Si/Ge/Si(001) Magnetron Sputter Heteroepitaxy: the Initial Stages of “Hut”- Cluster Overgrowth

We show that the capping of Ge/Si(001) hut clusters with epitaxial Si cannot be easily realised. Even for growth temperatures as low as 300δC, Ge segregation during the Si deposition leads to a Si 1- x Ge x alloy in both the wetting layer and the clusters which reduces the misfit of the system. In turn it is no longer beneficial to release the strain energy by forming small islands. On the contrary, there appears to be a metastable state for exhibiting a flat surface. Thus we find a 3D to 2D transition which at low Si coverages is probably thermally activated. At higher coverages, however, the transition becomes a barrier-less, resulting in the complete dissolution of the hut clusters. Annealing experiments prove that the resulting 2×8 reconstructed Si 1- x Ge x surface is not in thermodynamic equilibrium. During the melting of a hut cluster a number of new facets are formed, mainly at the edges of the clusters and at the top, where the apex becomes substituted by a 001 facet. Our results emphasize that the melting of the Ge/Si(001) hut clusters is a serious obstacle to their application, e. g. in electroluminescence devices. It has to be solved, e. g. by the use of surfactants.