Vertical ordering in multilayers of self-assembled Ge/Si(001) quantum dots

The effect of vertical ordering in multilayers of self-assembled Ge/Si(001) quantum dots was investigated by a combination of structural and optical characterizations via in situ reflection high-energy electron diffraction, transmission electron microscopy, atomic force microscopy, and photoluminescence spectroscopy. We show that the vertical ordering observed in quantum-dot multilayers is characterized not only by the alignment of islands along the growth direction but also by a reduction of the critical thickness. The better the vertical ordering, the more pronounced the reduction of the critical thickness is observed. Such an evolution of the critical thickness could be explained by elastic strain fields induced by buried islands and propagate through the spacer layers. An important result issued from this work is the realization of multilayers in which dots have equal size in all layers. Furthermore, experiments performed on the transformation of the island shape suggest that preferential nucleation induced by surface roughness may be the main mechanism responsible for the vertical ordering in quantum-dot multilayers. (C) 2002 American Vacuum Society.