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

The morphological stability of NiSi1−uGeu ternary alloy films formed by reacting Ni with single-crystal (sc) and polycrystalline (poly) Si1−xGeu is studied (u can be different from x). The agglomeration of NiSi1−uGeu films on Si0.7Ge0.3 occurs at 550°C after rapid thermal processing for 30 s, independently of the crystallinity of the Si1−xGeu. 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 Si1−xGex, the presence of Ge simultaneously gives rise to two effects of different origin: mechanical and thermodynamic. The main driving forces behind the agglomeration of NiSi1−uGeu on sc-Si1−xGex are found to be the stored strain energy in the Si1−xGex and the larger (absolute) free energy of formation of NiSi compared to NiGe. The latter constitutes the principal driving force behind the agglomeration of NiSi1−uGeu on poly-Si1−xGex and is not affected by the degree of crystallinity of Si1−xGex. The total free-energy change also includes terms corresponding to the entropy of mixing of Si and Ge in both Si1−xGex and NiSi1−uGeu. Calculations show that the strain energy and the total free-energy change can be 5–7 times (with 30at.% 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 Si1−xGex and NiSi1−uGeu. A structure likely to improve the stability of NiSi1−uGeu∕Si1−xGex is discussed.