The Dependence of Ion Beam Mixing on Projectile Mass
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THE DEPENDENCE
OF ION BEAM MIXING ON PROJECTILE MASS*
R. S. AVERBACK, L. J. THOMPSON AND L. E. REHN Materials Science and Technology Division, Argonne National Laboratory, 60439, U.S.A. 9700 S. Cass Ave., Argonne, Illinois
ABSTRACT Ion beam mixing in Pt-Si bilayered samples was measured during irradiation with projectiles ranging in mass from 4 amu (He) to 131 amu (Xe) at 10 K, 300 K and 373 K. Using deposited damage energy as a basis for comparing the different irradiations, it was found that the heavier ions were more efficient than the lighter ones for inducing mixing. Moreover, it was observed that the mixing was essentially independent of temperature below 373 K. These results are interpreted on the basis that the mixing is caused by the stimulated motion of defects during the cooling phase of energetic cascades.
INTRODUCTION Theoretical attempts to explain the low-temperature diffusion of The collisional success. solids during ion irradiation have had little mixing theory of Sigmund and Gras-Marti [l), for example, greatly underestimates the magnitude of diffusion; moreover, it incorrectly A second predicts a shift of light impurities relative to heavy ones [2]. collisional theory, which was proposed by Matteson et al. [31, and which is based on a "random flights" model, is able to predict the magnitude of mixing; however, it predicts also that light marker atoms should diffuse substantially more than heavy ones and this is contrary to observation To overcome the failures of ballistic mixing theories, Sigmund has [4]. recently proposed that the large magnitude of mixing arises from the In migration of defects produced in energetic displacement cascades [5]. reference 5, it seems to be implied that the defect motion results from a This would be difficult to reconcile with the thermally activated process. observations that the magnitude of mixing does not dramatically decrease when the specimen is cooled to very low temperatures, 10 K [6,7], for which The theory thermally-induced defect motion is thought to be suppressed. itself however, only requires that defects have some mobility within the An alternative mechanism to thermally-induced defect cascade region. motion is defect motion which is stimulated by the agitation of the lattice There is both experimental [8] during the cooling phase of the cascade. and theoretical [9,101 evidence that such stimulated motion is important in In this model both the production of defects and their cascades. To explore stimulated motion is required for low-temperature diffusion. this possibility, mixing was measured for the cases that: 1) defects are produced in isolated low-energy recoil events, and 2) defects are produced Specifically, the experiments in energetic displacement cascades. consisted of measuring the interdiffusion of Pt-Si diffusion couples during irradiation with projectiles ranging in mass from 4 amu (He) to 131 amu The He irradiation produces (Xe) at temperatures between 6 K and 373 K. defects (i.e. Frenkel pairs) mostly in low-energy recoil events, wh
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