Molecular dynamics simulation of ripple formation and propagation

Low and medium energy ion irradiation can induce different structures over the surface of semiconductors and metals depending on the parameters used for the irradiation of the surfaces. Different mathematical models have been developed in the last decades to explain the formation reasons of such structures, such as nano-dots or ripples, and to predict the pattern wavelength. These theories have been discussed and tested for several years. In this work, computational methods are used in order to predict and observe such effects. First, a mathematical model, which uses as an input the results from the computational methods, is applied to predict the pattern wavelength and at which angle the regime changes from stable to unstable. Moreover, a relaxation method to remove the background displacement in amorphous silicon, which affects the prediction is presented. Second, a simulation model of sequential irradiation consisting of the irradiation of a segment of the surface, and speeding-up the eventual modification of the surface is developed. The simulation outputs at ultra-low energy are compared with experimental results. The use of the same model at higher energies and applied to aluminum allows us to obtain conclusions on the reason of pattern formation in both materials at different energies and irradiation angles. The last part of this work contains the results obtained from homogeneously distributed irradiation. The irradiation is performed according to an accelerated molecular dynamics method which reduces the time span between impacts and allows us to reach higher fluences. This latter method allowed us to observe the direct ripple-formation and the propagation of the pattern on the surface for the first time. This study allows to explain and observe the different stages before the eventual ripple formation.Låg eller medium energis jonbestrålning kan orsaka strukturella variationer på ytan av halvledare och metaller beroende på parametervalen för ytbestrålningen. Under det senaste årtioendet har olika matematiska modeller utvecklats för att beskriva härkomsten av strukturerna, t.ex. nanokluster eller -krusningar, samt för att bestämma våglängden av mönstrena. Dessa teorier har diskuterats och testats under flera år. I det här arbetet används beräkningsmetoder för att förutse och observera dessa effekter. Först tillämpas en matematisk modell, som använder resultaten från beräkningsmetoderna, för att förutse våglängden av mönstrena och vid vilken vinkel regimen övergår från stabil till ostabil. Utöver det här presenteras även en metod för att ta bort amorfa kislets bakgrundsförskjutning som inverkar på förutsägelsen. Till näst utvecklades en simulationsmodell för att snabba upp eventuella ytförändringar under sekventiell bestrålning av ett ytsegment. Simulationsresultaten vid ultralåga energier jämförs med experimentella resultat. Användningen av samma modell vid högre energiregimer på aluminium möjliggör slutsatser som beskriver orsakerna till mönsterformationerna hos båda materialen vid olika energier och bestrålningsvinklar. Sista delen av det här arbetet innehåller resultat erhållna med en homogent fördelad bestrålning. Bestrålningen är utförd med en accelererad molekyldynamikmodell som kortar av simulationstiden mellan varje impakt och medför möjlighet till högre fluenser. Den sistnämnda metoden tillät oss att observera den direkta formationen av krusningar samt mönstrenas propagering på ytan för första gången. Studien möjliggör förklaringar och observationer av de olika stadierna före eventuell formning av krusningar

Formation of parallel and perpendicular ripples on solid amorphous surfaces by ion beam-driven atomic flow on and under the surface

The off-normal ion irradiation of semiconductor materials is seen to induce nanopatterning effects. Different theories are proposed to explain the mechanisms that drive self-reorganization of amorphizable surfaces. One of the prominent hypothesis associates formation of nanopatterning with the changes of sputtering characteristics caused by changes in surface morphology. At ultralow energy, when sputtering is negligible, the Si surface has still been seen to reorganize forming surface ripples with the wave vector is either aligned with the ion beam direction or perpendicular to it. In this work, we investigate the formation of ripples using molecular dynamics in all the three regimes of ripple formation: low angles where no ripples form, intermediate regime where the ripple wave vectors are parallel to the beam, and high angles where they are perpendicular to it. We obtain atom-level insight on how the ion-beam driven atomic dynamics at the surface contributes to organization, or lack of it, in all the different regimes. Results of our simulations agree well with experimental observations in the same range of ultralow energy of ion irradiation.Peer reviewe

Modeling of high-fluence irradiation of amorphous Si and crystalline Al by linearly focused Ar ions

Long time ion irradiation of surfaces under tilted incidence causes formation of regular nanostructures known as surface ripples. The nature of mechanisms leading to ripples is still not clear, this is why computational methods can shed the light on such a complex phenomenon and help to understand which surface processes are mainly responsible for it. In this work, we analyse the surface response of two materials, a semiconductor (silicon) and a metal (aluminium) under irradiation with the 250 eV and 1000 eV Ar ions focused at 70° from the normal to the surface. We simulate consecutive ion impacts by the means of molecular dynamics to investigate the effect on ripple formation. We find that the redistribution mechanism seems to be the main creator of ripples in amorphous materials, while the erosion mechanism is the leading origin for the pattern formation in crystalline metals.Peer reviewe

Effect of surface morphology on Tungsten sputtering yields

Nuclear fusion is one of the most promising concepts for future energy production, due to the almost endless source of fuel and the lack of greenhouse effects during operation. However, to successfully build a fusion reactor, the development of new materials and knowledge of their behavior are needed. One important structural part of the reactor and the reactor vessel is the wall facing the plasma. The wall will be bombarded by the products of the nuclear reaction, which will erode and degrade its performance. In this work, we study the sputtering of different tungsten surfaces under various conditions, obtaining a deeper understanding of the process using molecular dynamics simulations. Additionally, we present the evolution of W fuzz cells and the effect of surface feature height on the erosion and sputtering of the surfaces under ion irradiation.Peer reviewe

Punching of arbitrary face prismatic loops from hydrogen nanobubbles in copper

When a metal surface is exposed to prolonged irradiation with energetic H-, the ions are expected to penetrate into bulk and dissolve in the matrix. However, the irradiated surfaces exhibit dramatic morphological changes in the form of "blisters " covering the surface exposed to irradiation. Blistering is usually explained by accumulation of implanted gas in the bubbles near surface. However, the exact mechanism of continuous growth of a bubble after it reaches the measurable size is still not fully clear. Commonly such growth is related to prismatic loop punching, which is a short time scale process not easily accessible by experimental techniques. Even atomistic modelling of loop punching in FCC metals is somewhat cumbersome. Since the void surfaces in these metals yield easily through shear loops, these were debatably suggested to explain the plastic growth of a bubble in copper, without demonstrating the detachment of these loops from the void. We address the mechanisms of fast bubble growth in Cu which is associated with blistering of Cu surface exposed to H- irradiation. We observe the emission of a complete prismatic loop enclosed within the number of shear loops with the Burgers vectors aligned with the gliding direction of the prismatic loop. We show that the prismatic loops punched from the bubble surface do not need to be smaller than the bubble cross-section. These simulations capture the general trend of dislocation emission in the condition of hydrostatic pressure exerted by the accumulated gas on the wall of the bubble. (c) 2021 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc.Peer reviewe

The cluster species effect on the noble gas cluster interaction with solid surfaces

The effect of noble gas cluster species on the cluster interaction with solid surfaces was investigated. Processes of Ar, Kr and Xe clusters interaction with Cu and Mo surfaces were studied using molecular dynamics simulations. It is shown that lighter cluster front atoms undergo more backscattering from surface atoms, causing more intense multiple collisions between cluster atoms. This affects cluster penetration, energy exchange between the cluster and surface atoms, and cluster thermalization. The influence of energy per cluster atom on these effects is discussed.The effect of noble gas cluster species on the cluster interaction with solid surfaces was investigated. Processes of Ar, Kr and Xe clusters interaction with Cu and Mo surfaces were studied using molecular dynamics simulations. It is shown that lighter cluster front atoms undergo more backscattering from surface atoms, causing more intense multiple collisions between cluster atoms. This affects cluster penetration, energy exchange between the cluster and surface atoms, and cluster thermalization. The influence of energy per cluster atom on these effects is discussed.Peer reviewe

Comparative study regarding the sputtering yield of nanocolumnar tungsten surfaces under Ar+ irradiation

Nanostructured tungsten has been proposed as a promising option for plasma facing materials in future fusion reactors, because compared to conventional tungsten it shows advantages such as a better radiation resistance and, in particular, a retardation of tungsten-fuzz growth. Besides these aspects, the sputtering yield of nanostructured tungsten under ion bombardment is of interest, since it would affect the atomic density of tungsten emitted into the fusion plasma, which leads to radiative heat losses. In this work, we present a multiscale approach for investigating the sputtering yield of nanocolumnar tungsten surfaces under 1 keV and 2 keV Ar irradiation. Our results cover experiments and also computational simulations, which operate either on the basis of the binary collision approximation and ray tracing or use a full molecular dynamics implementation. In our studied case, both computational approaches can predict the sputtering yield of nanocolumnar tungsten surfaces very well. In comparison to flat W, we observe a much reduced dependence on the ion incidence angle, similar as reported for conventional rough surfaces in literature. However, an additional global reduction of the sputtering yield was identified, which can be attributed to geometrical redeposition effects between the separated nanocolumns. These results support the applicability of nanocolumnar tungsten as a first wall coating.Peer reviewe