InAs/AlGaAs quantum dots grown by a novel molecular beam epitaxy multistep design for intermediate band solar cells: physical insight into the structure, composition, strain and optical properties
In the present work, we investigate in detail the complex morphological, structural and chemical profile of a III–V multistep quantum dot (QD) system grown by molecular beam epitaxy and employed for the realization of enlarged bandgap intermediate band solar cells (IBSCs). The peculiar multistep design, used during the QD growth for the band-gap engineering of the IBSCs and involving a gradual compositional change from quaternary (AlInGaAs) to ternary (InGaAs) and binary (InAs) compounds, affects the morphological, chemical and structural properties of the QDs and then the electro-optical behavior of the IBSC device. These properties are assessed by combining and comparing scanning transmission electron microscopy experiments, based on high angle annular dark field imaging, with strain analysis and energy dispersive X-ray spectroscopy. The analysis demonstrates defect-free QDs embedded in the AlGaAs matrix, with a well-defined shape and faceting. The results of the STEM experiments are correlated with the complex carrier dynamics and electro-optical properties of the solar cells, showing the potential of the proposed structure for further improvement in the design of new IBSCs, by virtue of an extremely confined strain field and high energy barrier characterizing our QD system.