Selective preparation of nickel silicides and nickel germanides from multilayer reactants

This dissertation investigates the application of a multilayer-based synthetic technique to the phase selective preparation of nickel silicides and nickel germanides. This preparative technique employs modulated elemental multilayers with ultrathin repeating subunits as initial reactants. Complete interdiffusion is effected with moderate annealing, producing an amorphous intermediate state prior to nucleation of a crystalline phase. This technique enables the direct preparation of a variety of nickel silicide and nickel germanide phases without the formation of unwanted crystalline intermediates. The success of this preparative strategy requires that the repeating subunit of the multilayer reactant be below some critical thickness, such that complete mixing precedes first phase formation.; The variation in the product of equiatomic nickel-silicon multilayer reactions near the critical thickness is investigated. Using x-ray reflectivity (XRR) and x-ray diffraction (XRD), two reaction mechanisms intermediate between conventional interfacial reaction and the sub-critical thickness reaction are identified. By varying multilayer composition while maintaining the repeating subunit below the critical thickness, the selective formation of Ni₃₁ Si₁₂, δ-Ni₂Si, &thetas;-Ni₂Si, NiSi, and NiSi₂ is demonstrated. Of particular note is the preparation of &thetas;-Ni₂Si, which forms in conditions under which it is metastable with respect to a low temperature phase. Comparison with first phase nucleation in non-layered amorphous precursors suggests that the multilayer preparative method has a high propensity towards homogeneous phase formation.; The application of this synthetic strategy to nickel germanides enables the preparation of a number of high temperature nickel germanides at temperatures below 200°C. This preparative technique favors the formation of NiAs-type nickel germanides, suggesting that the ordering in the amorphous phase may closely reflect the NiAs structure.; Preliminary results on the preparation of ternary nickel-silicon-germanium compounds are discussed. The critical length scales in silicon-germanium multilayers are determined. The preparation of NiAs-type compounds of the form NiSi _1−xGe_x is reported.; This dissertation includes my previously published and co-authored materials.