The selective synthesis of molybdenum silicides from modulated elemental reactants

Elementally modulated reactants with repeat layer thicknesses of less than 30 angstrom were found to crystallize various molybdenum silicides depending on their composition. Reactants with compositions near 1:2 Mo:Si were found to nucleate h-MoSi2 at 400 C with an activation energy of 1.9 eV, even though h-MoSi2 is metastable with respect to t-MoSi2 below 1900 C. Reactants with compositions near 5:3 Mo:Si were found to nucleate Mo5Si3 at 650 C with an activation energy of 3.0eV. Reactants with compositions near 3:1 Mo:Si were found to nucleate Mo3Si at 720 C with an activation energy of 2.2 eV. The ability to control the crystalline product via the initial composition is distinctly different from the behavior reported for multilayers with larger repeat spacings, which were observed to form h-MoSi2 regardless of overall composition. The measured nucleation energy as a function of composition suggests that the phase selectivity is nucleation controlled. The selectivity depends on the elimination of large composition gradients at the reacting interfaces before nucleation of the crystalline product