In situ reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy and photoluminescence spectroscopy have been combined to analyze the Ge/Si (001) growth process in single and stacked layers. In a single laver, the existence of intermediate dusters between two-dimensional layers and three-dimensional islands is established. These clusters are shown to be metastable both in view of shape and optical properties. In stacked layers, the decrease of the Ge critical thickness in the upper layers is found to be the main parameter, leading to the increase of the island size and height. Such an evolution of the Ge critical thickness could be explained by elastic strain fields induced by lower Ge layers in the Si spacer layers. These results open the ways to the realization of stacked layers in which the islands have equal size in all layers and offer a promising opportunity for studying a real effect of electronic coupling between islands.