New insight into the kinetics of Stranski–Krastanow growth of Ge on Si(0 0 1)

In situ reflection high-energy electron diffraction along with atomic force microscopy and photoluminescence spectroscopy have been combined to investigate the kinetic pathway of the transition from two-dimensional (2D) to islanding growth in the Ge/Si(0 0 1) system. By performing experiments in the dynamic growth regime and under growth interruption, we can identify two main steps in the kinetics of Ge/Si(0 0 1) growth. First, the 2D wetting layers (WLs) are found to undergo a morphological instability well before reaching the critical thickness. This instability appears to be strain driven and is the driving force leading to the formation of intermediate clusters between 2D layers and 3D macroscopic islands. By controlling the degree of instability of the 2D layers, we show that islands with different sizes, shapes and optical properties can be formed. Second, when the Ge coverage reaches the critical thickness, 3D macroscopic islands are found to be spontaneously formed by consuming Ge from the 2D layers without needing to proceed via a precursor phase. Energetically, the spontaneous nucleation of 3D islands is a favorable process because it allows a reduction by a factor of almost two of the elastic energy of the remaining WL through the reduction of its thickness.