A Comparison Between High- and Low-Energy Ion Mixing
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A COMPARISON BETWEEN HIGH- AND LOW-ENERGY ION MIXING Y.-T. CHENG,* E.-H. CIRLIN,** B. M. CLEMENS,* AND A. A. DOW*+ * General Motors Research Laboratories, Warren, MI 48090-9055. ** Hughes Research Laboratories, Malibu, CA 90265. Diffusion in a ther:'nal spike is shown to be the dominant mechanism for both highand low-energy ion mizing. The similarity between high- and low-energy ion mizing is the result of the j•'actal nature of collision cascades. Recently, high-energy (e.g., several hundred keV) ion mixing has attracted much interest because of its technological importance in surface modification [1]. In contrast, there have been fewer studies of low-energy (e.g., several keV) ion mixing, in spite of its importance to the depth resolution of sputter depth profiling and low-energy ion beamassisted deposition. One of the basic questions common to both high-and low-energy ion mixing is, of course, the mechanism of the mixing process. In this paper, we provide experimental evidence that diffusion in a thermal spike is the dominant mechanism in both high- and low-energy ion mixing, and we present an interpretation based on the fractal nature of collision cascades. Over the years, several authors have proposed models of ion mixing. Ion mixing in the absence of radiation-enhanced diffusion has been categorized by two types of ion mixing models. One is based on the transport equations for collision cascades first derived by Lindhard et al. (LSS) [2] and Winterbon et al. (WSS) [3]. This type of model [4-7] takes into account the purely "ballistic" aspects of atomic collisions such as the atomic mass and density, but does not take into account the chemical properties of the solids. This type of model has been called the "ballistic" model of ion mixing. According to the ballistic model, the mixing rate d(4Dt)/d¢, which is the square of the width of the mixed interface per unit ion dose € in bilayer ion mixing experiments, is given by [4,7] d(4Dt) _ C" R2c de p Ed'
(1)
where e is the energy deposited per unit length due to nuclear collisions, p is the atomic density, Ed is the threshold displacement energy, Rc is the range associated with Ea, and C is a function of the atomic mass. The ballistic model was proposed for both high- and low-energy ion mixing. However, eq. (1) has been shown inadequate in describing the magnitude of either high- or low-energy ion mixing [1,8,9]. The second type of model is based on experimental observations from high-energy ion mixing where thermodynamic parameters, such as the heats of mixing AHmiz and the cohesive energy AHcoh, strongly influence the ion mixing efficiency [10-14]. It was shown that 600 keV Xe++ ion mixing in metallic bilayers consisting of elements with large and negative heats of mixing [15], such as Pt/Wi (-122 kJ/g.at) and Pt/Zr (-151 k3/g.at), mix more efficiently than bilayers with zero or small negative heats of mixing, such as Pt/Si (-7 k3/g.at) or Pt/Mo (-42 k3/g.at), under identical ion irradiation conditions. A phenomenological model was developed for the observatio
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