Stress-driven nucleation of coherent islands: theory and experiment

In order to determine the driving force of coherent island nucleation and growth, the formation of Ge islands on H-terminated Si(100) surfaces has been investigated by spectroscopic ellipsometry in real time. The process of island nucleation is found to be accompanied by a strong reduction of the wetting layer thickness. The results indicate that, in this case, islands are nucleated mainly due to the relaxation of the stress energy in the wetting layer. Thermodynamic and kinetic models have been developed for this process. It is shown that the free energy of stress-driven formation of islands contains a saddle point. The value of free energy at this point determines the nucleation process. The time evolution of the thickness of the wetting layer is described and the time dependencies of the nucleation rate and surface density of islands are calculated. Good agreement between this theory and real-time ellipsometric analysis of the growth process is found.