Mechanisms and kinetics of ion implantation
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I. INTRODUCTION Ion implantation has been widely applied to control modifications of surface-sensitive properties in recent years, because of its many beneficial features that cannot be matched by conventional diffusion or coating methods.1 By this process a selected atomic species is introduced into the surface layer of a material, to a depth that is, in principle, determined by the ion energy and the stopping power of the target. With the nearly unlimited choice of elements to be implanted and the easy control of their concentrations, ion implantation allows the convenient production of a wide range of surface alloys. Moreover, being a nonequilibrium technique, practically free from thermodynamic constraints, such as diffusivity and solubility, ion implantation also provides a flexibility in the selection of processing variables, including implantation rate and temperature. Ion implantation is, however, a violent process. At practical energies, between ~ 50 and 500 keV, the slowing-down of the implants in the target material generates displacement cascades and nonequilibrium point defects (interstitials and vacancies). The former is the physical origin of sputtering, when it takes place in the very near-surface region, and displacement mixing, which comprises recoil implantation and cascade mixing. Recoil implantation occurs when the lighter atoms are preferentially transported in the beam direction due J. Mater. Res. 1 (2), Mar/Apr 1986 http://journals.cambridge.org
to preferential momentum transfer, whereas cascade mixing is a random-walk process resulting from the movement of higher-order recoils. To a good approximation, displacement mixing can be assumed to be temperature-independent since the energies of the recoils involved are much larger than thermal energies. At sufficiently high temperatures, radiation-induced point defects (which are at concentrations significantly larger than their thermodynamic equilibrium values) migrate until they are eliminated by mutual recombination or annihilation at sinks. Their motion leads to radiationenhanced diffusion (RED) because the diffusion coefficients of atoms in the alloy are proportional to the defect concentrations. Nonuniform defect production and/or annihilation give rise to persistent defect fluxes in the solid during irradiation. Any preferential association of defects with a particular alloy constituent and/or preferential participation of a component in defect diffusion will couple a net flux of the alloying element to the defect fluxes, leading to radiation-induced segregation (RIS). The relative importance and time scale of each process depend mainly on the implanted material, the species and energy of the incoming ions, and implantation temperature. For example, in targets in which the atomic species are not significantly different in mass, recoil implantation may be neglected2; at high ion energies, sputtering is reduced3; at low temperatures (below ~ 100 °C) RIS is ineffective,4"7 whereas at higher temperatures, RIS and RED overwhelm the effects of dis
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