  The Si/SiGe heterosystem would be ideally suited for the realization of
complementary metal-oxide-semiconductor (CMOS)-compatible integrated light
sources, but the indirect band gap, exacerbated by a type-II band offset, makes
it challenging to achieve efficient light emission. We address this problem by
strain engineering in ordered arrays of vertically close-stacked SiGe quantum
dot (QD) pairs. The strain induced by the respective lower QD creates a
preferential nucleation site for the upper one and strains the upper QD as well
as the Si cap above it. Electrons are confined in the strain pockets in the Si
cap, which leads to an enhanced wave function overlap with the heavy holes near
the upper QD's apex. With a thickness of the Si spacer between the stacked QDs
below 5 nm, we separated the functions of the two QDs: The role of the lower
one is that of a pure stressor, whereas only the upper QD facilitates radiative
recombination of QD-bound excitons. We report on the design and strain
engineering of the QD pairs via strain-dependent Schr\"odinger-Poisson
simulations, their implementation by molecular beam epitaxy, and a
comprehensive study of their structural and optical properties in comparison
with those of single-layer SiGe QD arrays. We find that the double QD
arrangement shifts the thermal quenching of the photoluminescence signal at
higher temperatures. Moreover, detrimental light emission from the QD-related
wetting layers is suppressed in the double-QD configuration.
