Growth morphology and the equilibrium shape: The role of ‘‘surfactants’’ in Ge/Si island formation

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Abstract
Surface energy anisotropy, as opposed to surface energy, is shown to influence change of growth mode in Ge/Si by ‘‘surfactant’’ impurities. By annealing thin Ge/Si films, we find the equilibrium island shape, and hence the surface energy anisotropy; radical changes in shape are seen for impurity-terminated surfaces: Ge:Sb enhances (100) facets compared to clean Ge and Ge:In favors {311}. Thus Sb impurities favor large flat islands which would lead to earlier island coalescence and can aid planar (100) growth, while In (though otherwise a good ‘‘surfactant’’) leaves the film faceted. Island suppression by a ‘‘morphactant’’ thus depends on enhanced faceting onto (100), as well as reduced diffusion.