Don E. Harrison

Low energy ion impact phenomena on single crystal surfaces

The dynamics of a solid bombarded by a 600 eV Ar+ ion have been studied classically by computer simulation. The model uses a crystallite of about 250 atoms described by pair potentials derived from elastic constants and which reproduce the surface binding energy of the solid. The relative calculated yield of secondary atom emission from the three low index faces of Cu follow the previously determined experimental order (111) > (100) > (110). We find major differences in the sputtering mechanisms for these faces. On (110), the impacted atom is ejected most frequently, while on (111) and (100) it almost never leaves the solid. We report the energy distribution of the sputtered particles for each face. The simulation successfully predicts the shape of the curve including the low energy maximum which is observed experimentally near 2 eV. In addition our model shows that many low energy atoms attempt to leave the crystal but are subsequently trapped to the solid at large distances from their original sites. This mechanism of radiation enhanced diffusion inevitably occurs in conjunction with sputtering or any other heavy secondary particle emission or scattering process.

Application of molecular dynamics simulations to the study of ion-bombarded metal surfaces

Abstract This article has two primary objectives: to present a status report on our present understanding of the physics of atom ejection by ion bombardment and to indicate the contributions of molecular dynamics simulations to this research area. Because this application of molecular dynamics is relatively unfamiliar, basic simulation techniques useful in open systems are described in some detail. While this review discusses the current situation, explanations and historical background are necessarily included to place problems in perspective.

Sputtering models―A synoptic view

Abstract This article surveys the present state of sputtering theories. Similarities and differences between the statistical theories and atom ejection simulations are discussed in detail. The limitations of both approaches are considered, and the different viewpoints are contrasted. Some computer simulation model results for atom ejection from pure metal single crystals are summarized. The implications of these results, and their contribution toward the future development of sputtering theory is indicated.

Formation of small metal clusters by ion bombardment of single crystal surfaces

The mechanism for the formation of small metal clusters ejected from an ion bombarded metal surface is examined in detail. The analysis is performed by classical trajectory methods which determine the positions and momenta of all particles in a model microcrystallite as a function of time. The calculation utilizes pair potentials for Cu derived from elastic constants of the solid and is performed for 600 eV Ar+ ion at normal incidence to the crystal. The results show that cluster species do not leave the surface as intact parts of the solid but form in a region above the surface. A trajectory for Cu5 formation is traced in detail showing a typical mechanism which is valid for Cun formation where n?7.

Structure sensitive factors in molecular cluster formation by ion bombardment of single crystal surfaces

Abstract The dynamics of molecular cluster formation from a solid bombarded by a 600 eV Ar + ion have been studied classically by computer simulation. The dimers and trimers are found to establish their identity as clusters within interaction range of the solid, but not by a direct ejection of a bound molecule. The Cu 2 /Cu and Cu 3 /Cu ratios are found to be strongly dependent on crystal orientation. The (111) face is 2–3 times more likely to produce multimers than the (100) face. We find 9 trimers from (111) but none from (110). The relationship between cluster composition and the original arrangement of those atoms on the surface is presented in detail. We find that each multimer forms from atoms that originate within a roughly circular region of area ∼70 A 2 or less. This region is not necessarily centered on the ion impact point. A consequence of this observation is that dimers can consist of atoms that were several Angstroms apart on the surface but that most trimers contain at least one nearest neighbor pair of atoms. The calculated energy distribution for the dimers matches well with similar experimental studies.

Computer simulation of the sputtering of clusters

The formation of clusters of sputtered copper atoms from an argon‐bombarded (100) copper surface has been simulated with a computer program which includes interatomic attractive forces. Dimer formation is very common. Most commonly, dimers are formed from atoms which were next‐nearest neighbors in the crystal. Nearest‐neighbor atom clustering is rare, and dimer formation by a single atom moving in a channel has not been observed, and could not be forced by artificial means. All multimer formation mechanisms depend strongly upon both the relatively low speed of the sputtered atoms and the relatively high speed of the knock‐on atoms involved in the sputtering mechanisms. Strong evidence for trimer formation exists. Pairs of dimers with a common atom have been observed. No quadrimers have been identified, but structures which almost meet the stability criteria have been observed.

Algorithm for the Calculation of the Classical Equations of Motion of an N‐Body System

The equations of motion for N‐body systems are usually integrated by means of a central‐difference algorithm. An alternative average force algorithm is described in this paper. The new method is shown to have both theoretical and practical advantages when compared to the central‐difference method.

Computer simulation of sputtering II

Abstract The conclusions of the first paper in this series have been confirmed by simulations in which the copper target is represented by a composite potential function consisting of a Born-Mayer repulsive potential segment, a cubic potential matching segment, and a Morse potential attractive segment. Surface layer relaxation has been included, and surface layer atom binding energies for the most primitive planes of copper have been determined to be:E b(100)=2.4 ± 0.1 ev, E b(100)=2.1 ± 0.1 eV and E b(111)=2.4 ± 0.1 ev. For argon sputtering copper there is no detectable change in the spot patterns between the two models, and the sputtering yields agree within the uncertainty of the simulation. Sputtering yield vs. energy curves now agree quite closely with the experimental data. For sputtering at 5 keV the energy distribution of the sputtered atoms appears to have the form dN/dE ∼ E−1.4. The argon copper sputtering efficiency matches smoothly into the polycrystalline experimental data reported by H. H. A...

Computer simulations and rainbow patterns of alkali ion scattering from metal surfaces

Abstract In a previous article [Surface Sci. 172 (1986) 90] the necessity of 3-dimensional computer simulations has been shown for the modelling of the scattering of 10–100 eV K + from W(110). The measured triple differential cross section shows a complicated peak structure which can only be understood with the help of trajectory calculations. A simple model potential is given that can describe the scattering at 35 eV. The calculated trajectories show from which impact regions in the surface unit cell the peaks in the spectra originate. This mapping also allows a rainbow analysis for a triple differential cross section.

Ejection of molecular clusters from ion‐bombarded surfaces

We have modeled, using classical dynamics, the dissipation of momentum of a 600‐eV Ar+ ion as it bombards a metal single crystal. The model correctly predicts relative sputtering yields, secondary particle energy distributions, and angular distributions. In addition, it also gives considerable insight into the mechanism of molecular cluster formation. For the three low index faces of copper, for example, the observed dimers, trimers, and higher multimers form over the surface but within interaction range of the solid. The clusters show rearrangement of their constituent atoms from their original surface positions, but do arise from a localized region of radius ∠5 A. We have also examined oxygen atoms and CO molecules adsorbed on copper and nickel, respectively. For the chemisorbed O atoms, the clusters Cu2, CuO, O2, Cu3, Cu2O, CuO2, and O3 have all been observed to form over the surface, analogous to the clean metal case. For CO, however, most of the ejection occurs molecularly due to the strong carbon–ox...