Molecular Dynamics Simulation of Sputtering with Mmany-Body Interactions
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MOLECULAR DYNAMICS SIMULATION OF SPUTTERING WITH MANY-BODY INTERACTIONS' Davy Y. Lo*, Tom A. Tombrello*, Mark H. Shapiro*, and Don E. Harrison Jr.** *Division of Physics, Mathematics, and Astronomy, 200-36 California Institute of Technology, Pasadena, CA 91125 **Department of Physics, US Naval Postgraduate School, Monterey, CA 93940
ABSTRACT
Many-body forces obtained by the Embedded-Atom Method (EAM) [41 are incorporated into the description of low energy collisions and surface ejection processes in molecular dynamics simulations of sputtering from metal targets. Bombardments of small, single crystal Cu targets (400-500 atoms) in three different orientations ({100}, {110}, {111}) by 5 keV Ar+ ions have been simulated. The results are compared to simulations using purely pair-wise additive interactions. Significant differences in the spectra of ejected atoms are found. Introduction Computer simulations of sputtering generally have used pair potentials to describe the forces between atoms [1]. Pair potential models assume that the total potential energy of a system of atoms may be expressed as a sum of two-body terms. The pair potential model has been successful in describing bulk properties such as heat of sublimation, bulk modulus, and thermodynamic equations of state [2]. This is surprising because atoms are not point particles. However plausible this may be in cases where the atomic density is macroscopically uniform, the pair potential approximation is rather dubious for processes that involve extreme local non-uniformity of atomic densities such as vacancy formation, surface diffusion, and atomic ejection during sputtering. A simple example will illustrate the many-body forces in atomic interactions. The forces between two isolated atoms consist of the mutual repulsion of the ion cores and the attractive force of the chemical bond which depends directly on the electron distribution. Introduction of a third atom will disturb the original electron distribution and thereby change the force between the first two atoms. The extent of this many-body effect will therefore depend on the polarizability of the atoms. In particular, it will be important in metals. We can always write the total energy as a sum of pair potentials, but it will not consistently describe the forces in particular atomic configurations. This is seen when we compare a pair potential fitted to experimental bulk Cu data with a dimer potential fitted to experimental diatomic data (figurel). The bulk Cu potential has a well depth of .34 eV [21 while the potential energy of a Cu dimer in vacuum has a minimum of 2.4 eV [3]. The two pair potentials are drastically different. In the case of sputtering from metals, the ejection process at the surface will involve dynamical multimer atomic configurations where the many-body effect should play an important role. The inability of pair potentials to describe bulk and multimer energetics is also a drawback in multimer ejection studies. To include these many-body effects, we will use the Embedded- Atom Method
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