Kinetics of the Silicide Formation

The energetic balance requires a negative heat of formation for the silicide. However, all phenomenological rules, and in particular Ronay s rule, indicate that energetic arguments are not sufficient to justify the sequence of phases observed. As the surface/interface compound is not in thermodynamic equihbrium, kinetic aspects are critical to determine the actual phases observed. 

In general (Table 14.1), silicides reacting at low temperatures (quasi-noble metals) and forming as first reacted phase metal-rich compounds (MejSi) exhibit a behavior characterized by a time dependence of the growth [26]. In turn, the thickness of refractory metals silicides, which require higher temperatures to react. 

The different physical processes giving rise to the different kinetics are related also to different activation energies. Most compounds whose growth is controlled by transport exhibit an activation energy around 1.5 eV, while in the case of growth controlled by interfacial reactions the activation energy is in the range 1.5-3.5 eV. 

In any case, the activation energies measured seem to be related to diffusion processes. Until now [1], there is no experimental evidence indicating the existence of a specific energy barrier for the formation of the silicide. This means that, after waiting enough time, the silicide will be formed, independently of temperature. We conclude then that the definition of the temperature at which the reaction takes place depends on the minimum silicide thickness that can be detected in the experiment. 

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