Capping InAs quantum dots with an InGaAsSb strain-reducing layer to improve optical properties and dot-size uniformity

•The InGaAsSb layer was capped on InAs quantum dots (QDs) for long-wavelength emission.•Reduced In–Ga intermixing was observed in (In)GaAsSb capped QDs.•The InAs/InGaAsSb QDs show long 1st excited-state emission wavelength to 1.31 μm.•Increased luminescence was observed in InAs/InGaAsSb QDs by tailored band-alignment.

The superior operation characteristics of quantum dot (QD) lasers with excited-state (ES) emission, namely high saturation gain and modulation bandwidth, are advantageous for applying lasers in optical-fiber communication. The InAs QDs in this study were covered with an InGaAsSb capping layer to reduce In–Ga intermixing, thereby increasing the QD density and dot-size uniformity. However, the extension of ES emission to 1310 nm in InAs/GaAs QD heterostructures is difficult because of the internal compressive strain and In–Ga intermixing. This study used an InGaAsSb strain-reducing layer (SRL) to cap InAs QDs, elongating the ground-state and excited-state (ES) emission wavelengths to 1426 and 1310 nm, respectively. The optical properties of the InAs/(In)GaAsSb QD structures were measured using power-dependent photoluminescence and time-resolved photoluminescence measurements. The energy band alignment of the InAs QD heterostructure changed from a type-II to Quasi-type I confinement (transition state from type-II to type-I) when the GaAsSb SRL was replaced with an InGaAsSb layer under a constant Sb flux during the epitaxy process. Therefore, an enhanced radiative recombination efficiency was observed in the InAs/InGaAsSb QDs at the long first ES emission at 1310 nm.