The shape of epitaxial islands on (111) surfaces obeys usually the threefold symmetry of the substrate surface; i.e. triangular islands are observed. We find that this threefold symmetry of the shape of two-dimensional Si or Ge islands on Si(111) is broken if a monolayer of Bi is used as a surfactant, floating at the surface during growth. Islands with lower symmetry are observed using scanning tunneling microscopy. Arrow-shaped Si islands and rhomb shaped Ge islands feature only a single Cs symmetry plane. On both, the substrate surface and the 2D islands, a Bi induced (√3 x √3) reconstruction is present. A mutual shift between the reconstruction on the substrate and the island is imposed by the diamond crystal structure of the Si substrate. The observed symmetry breaking is explained by a reduced symmetry of the combined system of (i) the surface reconstruction of the substrate and (ii) the surface reconstruction on the islands.
The crystal shape of the islands is also related to the growth properties of the step edges. With a given atomic structure of the combined island-substrate system a large variety of step structures are possible. Only some of these steps, usually the slow growing ones, survive during growth and determine the island (crystal) shape. More precisely the kinematic Wulff theorem relates the step speeds to the observed shape of the islands. However, one additional information is needed for the determination of the step speeds: the position of the nucleation center inside the island. Here we used a marker technique to determine this nucleation point for the islands and hence determined the relation between the step speeds of the island resulting in a complete information on the growth kinetics of the islands.
Generally, on reconstructed surfaces the symmetry of epitaxially grown islands can break the symmetry of the underlying surface. This could be used to fabricate nanostructures directed along one specific direction on a substrate of higher symmetry.
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