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 Angstr om 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 be ta-molybdenum disilicide is metastable with respect to alpha-molybdenu m disilicide below 1900 degrees C. The activation energy of the nuclea tion event was found to be 1.9 eV. Modulated reactants with compositio ns near 5:3 Mo:Si formed Mo5Si3 at 650 degrees C with an activation en ergy of 3.0 eV. Modulated reactants with compositions near 3:1 crystal lize Mo3Si at 750 degrees C with an activation energy of 2.2 eV. Low-a ngle X-ray diffraction indicates that significant interdiffusion occur s during annealing below the formation temperatures of the compounds. Transmission electron microscopy data collected on samples annealed be low the formation temperatures indicate that the samples were amorphou s. The nucleation energy of the compounds was observed to increase as the stoichiometry of the amorphous phase varied from that of the nucle ating compound. This implies that the ability to control the crystalli ne product using the composition of the amorphous intermediate results from the composition dependence of the nucleation energy for crystall ization. Presumably, beta-MoSi2 forms because the nucleation barrier i s lower than that of the thermodynamically more stable alpha-MoSi2.