The nucleation of coherent semiconductor islands during the Stranski-Krastanov growth induced by elastic strains

The Ge island growth on the Si(100) and Si(111) surfaces was investigated through spectral ellipsometry in real time. It is found that both cases correspond to Stranski-Krastanov growth; i.e., a Ge wetting layer is initially formed, and only then do the islands of the new phase grow on the surface of this layer. However, the island nucleation on the (100) surface is accompanied by a substantial decrease in the wetting layer thickness, whereas, on the (111) surface, the islands nucleate and grow on the wetting layer of constant thickness. The Ge atoms on the (100) surface transfer from the wetting layer to islands, thus substantially decreasing the elastic energy of the system, but increasing the surface energy. For this reason, it is concluded that, in this case, it is the elastic energy which represents the fundamental driving force of the island nucleation. Thermodynamic and kinetic theories of island nucleation from the wetting layer under the effect of elastic energy are developed. A new notion of overstress is introduced by analogy with supersaturation and overcooling. The time evolution of the wetting layer thickness, the nucleation rate, and the island surface density of the new phase is described. The theoretical results are compared to experimental data obtained through ellipsometric simulation, and it is found that the theory and experiment are in good agreement.