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Description
In surface science, MgO is attractive due to its highly stable, atomically flat (100) vicinal surface for epitaxial growth. Annealing is commonly used to form well-defined terraces on MgO single-crystal substrates. In MgO crystals, however, impurities can redistribute during thermal treatment; calcium is commonly reported as one of the most prevalent impurities in MgO single crystals and is known to segregate to the surface upon annealing. It is also reported that mechanical surface polishing may increase CaO segregation and it would be linked to the formation of dislocations on the surface by polishing. As dislocations further enhance segregation attracting impurities such as Ca, promoting accumulation along dislocation lines and at surface.
To understand the segregation mechanism, MgO(100) single crystals were annealed at 1100 °C under 80 sccm O2 flow for different annealing time. Annealed samples characterized by terraces separated by atomic steps and a secondary phase distinct from the bulk MgO. AFM phase images showing a phase contrast of ~20°, which corresponds to the existence of a different material om the MgO single crystal surface. TEM elemental mapping identify them as CaO particles. TEM images also provide insight into the underlying lattice structure, a high density of dislocations below the MgO surface becomes evident. These dislocations are concentrated near the segregated CaO particles, suggesting a strong interaction between lattice defects and surface impurities. Ca atoms migrate to the surface along dislocation cores, forming CaO particles that act as pinning centers at the edges of MgO terraces. In some of these pinned regions, step bunching is observed, which serves as further evidence for the presence of dislocations beneath the CaO particles. This behavior also demonstrates the role of pipe diffusion in facilitating the transport of Ca to the surface, highlighting the strong coupling between dislocation-mediated diffusion and surface morphology evolution.
| Keyword-1 | Surface segregation |
|---|---|
| Keyword-2 | Pipe diffusion |
| Keyword-3 | Atomic force microscopy (AFM) |