A Ni/GaAs/Si(001) structure was used to investigate solid-phase reactions of Ni with GaAs and Si. Cross-sectional transmission electron microscopy and secondary-ion-mass-spectrometry depth profile data reveal that a Ni/GaAs/Si(001) structure converts to a crystalline GaAs/amorphous NiSi/Si(001) after low temperature (⩽350 °C) annealing. We demonstrate that the reaction is driven by the decomposition of aNixGaAsintermediate which is induced by the proximity of the Si substrate. Two models are suggested to explain the mechanism of a crystalline GaAs layer nucleated fromNixGaAson the amorphous NiSi layer. ©1998 American Institute of Physics.