Strain Relief by Ion Beam Mixing. Molecular Dynamics Simulations Applied to Metallic Hetero-Structures
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STRAIN RELIEF BY ION BEAM MIXING. MOLECULAR DYNAMICS SIMULATIONS APPLIED TO METALLIC HETERO-STRUCTURES.
A.M.MAZZONE C.N.R. - Istituto LAMEL-Via de' Castagnoli 1 - 40126 - Bologna - Italy.
ABSTRACT This work presents molecular dynamics simulations of low-energy (40-80 eV) ionbeam mixing of thin metallic hetero-structures. The results indicate that the propagation of the cascade may maintain or even restore crystallinity in disordered interfacial regions. INTRODUCTION Owing to the current trends towards the use of superlattices and microstructures in electronic device fabrication, multilayers of thickness in the range tens-hundreds A are being actively investigated. The technolgies for the fabbrication of these structures involve the use of charged (or even neutral) beams either to induce ion beam mixing or for beam-assisted deposition. It is known that ion energies in the range of few eV play a predominant role in the formation of defects [1] and in beam-assisted technologies the phenomena taking place in the eV range control the yield of the processing. Today many aspects of these phenomena are incompletely understood and the field is being actively investigated. A critical problem is represented by elastic effects. On one side, in fact, it is known that the ion generates, via impact collisions, atomic displacements and lattice disorder. On the other side strain relief has been observed in thin hetero-structures under mixing conditions [2]. The purpose of this work is to offer a description on an atomic scale of these phenomena which are normally treated on the basis of standard elasticity theory. A molecular dynamics simulation method is used to analyze the effect of the ion beam on thin heterostructures containing a disordered interfacial layer. The systems analyzed are Fe/Ag and Au/Ag irradiated with As+ and Au+ ions of energy in the range 40/80 eV. The results of the simulations indicate that the propagation of the cascade may anneal excess of the atomic concentration restoring a crystalline order.The effect was not previously known and it represents a plausible explanation for the experimental results [2] on strain. THE SIMULATION METHOD The simulation method is the one previously used in [3]. The formation of the heterostructure is simply simulated by bringing into contact two metallic sublattices and allowing the hetero-structure to reach an equilibrium configuration under the action of the lattice forces. As discussed in [3], this process represents a simplified approach to the description of normal multi-layer growth. However an attempt of a more detailed account may be unrealistic as it is known that the state of the interface is case-dependent and may be altered by uncontrollable factors such as contaminants and impurities. In the simulation cell each sublattice has a cubic form and the two cubes overlap. The number of atoms in each sublattices (approximately 1000) is chosen in such a way that the two cubes have an identical linear dimension, that is approximately 25 A. The thickness of the cell Mat. R
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