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The growth kinetics of an amorphous (a‐)interlayer in polycrystalline Zr and Hf thin films on (111)Si have been investigated by cross‐sectional transmission electron microscopy. The growth of the a‐interlayer in group‐IVb metals and silicon systems was found to exhibit similar behaviors. The growth was found to follow a linear growth law initially. The growth rate then slows down and deviates from a linear growth law as a critical thickness of the a‐interlayer was reached. Crystalline silicide (ZrSi or HfSi) was found to nucleate at the a‐interlayer/Si interface in samples after prolonged and/or high‐temperature annealing. Silicon atoms were found to be the dominant diffusing species during the formation of amorphous alloys. The activation energy of the linear growth and maximum thickness of the a‐interlayer were measured to be 1.4 eV, 17 nm and 1.2 eV, 27 nm in Zr/Si and Hf/Si systems, respectively. The correlations among the differences in atomic size between metal and Si atoms, growth rate and activation energy of the linear growth, critical and maximum a‐interlayer thickness, the largest heat of formation energy for crystalline silicides, the calculated free‐energy difference in forming amorphous phase, as well as the atomic mobility in Ti/Si, Zr/Si, and Hf/Si systems are discussed.