Understanding ion beam synthesis of nanostructures: Modeling and atomistic simulations
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Understanding ion beam synthesis of nanostructures: Modeling and atomistic simulations M. Strobel1,2,3 K.-H. Heinig1 and W. M¨oller1 1 Forschungszentrum Rossendorf, Institut f¨ ur Ionenstrahlphysik und Materialforschung P.O. Box 510 119, D – 01314 Dresden, Germany 2 CNR–IMETEM, Stradale Primosole 50, I – 95121 Catania, Italy 3 MIRIAM, University of Milan, Via C. Saldini 50, I – 20133 Milano, Italy ABSTRACT Ion implantation, specified by parameters like ion energy, ion fluence, ion flux and substrate temperature, has become a well-established tool to synthesize buried low-dimensional nanostructures. In general, in ion beam synthesis the evolution of nanostructures is determined by the competition between ballistic and thermodynamic effects. A kinetic 3D lattice Monte-Carlo model is introduced, which allows for a proper incorporation of collisional mixing and phase separation within supersaturated solid-solutions. It is shown, that for both the ballistically and thermodynamically dominated regimes, the Gibbs-Thomson relation is the key ingredient in understanding nanocluster evolution. Various aspects of precipitate evolution during implantation, formation of ordered arrays of nanophase domains by focused ion implantation and compound nanocluster synthesis are discussed. INTRODUCTION The ability both to control matter and to design functional units on the nanoscale will be the basis for improvements and innovations in various areas of modern technologies. A promising route towards tailored nanophase materials is the intelligent exploitation of non-equilibrium processing techniques. In the inorganic world, besides thin-film deposition methods and surface patterning procedures with electromagnetic radiation or particles, high-fluence ion implantation has become an established tool in order to synthesize nanostructures (buried clusters, wires or layers) close to the surface of a substrate. During this process [commonly referred to as ion beam synthesis (IBS)] second-phase precipitates evolve from a time- and depth-dependent supersaturated solid solution of impurity atoms via nucleation and growth [see Figure 1(a)]. Depending on their local density and spatial distribution these second-phase domains, consisting either entirely of atoms of the implanted species or forming a compound of implanted and substrate atoms, may coarsen or coalesce to form larger domains, i.e. wires or layers, mainly by surface tension-driven diffusional redistribution. In general, implantation conditions (ion energy E, ion flux j, ion fluence F , and substrate temperature T ) as well as materials properties (i.e. the solubility c∞ (T ) and diffusivity D(T ) of the impurities within the substrate) determine, if the nanocluster formation occurs already during implantation or, if applied, during a subsequent heat treatment. In the latter process, the mean cluster size R can be adjusted via thermally activated Ostwald ripening (OR). This proceeds, however, on the expense of a characteristic, disperse size distribution (Lifshitz-Slyozov-Wagner distribution [1,
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