Properties of wetting layer states in low density InAs quantum dot nanostructures emitting at 1.3 µm: Effects of InGaAs capping

In this work we study the properties of energy levels of the two-dimensional quantum system composed by wetting layers and thin capping layers in low density InAs/InGaAs quantum dot structures, that can be used as single photon sources at the fiber-optic wavelength of 1.3 ?m. We show how, thanks to the low density of quantum dots, x-ray characterization of structures allows to extract thicknesses and compositions of the InAs wetting layer and the quantum well formed by the InGaAs capping layer, resulting in substantial deviations from the simplified picture of a wetting layer consisting of a 1.6 monolayer thick InAs square well. The agreement between model calculations of quantum confined energy levels based on x-ray data and photoluminescence peak energies substantiates the validity of this calculation, that also allows to investigate on carrier localization. The increase in In composition in the InGaAs capping layer results in reduced localization of heavy holes in the wetting layer, that are pushed into the InGaAs quantum well, while the electron, being already strongly delocalized in the quantum well, is relatively less affected. We discuss consequences on carriers' overlap in the wetting layer and on spatial separation between wetting layer and quantum dot carriers and we consider implications for control of properties of quantum dot structures suitable as single photon sources in the telecommunication windows.