Microstructure of buried CoSi2 layers formed by high-dose Co implantation into (100) and (111) Si substrates

Heteroepitaxial Si/CoSi₂/Si structures have been synthesized by implanting 170-keV Co⁺ with doses in the range 1-3×10¹⁷ Co⁺ions/cm² into (100) and (111) Si substrates and subsequent annealing. The microstructure of both the as-implanted and annealed structures is investigated in great detail by transmission electron microscopy, high-resolution electron microscopy, and x-ray diffraction. In the as-implanted samples, the Co is present as CoSi₂ precipitates, occurring both in aligned (A-type) and twinned (B-type) orientation. For the highest dose, a continuous layer of stoichiometric CoSi₂ is already formed during implantation. It is found that the formation of a connected layer, already during implantation, is crucial for the formation of a buried CoSi₂ layer upon subsequent annealing. Particular attention is given to the coordination of the interfacial Co atoms at the Si/CoSi₂ (111) interfaces of both types of precipitates. We find that the interfacial Co atoms at the A-type interfaces are fully sevenfold coordinated, whereas at the B-type interfaces they appear to be eightfold coordinated.

It is shown that these interface configurations introduce defects in the three-dimensional CoSi₂ precipitates and Si matrix. As a result, the nuclei are subjected to compressive strain. It is argued that the combination of interface energy and strain results in a larger stability of small B-type nuclei as compared to A type. When the precipitates grow beyond a critical size of some 20-30 nm, A-type precipitates become more stable, finally resulting in a buried layer of aligned orientation if the layer thickness is larger than about 30 nm. If smaller, it is argued that upon prolonged annealing the layer will have a twinned orientation (B type). Annealed layers of aligned orientation in (100) Si are found to contain interfacial dislocations of edge type with Burgers vectors b=a/4<111> and b=a/2<100>. These dislocations are associated with boundaries separating domains having different interface structures. For (111) Si, there exist edge-type dislocations with Burgers vector b=a/2<110>. The final state of strain can be attributed to the difference in thermal expansion between CoSi₂ and Si. The strain at room temperature corresponds to a fully relaxed layer at about 700 °C. Below this temperature, dislocations become immobile.