Atomic structures at cobalt silicide-silicon interfaces

The structure and chemistry at CoSi-Si(111) and CoSi₂-Si(111) interfaces has been investigated in cross-section using high-resolution electron microscopy. After low-temperature annealing, evidence is shown for epitaxial CoSi growth; the corresponding orientation relationships are explained on the basis of a lattice-matching model. During the early stages of the CoSi-to-CoSi₂ transformation, an epitaxial CoSi/CoSi₂/Si system is observed, suggesting that CoSi2 nucleates at an epitaxial CoSi-Si interface. The predominant orientation is such that (021)_CoSi planes are parallel to (220)_CoSi2 planes, the CoSi₂-Si interface being of B type. Based on the experimental observations and arguments related to molecular volume, the growth of CoSi₂ is shown to proceed at the expense of both CoSi and Si. Models to understand the atomic configurations at epitaxial CoSi-Si interfaces are proposed and then used for image simulations. Comparisons between experimental and calculated images show that the terminal CoSi plane at the interface is a Co plane and that there is a good match between the atomic densities on both sides of the interface. On the basis of image simulations of various interface bonding models evidence is shown for sevenfold Co coordination at the A-type CoSi₂-Si interfaces and for sevenfold coordinatation at the B-type interfaces. Finally, an atomic-scale investigation of embedded interfacial dicontinuities, in particular on [111] steps, is reported. It is shown that the interfacial atomic configuration can be assessed even in such complex cases.