The (2×2) reconstructions on the surface of cobalt silicides: Atomic configuration at the annealed Co/Si(111) interface

- A '(1×1)'-RC phase is the most appropriate template for growth of Co silicides.
- Our STM observations support the existence of two (2×2) adatom structures.
- Two adatom models of the (2×2) surface structures are developed.
- These structures are assigned to CoSi2 and CoSi with a (2×2) adatom termination.
- In both case, interfacial Co atoms are coordinated to eight Si atoms.

We have used scanning tunneling microscopy (STM) and ab initio total-energy calculations to characterize surface and interfacial structure of Co-Si(111) system. It has been found experimentally that two different types of the (2×2) surface structures occur. The coexistence of two phases is demonstrated by the example of STM image of the surface formed at the early stages of cobalt silicide formation under moderate annealing temperatures (500 °C). The measured height difference between the adjacent (2×2) reconstructed patches equal to about 1.0 Å (as determined from the filled-state STM images). In addition, the shift of the atomic rows by half of the row spacing is observed. Two adatom models of the (2×2) surface structures are developed. According to our data, these structures are assigned to CaF2-type CoSi2 and CsCl-type CoSi with a (2×2) array of Si adatoms on their surfaces. If the latter is the case, it has а coherent double interface CoSi/CoSi2/Si(111) with a two-layer CoSi2. Both of these interfaces are characterized by the eightfold cobalt coordination and incorporate a grown-in stacking fault.

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