InAlGaAs encapsulation of MOVPE-grown InAs quantum dots on InP(0 0 1) substrate

Stranski-Krastanow (S-K) grown self-assembled quantum dots (QDs) are considered as an efficient active layer for many optoelectronic applications, such as semiconductor laser, photodetector, due to the possibility to confine carriers in three dimensions. Towards the device application, the subsequent growth of an encapsulation layer is a key step to obtain the desired QDs-based heterostructures. In this work, we have studied step-by-step the encapsulation process of S-K grown InAs QDs/InP(0 0 1) by InAlGaAs (lattice matched to InP) in MOVPE. A detailed investigation with Electron Channeling Contrast Imaging (ECCI) and cross-sectional Transmission Electron Microscopy (TEM), indicated that after the InAlGaAs capping the partially relaxed, dome shaped InAs QDs are transformed into fully biaxially strained, truncated pyramids however without introducing any defects in the heterostructure. The study also presents the important role of the capping layer thickness on the QDs photoluminescence efficiency. The gradual change of the surface morphology during the encapsulation process, has been investigated step-by-step in-depth by atomic force microscopy. The morphological changes can be explained by a capping model based on the localized growth rates. We show that two growth steps of Non-uniform capping, where growth rates are different from each other, result in the successful planarization of the InAlGaAs capping without forming any defect.