Anisotropic Transport of Impurities in Ion Mixing
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ANISOTROPIC TRANSPORT OF IMPURITIES IN ION MIXING S. Matteson, J.A. Keenan, and R.F. Pinizzotto Texas Instruments Incorporated P.O. Box 225936 MS-147 Dallas, TX 75265
ABSTRACT Experimental investigations of the redistribution of a thin (lnm) Sn marker layer lying between two thicker dissimilar layers (Ge and Si) during 360 keV As ion irradiation are reported. Several permutations of layer arrangements were tested, i.e. Si/Sn/Ge, Ge/Sn/Si, Si/Sn/Si and Ge/Sn/Ge. It was also found that in the dissimilar "diffusion" couples, the Sn drifts in the Ge rich direction, regardless of whether the Ge is on the surface side or the substrate side of the marker. This phenomemon of anisotropic transport is interpreted as a drift induced by a gradient in the "diffusion" coefficient. The radiation resistance of concentrated alloys is discussed in the light of this phenomenon. INTRODUCTION Ion Mixing or Ion Beam Mixing has1 received much attention in the last five years, both experimental and theoretical. -4 The nature of the processes at work in Ion Mixing has so far eluded precise description. It appears that, in general, thin impurity distributions are redistributed by ion mixing obeying approximately gaussian statistics, i.e. the width of the redistributed impurity profile grows as the square root of the incident ion fluence and is often independent of temperature. The rate of the increase in the width of the distribution is directly related to the energy deposited in atomic displacements. 5 However, the precise functional relationship is not well established. 6Moreover, small shifts in the peak position of the distributions have been reported. Attempts at describing the atomistic origins of Ion Mixing have taken three forms. Random walk models have been proposed which correctly predict the gaussian-like form of the redistribution profiles. The significant contribution of ballistic mixing in these models comes from low energy recoils of impurity atoms. The range of low energy displaced atoms is not known accurately but is essential for quantitative predictions. Some estimates suggest that simple ballistic mixing is insufficient to account for the observations. Recoil mixing, an alternative model of Ion Mixing, treats the relative displacement of the impurity atom in the host matrix by examining the high energy recoils of the various atoms. Anisotropy is an expected outcome of such processes. A prediction of this model is that the profiles would be non-gaussian. 3 This is not generally observed. More recent models have suggested7 that a relaxation mechanism may overshadow the prompt ballistic displacements. Indeed, the displaced impurity atom may "wander" some distance from its original site before it is trapped again. This process could be expected to appear temperature independent, as well as ion dose rate independent, if the trapping sites were not mobile and if the relaxation occurred over a time which was short relative to the mean time between displacements for an atom. There is an apparent contradiction between gaussian-l
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