Investigation of a Thermal Spike Model for Ion Mixing of Metals with SI
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iNVESTIGATION OF A THERMAL SPIKE MODEL FOR ION MIXING OF METALS WITH SI
U. SHRETER, FRANK C. T. SO, B. M. PAINE AND M-A. NICOLET California Institute of Technology, Pasadena, CA 91125
ABSTRACT A model for ion mixing of bilayer systems in a thermal spike is described. Normal thermal reactions, at a constant average temperature, are assumed for the duration of the spike. New experimental results on mixing of a NbiSi couple with Xe ions are shown to agree with the prediction of the model that the apparent activation energy in ion mixing can depend upon the ion species. Calculations of spike mixing tor Cr and Ni with Si are presented which show that a substantial part of the experimentally observed mixing rates, and their temperature dependence, can be attributed to thermal spike effects.
INTRODUCTION Atomic mixing induced by heavy-ion irradiation has been attributed to two main mechanisms: collisional mixing, and delayed thermal diffusion of irradiation-induced defects (radiation-enhanced diffusion) [1,23. Mixing in metal-Si bilaver systems has been interpreted as a collisional process in the temperature-independent regime (below 300 K) and radiation-enhanced diffusion in the temperature-activated regime (above 300 K). but recent investigations of ion mixing phenomena suggest that the mixing mechanisms may be more complex than this. Radiation-enhanced diffusion would be expected to vary with the flux of the irradiation, but nverback et. al. [3) have shown that mixing of NiiSi bilayers in the high temperature regime is independent of flux. They therefore argued that the transport mechanism cannot be normal radiation-enhanced diffusion but rather ,s rapid migration over distances of the order of the cascade volume, i.e. it is essentially an intra-cascade phenomenon. Also, Averback [4] has determined that mixing of FtiSi bilayers at very low temperatures depends superlinearly on the density of energy deposited in atomic displacements. This is not consistent with a collisional mechanism based on two body collisions and suggests a co!lective motion or 'spike". In recent work we have determined that in the formation of CrSi 2 by ion mixing the irradiation induces an inter-acial reaction that is temperature-dependent [5] and therefore is clearly not a purely collisional erfect, and also does not necessarily invoive radiation-enhanced diffusion. mdditionaI clues as to the nature of the mixing processes in metal3i systems can be obtained by comparing the formation of binary compounds by thermal annealing and by ion mixing. There are strong correiations in the first phases that are formed and in the reaction Linetics [o&, suqgesting that the ion mixing indeed involves thermal diffusion and reactions. However, the temperature dependences of the two processes are different. Ion mixing induces reactions at lower ambient temperatures and with lower apparent activation energies than thermal annealing.
Mat. Res. Soc. Symp. Proc. Vol. 27 (1984) QElsevier Science Publishing Co., Inc.
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These observations suggest that a thermal
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