Spectroscopic studies of stacked InGaAs QDs structures with and without strain coupling

InGaAs/GaAs quantum dots are of great interest for studying light-matter interactions on a basic level also leading to new device applications like quantum dot (QD) lasers or single-photon sources. Here we present spectroscopic studies of stacked QDs structures which consist of a number of alternating layers of InGaAs QDs and barriers of GaAs. The samples were grown with solid source molecular beam epitaxy (MBE). We investigated e.g. stacks with 30 layers of QDs with high dots density with absorption spectroscopy and photoluminescence (PL) spectroscopy in the regime where no strain coupling can be expected, i.e. with GaAs barrier thicknesses of 50 nm. Then we reduced the thicknesses of the barrier layers for different stacks down to regions where the formation of islands is influenced by the localized strain fields caused by the underlying dots layer. Absorption spectroscopy and PL spectroscopy provide here together a complementary analysis as for example absorption spectroscopy gives direct information about the wetting layer states whereas with PL spectroscopy in most cases the wetting layer cannot be analysed. Morphology data was gained by atomic force microscopy (AFM).