Ripple structures on ion bombarded surfaces arising from the sputter yield dependence on incidence angle

It is shown that ripple structures on oblique incidence ion bombarded surfaces can be stable features under ion erosion without the necessity to invoke mass redistribution, surface diffusion or micro-roughening due to surface curvature dependent energy deposition. Instead the patterns are predicted to be a natural consequence of non-linear effects due to the dependence of the sputtering yield on the angle of incidence

Ion beam induced surface pattern formation and stable travelling wave solutions

The formation of ripple structures on ion bombarded semiconductor surfaces is examined theoretically. Previous models are discussed and a new nonlinear model is formulated, based on the infinitesimal local atomic relocation induced by elastic nuclear collisions in the early stages of collision cascades and an associated density change in the near surface region. Within this framework ripple structures are shown to form without the necessity to invoke surface diffusion or large sputtering as important mechanisms. The model can also be extended to the case where sputtering is important, and it is shown that in this case certain 'magic' angles can occur at which the ripple patterns are most clearly defined. The results are in very good agreement with experimental observations

A travelling wave model of ripple formation on ion bombarded surfaces

We present a mathematical model describing surface modification resulting from atomic motion after ion bombardment. The model considers only the defect production and recovery process induced by the local atom rearrangement and is essentially independent of surface topography changes formed by both sputtering and surface diffusion. A stable analytic, travelling wave solution is presented for a specific incident angle, which agrees with experimental observation excellently. © 2013 Elsevier B.V. All rights reserved

Markov-chain model of classified atomistic transition states for discrete kinetic Monte Carlo simulations

Classical harmonic transition state theory is considered and applied in discrete lattice cells with hierarchical transition levels. The scheme is then used to determine transitions that can be applied in a lattice-based kinetic Monte Carlo (KMC) atomistic simulation model. The model results in an effective reduction of KMC simulation steps by utilizing a classification scheme of transition levels for thermally activated atomistic diffusion processes. Thermally activated atomistic movements are considered as local transition events constrained in potential energy wells over certain local time periods. These processes are represented by Markov chains of multidimensional Boolean valued functions in three-dimensional lattice space. The events inhibited by the barriers under a certain level are regarded as thermal fluctuations of the canonical ensemble and accepted freely. Consequently, the fluctuating system evolution process is implemented as a Markov chain of equivalence class objects. It is shown that the process can be characterized by the acceptance of metastable local transitions. The method is applied to a problem of Au and Ag cluster growth on a rippled surface. The simulation predicts the existence of a morphology-dependent transition time limit from a local metastable to stable state for subsequent cluster growth by accretion. Excellent agreement with observed experimental results is obtained

Modelling the sputtering of Au surfaces using a multi time-scale technique

We present results from an atomistic computer simulation model of the sputtering of gold crystal surfaces under 500 eV ion bombardment by Au and Ar ions for doses up to 1014 ions cm−2. The multi time-scale technique uses molecular dynamics to calculate the fast ballistic collision processes in the early stages of the cascade, whereas an on-the fly kinetic Monte Carlo technique is used to model the relaxation and diffusion processes between successive ion impacts when the defect motion has begun to be dominated by rare events. The results indicate a large amount of crystalline recovery between impacts, some facetting of the crystal surfaces but no large sub-surface defect accumulation. Because of this recovery process, sputtering yields and energy distributions are in good agreement with those obtained assuming a perfect crystal surface and also with those experimentally measured

Structural changes at grain boundaries in bcc iron induced by atomic collisions

Symmetrical tilt and twist grain boundary structures have been simulated in bcc iron using a many-body potential of the Finnis-Sinclair form. Initial structures were relaxed to the local minimum energy configuration using molecular dynamics. The width and relative energies of the resulting grain boundaries have been calculated. Collision cascades have been initiated in the structure by imparting initial energy to a single Fe atom and the interaction of the cascade with the grain boundary has been studied again using molecular dynamics simulations. The cascades were chosen where the primary knock-on atom (PKA) had initial energy of 1 keV and the orientation and distance of the PKA were changed in order to generate some statistical information concerning the radiation damage near the interface. The results show an increased radiation damage in the grain boundary region compared to the bulk material. The interstitials that form in the boundary region seem to be stable and do not move away from the boundary during the recrystallisation phase of the collision cascade. Clusters of interstitials are easily produced at the boundary in either structure but the defects induced near the twist boundary are more extensive than those near the tilt boundary

Critical island size for Ag thin film growth on ZnO (000-1)

Island growth of Ag on ZnO is investigated with the development of a new technique to approximate critical island sizes. Ag is shown to attach in one of three highly symmetric sites on the ZnO surface or initial monolayers of grown Ag. Due to this, a lattice based adaptive kinetic Monte Carlo (LatAKMC) method is used to investigate initial growth phases. As island formation is commonly reported in the literature, the critical island sizes of Ag islands on a perfect polar ZnO surface and a first monolayer of grown Ag on the ZnO surface are considered. A mean rate approach is used to calculate the average time for an Ag ad-atom to drop off an island and this is then compared to deposition rates on the same island. Results suggest that Ag on ZnO (0 0 0 View the MathML source1¯) will exhibit Stranski–Krastanov (layer plus island) growth

Borosilicate glass potentials for radiation damage simulations

Three borosilicate glass (SiO2-B2O3) fixed charge potentials from the literature are compared (Delaye and Ghaleb, 1996; Kieu et al., 2011; Rushton, 2006) and their suitability for use in simulations of radiation damage is assessed.For a range of densities, we generate glass structures by quenching at 5×1012 K/s using constant volume Molecular Dynamics. In each case, the bond lengths, mean bond angles, bulk modulus, melting point and displacement energy thresholds are calculated, and where possible compared to experimental data. Whereas the bond lengths and mean bond angles are reasonably well predicted, we find that the potentials predict melting temperatures, bulk moduli and densities that are higher than experimental data.The displacement energy thresholds are generally lower than those for ionic crystalline materials, but show a wider spread of values. However, the barriers for atomic rearrangements, after atoms have been displaced in the equilibrium structures, are very high. This indicates, that the radiation damage produced in the ballistic phase of a collision cascade, is likely to persist for extended time scales. This is in contrast to crystals, where interstitials and vacancies can diffuse rapidly between successive radiation events

Atomistic-scale modelling of nanoindentation into optical coatings

Simulations of nanoindentation into a typical optical-coatings stack employed in energy efficient glazing have been performed using classical molecular dynamics (MD) and a coupled finite element/MD methodology. The coatings stack consists of a low-emissivity material, Ag, sandwiched between two layers of a transparent conducting oxide (TCO), ZnO. Simulations into both the ZnO and the coatings stack show a strong interaction between the tip symmetry and crystal symmetry in the observed displacement field. A large amount of elastic recovery is observed for both the ZnO system and the coatings stack, but with an impression left on the surface that looks like a crack but extends no further than the tip imprint at maximum depth. The full stack is observed to have a lower hardness once there is a significant penetration of the displacement field into the Ag, when compared to the pure ZnO system. A comparison between the coupled finite element/MD methodology and the fixed boundary MD-only model shows that the boundary conditions have little influence on the calculated results

Variable step radial ordering in carbon onions

We analyse the radial distribution of atoms in carbon onions optimised using a combination of classical molecular dynamics and density functional theory. X-ray diffractograms of thermally annealed nanodiamonds are also exploited together with high-resolution transmission electron microscopy data published elsewhere. The internal radial ordering of atoms inside the onions was determined showing a gradual change of intershell separation as a function of radius. This change may result in a twisted internal structure of the onions. The influence of atomic defects appearing in the shells altering their curvature on the formation of twisted onions is discussed