation. The lower temperature morphology is attributed to the presence of diffusion barrier across step edges. The time evolution of this morphology is also studied and the bidirectional modulation morphology is observed to change back to ripple morphology as the ion fluence increases.

Hydrodynamic significance of variable ripple morphology across deep-water barchan dunes in the Faroe–Shetland Channel

A combination of high-resolution (30 kHz) sidescan sonar and seafloor photography has been used to image barchan dunes and sand ripples in the Faroe–Shetland Channel. The barchan dunes display body lengths and horn-to-horn widths of up to 120 m, and the horns point downcurrent to the southwest. The seafloor surrounding the dunes is variable, with gravel patches, sand with gravel streaks, and uniform rippled sand all observed. The pattern of sand ripples across the dune surface is strongly controlled by the dune morphology. On the lower stoss slopes of the dunes, straight-crested transverse ripples gradually merge upslope into sinuous-crested transverse ripples. Most of the upper dune surface is dominated by short-crested linguoid ripples. This sequence represents an increase in height above seafloor and increasing flow velocity. The overall pattern of ripples across the dune surface indicates that the bottom current flow is modified by the dune morphology, with flow concentrated over the dune crest and along the inner edge of the horns. It is also possible to estimate flow velocity across the dune field, using published analogues and short-term current meter observations. Generally, published examples of submarine barchans suggest a flow velocity of 40–80 cm/s. This range of values is in agreement with near-bed current meter measurements in the study area, which indicate peak flow velocities of 50–60 cm/s. However, in the longer term higher flow speeds may be possible, especially considering the presence of erosional furrows adjacent to the dune field.

Stereo–video observation of nearshore bedforms on a low energy beach

A new technique utilising underwater stereo–video was tested for its ability to measure nearshore bed morphology at high-frequency and high-resolution. A stereo–video system was used to monitor small-scale ripple morphology (wavelengths 0.05–0.10 m, heights ∼5 mm) in the nearshore of a low energy beach in southwestern Australia. Waves, currents and suspended sediment concentrations were also monitored. Bed morphology could be measured with a mean accuracy of 2 mm with the stereo–video system. Measurements in the field were limited to ripple crest positions since targets over the entire ripple profiles could not be identified in both the left and right stereo images. This limitation could be overcome in the future by use of laser stripes to illuminate the profiles and the generation of epipolar lines within the analysis software to match corresponding targets in each of the images. Ripple crest positions were measured at 1 Hz and the relationship between ripple movement and near-bed hydrodynamics was investigated to test the applicability of the stereo–video system to make high-frequency bedform observations. Ripple crests were found to oscillate at the same frequency as the swell waves. The magnitude of the ripple movement at swell wave frequencies was an order of magnitude greater than ripple movement at wind wave frequencies. Net onshore migration of the ripples corresponded to onshore wave asymmetry, very weak mean cross-shore currents and net onshore suspended sediment flux measured at an elevation of 0.05 m.

Evolution of surface morphology of ion sputtered GaAs( [formula omitted

In order to explore possible route to fabricate nano-scale semiconductor dots, a series of ion bombardment experiment on GaAs(1 0 0) was undertaken using a high current isotope separator and ion implanter with 40 Ar + ions of an energy of 60 keV incident at an angle of 60° with respect to surface normal. Detailed surface topographical features of the bombarded samples were characterised by atomic force microscopy. To observe the growth of topography with time, the samples were bombarded at a number of doses. At a dose of 1×10 17 ions/cm 2, no observable topography was developed. At a dose of 2×10 17 ions/cm 2, the topography started to develop in the form of roughness along with islands or dots formation on the crest of waves or hillocks. Similar kind of topography has been observed up to a dose of 1×10 18 ions/cm 2, remarkable with the formation of nano-dots with the maximum dimension of a few hundred nanometer. At the dose of 3×10 18 ions/cm 2 the surface became populated with ripple morphology without formation of any island or dot, in contrast with lower doses.

Nanostructured Polymer Brushes With Reversibly Changing Properties

AbstractWe investigated the interplay between different mechanisms of the lateral and vertical segregation in the synthesized via “grafting from” approach symmetric A/B (where A and B are poly(styrene-co-2,3,4,5,6-pentafluorostyrene) and poly(methylmethacrylate), respectively) polymer brushes upon exposure to different solvents. We used X-ray photoemission electron spectroscopy and microscopy (X-PEEM), AFM, water contact angle measurements, and oxygen plasma etching to study morphology of the brushes. The ripple morphology after toluene (nonselective solvent) revealed elongated lamellar-like domains of A and B polymers alternating across the surface. The dimple-A morphology consisting of round clusters of the polymer A was observed after acetone (selective solvent for B). The top layer was enriched with the polymer B showing that the brush underwent both the lateral and vertical phase segregation. A qualitative agreement with predictions of SCF theory was found.

Si1−xGexgrowth instabilities on vicinal Si(001) substrates: Kinetic vs. strain-induced effects

A comparative study of kinetically and thermodynamically driven instabilities on vicinal Si(001) surfaces during overgrowth with ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ is reported. We mapped out a wide range of the multidimensional growth parameter space and found, in contrast to previous reports, no evidence for strain-induced step bunching. At low Ge concentrations strain is insufficient to promote strain-induced step bunching, and the modified surface kinetics in the presence of segregated Ge leads to a smoother rather than rougher morphology. High Ge concentrations around 50% could be expected to provide enough strain, but near equilibrium hut cluster formation is the more effective strain-relaxation mechanism. We found the characteristically rippled step-bunching morphology only in a kinetically limited growth regime, where strain is of limited relevance, and in experiments where the SiGe layers replicate an underlying ripple morphology.

Influence of compositional macrosteps on the reduction of the critical thickness by generation of 〈010〉 misfit dislocations in InGaAs/GaAs quantum wells

A reduction of the critical thickness has been seen to occur in InGaAs single quantum wells grown by MOVPE on tilted GaAs substrates by the generation of 〈010〉 aligned misfit dislocations. The latter ones have been generated by glide of half loops on {110} planes instead of the usual {111} slip planes. The single quantum wells also contain compositional macrosteps which produce periodic thickness oscillations giving rise to a ripple morphology of the layer surface. Since the glide of half loops in the secondary {110} slip planes can occur only under conditions of high strain, it is hypothesized that their propagation, hence the formation of 〈010〉 misfit dislocations, takes place at the observed compositional macrosteps where enhanced concentration of strain is likely to occur.

Cooperative nucleation leading to ripple formation in InGaAs/GaAs films

In 0.25 Ga 0.75 As epilayers were grown on GaAs (001) substrates (1.8% misfit strain) by molecular beam epitaxy to investigate the two-dimensional to three-dimensional transition as a function of thickness (t⩽30 MLs). Tapping-mode atomic force micrographs show the evolution of the morphology as a function of thickness. As the film is deposited, the nucleation of 3D islands followed by cooperative nucleation of pits is observed. As the thickness increases, both islands and pits continue to nucleate and grow until they coalesce, resulting in a fully formed ripple morphology running along the [11̄0]. The ripples also exhibit a secondary alignment roughly along the 〈310〉 which is attributed to the nucleation of islands with {136} faces.

Tidal asymmetry in sediment resuspension on a macrotidal beach in northwestern Australia

A field experiment was conducted on a macrotidal beach in northwestern Australia to investigate the relation between sediment resuspension and sea bed morphology. Continuous measurements of waves, currents and suspended sediment concentrations were carried out over a period of several hours around high tide, concurrent with half-hourly visual observations of the sea bed morphology. The incident waves had a significant wave height of 0.35 m and period 11 s. Most of the data were collected outside the surf zone where sediment resuspension was mainly by wave groups, and suspended sediment transport was primarily accomplished by mean longshore and cross-shore currents. The sediment resuspension process was further characterised by a pronounced tidal asymmetry with suspended sediment concentrations and transport rates during the falling tide significantly larger than during the rising tide. The asymmetry is attributed to the combined effects of a larger bed roughness and stronger mean nearshore flows during the falling tide. The increase in bed roughness occurred during the rising tide under the influence of decreasing bed shear stresses when post-vortex ripple morphology developed on a previously plane bed. Around high tide, the ripples were fully developed and had attained a ripple height of approximately 0.005 m and a ripple length of 0.06–0.08 m. Application of the Van Rijn [Van Rijn, L.C., 1989. Handbook Sediment Transport by Currents and Waves. Delft Hydraulics, Report H461] sediment transport model further confirmed that enhanced sediment resuspension during the falling tide was primarily the result of the development of ripple morphology, with increased mean current strengths playing a secondary role. The results have major implications for modelling suspended sediment transport on macrotidal beaches.

Use of substratum ripples for flow refuging by Atlantic cod, Gadus morhua

The ability to maintain position in a current without actively swimming (station-holding) was measured on substratum ripples for Atlantic cod, Gadus morhua, a bentho-pelagic fusiform species. The current velocities tested ranged from 0–111 cm sec-1. Ripples were sinusoidal, with twelve combinations of ripple wavelength (10, 25, 50, 125 cm) and ripple amplitude (1.0, 2.5, 5.0 cm). Ripple wavelengths were chosen to approximate 0.5, 1.0, 2.0 and 5.0 times fish total length. The potential of ripples to locally retard current and thereby provide a refuge from the flow was measured as a velocity ratio, utrough/ufree-stream, where utrough is the flow velocity measured at a height of 0.5 cm from the bottom of a trough and ufree-stream the flow velocity measured at a height of 10 cm above ripple crests. Cod usually swam steadily above substratum ripple crests in the free-stream flow. They used substratum ripples to hold station on only 3 of the 12 ripples tested by refuging from the flow in the ripple troughs (flow refuging). These ripples had wavelengths approaching twice the body length, with ripple amplitudes that produced velocity ratios of 0.44–0.65, thus providing at least a 35% flow reduction in the troughs. In addition, these ripples were only used at intermediate velocities starting at 49–78 cm sec-1 and ending at 81–109 cm sec-1 depending on the ripple morphology, suggesting there may be costs involved in flow refuging, probably in stability control. Flow refuging on substratum ripples could dramatically impact the physiology and ecology of cod in high current velocities by providing areas of retreat for energetic savings, but also offering opportunities for enhanced feeding and migration.

Scaling Laws of the Ripple Morphology on Cu(110)

The evolution of the Cu(110) surface morphology during low temperature (180 K) ion sputtering was studied as a function of the incident ion beam angle $\ensuremath{\theta}$ by means of scanning tunneling microscopy. The morphology was dominated by a ripple structure with the wave vector parallel or perpendicular to the direction of the incident beam. The time evolution of the interface shows that the ripple wavelength increases in time following a scaling law $\ensuremath{\lambda}\ensuremath{\propto}{t}^{z}$, with $z\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.26\ifmmode\pm\else\textpm\fi{}0.02$. These results are ascribed to the effect of a Schwoebel barrier on the interlayer diffusion of the recoiling atoms produced during ion sputtering.

Viscoelastic buckling and plastic flow?deterministic mechanisms for ripple initiation on ion-bombarded amorphous solids

The great majority of current theoretical studies of the initiation or ripple morphology on ion-bombarded amorphous solids advocate the need for stochastic sputtering processes to initiate random surface structures from which ripples evolve. The present approach considers the initiation to be deterministic and resulting from viscoelastic buckling of a surface layer that is compressively stressed. The orientation of the ripple wave vector parallel to the incident ion projection on the surface is believed to result from beam-induced plastic flow preferentially in this direction.

Kinetic Pathways to Strain Relaxation in the Si-Ge System

The strain-induced transition of a planar film to a three-dimensional island morphology is presently a significant issue in the growth of semiconductor thin films. Strain-induced roughening can be problematic in the fabrication of coherently strained device structures where it is important to understand the early stages of the transition to avoid or suppress three-dimensional (3D) growth. On the other hand, the strain-driven transition is beneficial for the self-assembly of quantum dots where it is necessary to control the size distribution and self-organizing behavior of the islands. In both cases, it is clearly important to identify and understand the kinetic pathways to island formation. From a more basic perspective, the strain-induced transition of epitaxial films allows us to study in detail the interplay between elastic stresses and surface energy in a carefully controlled experimental environment. One would therefore hope that the lessons learned from model semiconductor systems will be of relevance to understanding related phenomena in other areas of materials science and physical metallurgy.Ihe strain-induced two-dimensional (2D)-to-3D transition in the Si-Ge system is manifested by a rich variety of observed surface morphologies. In the case of pure Ge on Si(OO1), the 4% misfit strain induces the formation of so-called hut clusters with curious elongated shapes. Such islands form almost immediately after the deposition of a wetting layer. In the case of lower misfit alloys, a more gentle ripple morphology can result that develops far from the interface. A general trend in all of the experiments is the decreasing size of typical morphological features with increasing misfit stress. In this article, largely guided by our experimental results, we adopt a nucleation and growth description of the 2D-to-3D transition. This approach appears particularly well-suited to explaining the wide spectrum of morphological development present in the Si-Ge system.

Stochastic model for surface erosion via ion sputtering: Dynamical evolution from ripple morphology to rough morphology.

Surfaces eroded by ion sputtering are sometimes observed to develop morphologies which are either ripple (periodic) or rough (nonperiodic). We introduce a discrete stochastic model that allows us to interpret these experimental observations within a unified framework. We find that a periodic ripple morphology characterizes the initial stages of the evolution, whereas the surface displays self-affine scaling in the later time regime. Further, we argue that the stochastic continuum equation describing the surface height is a noisy version of the Kuramoto-Sivashinsky equation.

A mechanism of ion bombardment induced ripple topography

Formation and development of ion bombardment induced morphology on the surfaces of amorphous solids have been investigated by a computer simulation code. The code calculates the surface topography depending on the sputtering yield-incident angle function, distribution of sputtering yield along surface, reflection and shadow effect. It has been shown that the main cause of the formation of the ripple morphology on the surface is the distribution of the sputtering yield along the surface depending on the incident point and incident angle (“downstream” effect). The wavelength of the ripple depends on the range of the sputtering plot on the surface, and is 10–13 times the distance between the impact point and the point of the maximum sputtering yield.

Tight Trough Ripple Marks: Pressure-Solution Modification of Primary Structures

ABSTRACT Ripple-mark-like structures with rounded crests and troughs are preserved in deltamargin sandstones of the Cerro Huerta Formation (Upper Cretaceous) in northeastern Mexico. The structures resemble substratal ripple-mark impressions and mullion structures. However, the internal laminae, symmetry, and lung, parallel crests, as well as an association with normal marks, show that these peculiar structures are modified wave-ripple Ripple indices and the internal stratification indicate that the structures were not produced by syndepositional modification or by soft-sediment deformation. Rather, these bedding-surface structures resulted from modification of oscillatory marks by pressure solution. The tight trough ripple-marked sandstones lie near the center of a broad, open syncline. The trough morphology exists only in marks aligned parallel or subparallel to the local orientation of the regional axial-plane cleavage. Pressure-solution cleavage folia extend along the troughs of the tight trough marks. Apparently, the troughs of the marks controlled the location of cleavage. In this structural setting the maximum compressive stress would be oriented along layering normal to the fold axis. During folding stress (force/area) would be highest at the troughs (least cross-sectional area), and, therefore, cleavage would initiate at the troughs. Thus, volume loss during formation of the pressure-solution cleavage was localized in the troughs, acc unting for the tight trough ripple morphology.

Wind sedimentation tunnel experiments on the origins of aeolian strata

ABSTRACTThe origins and sedimentary features of grainfall‐, avalanche‐, and ripple‐produced strata have been studied experimentally in a wind sedimentation tunnel. Rate of deposition, wind velocity and wind duration have been shown to control specific sedimentary features of these types of strata.Grainfall‐produced strata were deposited on a horizontal surface, and surfaces sloping up to the angle of initial yield for dry sand (about 34°). Thickness of a grainfall‐produced stratum depended upon rate of deposition and duration of a specific wind event. Grainfall‐produced strata were both non‐graded and graded. Graded strata were produced by changes in wind velocity which controlled size of sand in transport and flying distances of individual grains. Distinctive features of grainfall‐produced strata are: (a) gradual thinning, or tapering downwind (e.g. down the simulated slipface and across the simulated interdune; (b) extreme variability of thickness from less than 1 mm (wind gusts of a few seconds) to 10 cm or more (sustained gusts).Aeolian avalanche‐produced strata were formed when grainfall‐produced strata steepened above the angle of initial yield and sheared downslope. A rapid transition in sedimentary features from top to bottom of the slipface characterized avalanche‐produced strata of the slump degeneration type in dry sand derived from grainfall deposition. Fadeout laminae formed near the top of the simulated slipface and about 1 m farther down the slipface were flame structures and drag folds. Near the base of the slipface, the avalanche truncated and then overrode grainfall‐produced deposits. Distinctive features of avalanche‐produced strata for a 2.5 m long slipface are the deformation structures, a thickness of 1 or 2 cm, sandflow toes, and steep dip (34°). Each avalanche‐produced stratum was roughly tabular in cross‐section parallel to wind direction, with gradual pinchout upslope.Aeolian ripple‐produced strata were deposited on horizontal surfaces, and surfaces sloping to as much as 28°. Thickness of a ripple‐produced stratum depended upon rate of deposition, morphology of the ripple, and rate of ripple migration. A maximum thickness of several centimetres was observed for a single ripple‐produced stratum. Shape and attitude of ripple foresets was controlled by ripple morphology. Distinctive features of aeolian ripple‐produced strata are: (a) presence of ripple foresets; (b) abrupt changes in thickness of a stratum or pinchout over downwind distances of a few centimetres; (c) low average foreset‐to‐diastem angle (10–15°); (d) low ripple‐climb angle (<10°).

Experimental studies on meltback morphology of InP

The meltback morphology of InP was experimentally investigated. As superheating and meltback time were increased, the surface became smoother to give macroscopically very flat appearance in contrast to the ripple morphology of the LPE growth. These observations were studied based on the morphological stability theory, which predicted stable and unstable behaviors, respectively, for the metling and growing interfaces. The agreement between the experiment and the theory strongly suggests that the meltback morphology is governed by constitutional superheating.

Ion bombardment induced ripple topography on amorphous solids

Abstract Earlier studies of the ion bombardment induced ripple morphology on the surfaces of amorphous solids when compared with geomorphological effects are shown to possess many similar features. The present study, with 40 keV Ar+ ion bombarded Si suggests that analogies are incomplete, however, and that greater similarities with the process of macroscopic sandblasting (corrasion) exist. It is shown that the genesis of wave like structures on Si is from isolated features, which have the appearance of ripple trains, which are faceted. It is suggested that these features result from particle flux enhancement processes near surface dimples generated by stress induced surface lifting.