Morphological evolution of self-assembled SiGe islands based on a mixed-phase pre-SiGe island layer grown by ion beam sputtering deposition

We present a study of the evolution of self-assembled SiGe islands grown on a microcrystalline Si (μc-Si)-based mixed-phase pre-SiGe island layer. Using atomic force microscopy, lots of new short islands with smaller diameters, high islands with transition dome (TD) shape, and super islands with ∼20 nm in height are observed after the Ge layer deposition. This anomalous experimental finding is well clarified by a model of regeneration and secondary growth. It is found that the density of the super islands increases rapidly at the initial stage when the Ge coverage exceeds ∼1.2 nm. This is essentially the result of the selective aggregation of Ge adatoms on the pre-SiGe island layer. The Ge content and stored strain in SiGe islands calculated based on Raman spectra decrease with the increase of the Ge coverage. It is demonstrated that the chemical potential difference-induced lateral atomic migration (LAM) from amorphous SiGe alloy into SiGe islands can be responsible for this phenomenon. The LAM also leads to the formation of very large dome islands. Finally, the overlap behavior of neighbor islands in the sample with 2.2 nm-thick Ge layer is explained by the combined action of denser nucleation centers, faster growth rates of super islands in lateral direction, and coarsening of neighbor small islands.