The nature of cluster ion–surface interactions changes dramatically with the kinetic energy and mass of the incoming cluster species. In this article we review some recent work on the nature of cluster–surface interactions spanning an energy range from a few tens of meV/atom to several MeV/cluster and cluster sizes in the range of 1–300000 atoms/cluster. We describe five possible distinct outcomes of a single cluster impact event: (i) deposition into a non-epitaxial configuration, (ii) deposition into an epitaxial configuration, (iii) crater formation by liquid flow, (iv) crater formation by hydrostatic pressure, (v) implantation.
We compare systematically the threshold displacement energy surface of 11 interatomic potentials in Fe. We discuss in detail different possible definitions of threshold displacement energies, and how they relate to different kinds of experimental threshold displacement energies. We compare the threshold results to experiments, and find that none of the 11 tested potentials agrees fully with experiments. However, all the potentials predict some qualitative features in the same way, most importantly that the threshold energy surface close to the 1 0 0 crystal direction is flat and that the largest threshold energies occur around very roughly the 1 2 3 crystal direction.
We compare systematically the threshold displacement energy surface of 11 interatomic potentials in Fe. We discuss in detail different possible definitions of threshold displacement energies, and how they relate to different kinds of experimental threshold displacement energies. We compare the threshold results to experiments, and find that none of the 11 tested potentials agrees fully with experiments. However, all the potentials predict some qualitative features in the same way, most importantly that the threshold energy surface close to the 100 crystal direction is flat and that the largest threshold energies occur around very roughly the 123 crystal direction. [All rights reserved Elsevier]
Molecular dynamics computer simulations have proven to be an extremely useful method to examine the basic mechanisms of ion-induced damage production in materials. In the particular case of insulators with a high degree of ionic bonding, there are some complications in the simulations associated with how the charges on atoms should be handled. In this article I review different approaches of handling the ionicity and discuss the limits of their applicability. I also present some of our recent simulation results in GaN and hydrogenated amorphous carbon. Comparison of defect production in collision cascades in GaN simulated with two different models, one with explicit ionic interactions and another without, gives the surprising result that the primary damage production does not seem to be affected by explicit ionic interactions. This indicates that at least in GaN it is justified to use fast short-range interaction models in simulations of the primary states of radiation damage
Recent work on the sizes of craters produced by ion impacts of solids has shown that the size of the crater scales with the inverse square of the cohesive energy. This observation is in contrast to the size of craters produced in macroscopic impacts, which scale directly with the inverse of the cohesive energy. It has relied on the assumption that the melting temperature is proportional to the cohesive energy. Using computer simulations, we now show that the size scales in fact with the inverse of the product of the melting temperature and cohesive energy. This provides direct proof that the reason to the different behavior of macroscopic and ion-induced cratering is flow of the liquid produced by the ions
K. Nordlund
P. Partyka
Y. Zhong
I.K. Robinson
R.S. Averback
P. Ehrhart
Diffuse X-ray scattering (DXS) at glancing incidence is a potentially powerful means for elucidating damage structures in irradiated solids. Fundamental to the analysis of diffuse X-ray scattering data is a knowledge of the atomic displacement field around defects, which for implantation damage in crystals like Si has been difficult to obtain using analytical solutions of elastic continuum theory. We present a method for predicting the diffuse scattering pattern by calculating the displacement field around a defect using fully atomistic simulations and performing discrete sums for the scattering intensity. We apply the method to analyze experimental DXS results of defects produced by 4.5 keV He and 20 keV Ga irradiations of Si at temperatures of 100-300 K. The results show that the self-interstitial in ion-irradiated Si becomes mobile around 150 K, and that amorphization of silicon by light and medium-heavy projectiles occurs homogeneously through the buildup of interstitial clusters, and not within single cascade events
We present an efficient molecular dynamics method for calculating ion ranges and deposited energies in the recoil energy region 100 eV to 100 keV. The method overcomes some of the drawbacks of the binary collision approximation methods conventionally used to calculate ion ranges. We describe principles by which one can simulate implantation into polycrystalline materials, and study the effect of the crystal structure on the ion range. Application of the simulation method to practical cases is demonstrated by analyzing experimental range results of 50 keV silicon self-ion-implantation measured at our laboratory
Molecular dynamics (MD) computer simulations, while very extensively used in chemistry and materials physics, have largely been absent in the theoretical treatment of ion beam analysis. Instead the computationally more efficient binary collision approximation (BCA) methods are widely used. In this paper I compare the two methods regarding the level of physical approximation versus accuracy, using a simple model study as an illustrative example. I then show, based on results in the literature, that although in most cases BCA methods are well sufficient for ion beam analysis, there are special conditions where MD methods are required even for keV and MeV kinetic energy processes.
Molecular dynamics (MD) computer simulations, while very extensively used in chemistry and materials physics, have largely been absent in the theoretical treatment of ion beam analysis. Instead the computationally more efficient binary collision approximation (BCA) methods are widely used. In this paper I compare the two methods regarding the level of physical approximation versus accuracy, using a simple model study as an illustrative example. I then show, based on results in the literature, that although in most cases BCA methods are well sufficient for ion beam analysis, there are special conditions where MD methods are required even for keV and MeV kinetic energy processes. [All rights reserved Elsevier].
K. Nordlund
J. Tarus
J. Keinonen
M. Ghaly
R.S. Averback
Although ion beam mixing has been studied intensively over the last 20 years, many questions about the fundamental mechanisms involved during mixing remain unresolved. We review here recent simulation and experimental work which provides answers to some of the lingering questions about mixing in elemental materials. The results make clear the specific role which thermodynamic material properties, the nature of atomic bonding and electron-phonon coupling can have on ion beam mixing. Agreement obtained by direct comparison of simulated and experimental mixing coefficients gives confidence in our results, indicating that the experimental mixing values in heavy metals can be understood predominantly on the basis of atomic motion in liquid-like zones, and that the role of the electron-phonon coupling on ion beam mixing is much smaller than previously thought
The repulsive part of the interatomic potential affects the outcome of computer simulations of many irradiation processes of practical interest, like sputtering and ion irradiation range distributions. The accuracy of repulsive potentials is studied by comparing potentials calculated using commonly available density-functional theory (DFI) and Hartree-Fock (HF) methods to highly accurate fully numerical HF and Hartree-Fock-Slater (HFS) calculations. We find that DFT calculations utilizing numerical basis sets and HF calculations using decontracted standard basis sets provide repulsive potentials which are significantly improved compared to the standard universal ZBL potential. The accuracy of the calculated potentials is almost totally governed by the quality of the one-particle basis set. The use of reliable repulsive potentials open up new avenues for analysis of ion irradiation experiments
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