MECHANISMS OF ISLAND VERTICAL ALIGNMENT IN Ge/Si(001) QUANTUM-DOT MULTILAYERS

摘要

The effect of vertical ordering in superlattices 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 phenomenon of vertical ordering is characterized not only by the alignment of islands along the growth direction but also by a reduction of the critical thickness in subsequent layers. The better the vertical ordering, the more pronounced the reduction of the critical thickness is observed. As a result of this finding, we demonstrate that quantum-dot superlattices in which dots having equal size in all layers can be formed. Furthermore, experiments dealing with the transformation of island shape versus the spacer-layer thickness reveal that preferential nucleation induced by surface roughness may be the main mechanisms responsible for the vertical ordering observed in quantum-dot superlattices.