On the thermodynamical driving force during ion mixing of the Co-Si system

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Abstract
The relative importance between the thermodynamical driving force and kinetics in thermal annealing and ion mixing in the thermally activated regime has not been clarified. To probe the role of the thermodynamical driving force in reactions between metals and silicon, the Co-Si system was chosen for investigation. In general, three silicide phases are formed during thermal annealing of samples consisting of Co thin films deposited on Si substrates, i.e., Co2Si (the first phase to form with a heat of formation, ΔHf=−9 kcal/g atoms), CoSi (ΔHf=−12 kcal/g atoms), and CoSi2 (the last phase to form, with ΔHf=−8.2 kcal/g atoms). Previous experiments have shown that annealing a sample of Si/CoSi/Co converts CoSi into Co2Si instead of a continuous growth of CoSi. This type of reaction is apparently unrelated to the magnitude of the thermodynamical driving force since ΔHf of CoSi is significantly larger than those of Co2Si and CoSi2, but is kinetically restricted instead. Under ion mixing conditions the kinetic restriction is expected to relax due to enhanced atomic mobilities under ion irradiation; the ion-induced reactions should then be driven by thermodynamics; i.e., growth of the phase with the largest ΔHf is favored. In this work, phase formation induced thermally and with ion mixing in the Co-Si system was investigated using Rutherford backscattering spectrometry and x-ray diffraction (Read camera). It was found that in thermal annealing, Co2Si is the first phase to form and Co is the dominant moving species in the formation of Co2Si, in agreement with previous results. In ion mixing, both CoSi and Co2Si are observed to form. At low temperatures, the formation of CoSi dominates. As the substrate temperature is increased, the formation of Co2Si becomes more significant. Co and Si are found to be the dominant moving species in the ion-beam-assisted formation of Co2Si and CoSi, respectively. By introducing the concept of an effective heat of formation, initial phase formation in both ion mixing and thermal annealing can be rationalized in terms of the thermodynamical driving force.

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