Mixing entropy and the nucleation of silicides: Ni–Pd–Si and Co–Mn–Si ternary systems

Abstract

Nucleation can play an important role during the formation of silicides, especially when the difference in Gibbs free energy ΔG between the existing and newly formed phase is small. In this work, it is shown that the addition of elements that form a solid solution with either the existing or nucleating phase influences the entropy of mixing and thus changes ΔG. In this way, the height of the nucleation barrier may be controlled, thus controlling the nucleation temperature. The influence of mixing entropy on silicide nucleation is illustrated by experiments for two ternary systems: Co–Mn–Si and Ni–Pd–Si. It is shown that the nucleation temperature of CoSi2 is increased by the addition of Mn, the nucleation temperature of MnSi1.7 is increased by the presence of Co, the nucleation temperature of NiSi2 is increased by the addition of Pd, and the nucleation temperature of PdSi is decreased by the addition of Ni. In all four cases, the effect of the alloying element on the nucleation temperature can be explained by a model on the basis of the concept of mixing entropy.

On the nucleation of PdSi and NiSi2 during the ternary Ni(Pd)/Si(100) reaction

“…The PdSi formation at temperatures significantly below the widely accepted nucleation temperature in the pure Pd-Si system does not result from the effect of entropy of mixing due to the direct formation of a Ni 1-x Pd x Si solid solution, as previously suggested. 8,[10][11][12][13][14] Rather, it is triggered by a reduction in surface energy, induced by the presence of the isomorphous NiSi phase. The coexistence of NiSi and PdSi transforms into a Ni 1-x Pd x Si solution at higher temperatures, which in turn results in an improved monosilicide stability by postponing NiSi 2 nucleation due to entropy of mixing.…”

“…As NiSi and PtSi, PdSi also shares the same crystal structure and alloying Ni with Pd has been shown to delay NiSi 2 nucleation. 8,9 The effect is generally explained similar to Pt by stating that a Ni 1-x Pd x Si solid solution forms. 8,[10][11][12][13][14] On the other hand, significant differences between the Pt-Si and Pd-Si solid phase reaction imply that the formation of a Ni 1-x Pd x Si solid solution cannot be simply assumed.…”

Effects of Ti Interlayer on Ni/Si Reaction Systems

“…18 The relation between the thermal stability and the addition of elements has also been explained. 19 In this study, the effects of Ti interlayer with different thickness ratios to the Ni film were studied. The stability and microstructure of the Ni/Si reaction are presented.…”

Texture in thin film silicides and germanides: A review

“…The nucleation at this interface can be attributed to the fact that the nucleation barrier is the smallest where the inhomogeneity in the metal distribution is the largest. 44,163,164 As this interface region is not in direct contact with the Si substrate, the nucleating grains can only develop a preferential orientation related to that of the CoSi and NiSi films, not directly to that of the Si(001) substrate. Increasing the Ni content in this concentration range gradually widens and moves the CoSi/NiSi interface region away from the substrate, resulting in an increasingly random texture as more and more Co 1Àx Ni x Si 2 grains will nucleate in that region.…”