Shape of Ni-containing nanoislands grown on an Ag-terminated Ge(111) surface

By using scanning tunneling microscopy we found that Ni-containing islands grown on an Ag/Ge(111)-√3 × √3 surface adopt hexagonal (isotropic) shapes when the size is smaller than the critical value ~12.7 nm. Above this critical size islands either grow with no change in shape or become elongated (anisotropic). The shape transformation in the latter case indicates that even on an isotropic surface the influence of surface stress anisotropy on the island growth is significant. Besides, we compared the thermo-evolutions of the Ni-containing islands grown on the multiphase Ag/Ge(111) and the single Ag/Ge(111)-√3 × √3 surfaces and found that the formation of hexagonal and elongated islands is a common feature. This observation made us conclude that the surface stress anisotropy also exerts an impact on the island shape on an inhomogeneous surface.