Morphological instability of NiSi1-uGeu on single-crystal and polycrystalline Si1-xGex.

The morphological stability of NiSi_1-uGe_u ternary alloy films formed by reacting Ni with single-crystal (sc) and polycrystalline (poly) Si_1-xGe_u is studied (u can be different from x). The agglomeration of NiSi_1-uGe_u films on Si_0.7Ge_0.3 occurs at 550°C after rapid thermal processing for 30 s, independently of the crystallinity of the Si_1-xGe_u. This behavior distinctly different from NiSi: NiSi films on poly-Si display a poorer morphological stability and degrade at lower temperatures than NiSi on sc-Si. On strained Si_1-xGe_x, the presence of Ge simultaneously gives rise to two effects of different origin: mechanical and thermodynamic. The main driving forces behind the agglomeration of NiSi_1-uGe_u on sc-Si_1-xGe_x are found to be the stored strain energy in the Si_1-xGe_x and the larger (absolute) free energy of formation of NiSi compared to NiGe. The latter constitutes the principal driving force behind the agglomeration of NiSi_1-uGe_u on poly-Si_1-xGe_x and is not affected by the degree of crystallinity of Si_1-xGe_x. The total free-energy change also includes terms corresponding to the entropy of mixing of Si and Ge in both Si_1-xGe_x and NiSi_1-uGe_u. Calculations show that the strain energy and the total free-energy change can be 5–7 times (with 30 at.% Ge) the surface/interface and grain-boundary energies in a NiSi film or the grain-boundary energy in an underlying poly-Si. These latter energies are responsible for the agglomeration of NiSi on sc- and poly-Si. The agglomeration takes place primarily via the interdiffusion of Si and Ge between Si_1-xGe_x and NiSi_1-uGe_u. A structure likely to improve the stability of NiSi_1-uGe_u/Si_1-xGe_x is discussed. [ABSTRACT FROM AUTHOR]