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The Si/SiGe heterosystem would be ideally suited for the realization ofcomplementary metal-oxide-semiconductor (CMOS)-compatible integrated lightsources, but the indirect band gap, exacerbated by a type-II band offset, makesit challenging to achieve efficient light emission. We address this problem bystrain engineering in ordered arrays of vertically close-stacked SiGe quantumdot (QD) pairs. The strain induced by the respective lower QD creates apreferential nucleation site for the upper one and strains the upper QD as wellas the Si cap above it. Electrons are confined in the strain pockets in the Sicap, which leads to an enhanced wave function overlap with the heavy holes nearthe upper QD's apex. With a thickness of the Si spacer between the stacked QDsbelow 5 nm, we separated the functions of the two QDs: The role of the lowerone is that of a pure stressor, whereas only the upper QD facilitates radiativerecombination of QD-bound excitons. We report on the design and strainengineering of the QD pairs via strain-dependent Schr\"odinger-Poissonsimulations, their implementation by molecular beam epitaxy, and acomprehensive study of their structural and optical properties in comparisonwith those of single-layer SiGe QD arrays. We find that the double QDarrangement shifts the thermal quenching of the photoluminescence signal athigher temperatures. Moreover, detrimental light emission from the QD-relatedwetting layers is suppressed in the double-QD configuration.

Photoluminescence enhancement by deterministically site-controlled, vertically stacked SiGe quantum dots