Variation of the nucleation energy of molybdenum silicides as a function of the composition of an amorphous precursor

Modulated elemental reactants containing alternating elemental layers of molybdenum and silicon with overall thicknesses less than 50 Angstrom were found to crystallize various molybdenum silicides depending on their compositions. Modulated reactants with compositions near 1:2 Mo:Si formed beta-molybdenum disilicide at 400 degrees C, even though beta-molybdenum disilicide is metastable with respect to alpha-molybdenum disilicide below 1900 degrees C. The activation energy of the nucleation event was found to be 1.9 eV. Modulated reactants with compositions near 5:3 Mo:Si formed Mo5Si3 at 650 degrees C with an activation energy of 3.0 eV. Modulated reactants with compositions near 3:1 crystallize Mo3Si at 750 degrees C with an activation energy of 2.2 eV. Low-angle X-ray diffraction indicates that significant interdiffusion occurs during annealing below the formation temperatures of the compounds. Transmission electron microscopy data collected on samples annealed below the formation temperatures indicate that the samples were amorphous. The nucleation energy of the compounds was observed to increase as the stoichiometry of the amorphous phase varied from that of the nucleating compound. This implies that the ability to control the crystalline product using the composition of the amorphous intermediate results from the composition dependence of the nucleation energy for crystallization. Presumably, beta-MoSi2 forms because the nucleation barrier is lower than that of the thermodynamically more stable alpha-MoSi2.