Nucleation and growth of self-assembled Ge/Si (001) quantum dots in singleand stacked layers

In situ reflection high-energy electron diffraction, atomic force microscop y, transmission electron microscopy and photoluminescence spectroscopy have been combined to analyze the Ge/Si (001) growth process in single and stac ked layers. In a single laver, the existence of intermediate dusters betwee n two-dimensional layers and three-dimensional islands is established. Thes e clusters are shown to be metastable both in view of shape and optical pro perties. In stacked layers, the decrease of the Ge critical thickness in th e upper layers is found to be the main parameter, leading to the increase o f the island size and height. Such an evolution of the Ge critical thicknes s 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 st acked layers in which the islands have equal size in all layers and offer a promising opportunity for studying a real effect of electronic coupling be tween islands. (C) 2000 Elsevier Science B.V. All rights reserved.