The evolution of the surface morphology and microstructures of Ge quantum dots (QD) on Si (001) during Si overgrowth was investigated. With increasing Si coverage to 28 eq-monolayers (ML), the uncapped Ge QDs were found to change their shapes from bimodal system to truncated pyramids and eventually to the nanorings. The results show that the formation of nanorings is closely correlated with a strain-driven process. It was found that the nanoring could be controlled by varying the composition of capping layers.
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