This study demonstrated the feasibility of fabricating a highly stacked vertically aligned InGaAs/GaAs(Sb) quantum dot (QD) structure with an AlGaAsSb spacer layer for improving the device performance of QD intermediate-band solar cell (QD-IBSC) devices. The power-dependent photoluminescence measurements of the proposed structure revealed a blueshift in the QD ground-state emissions when the excitation power was increased, indicating the formation of an intermediate band inside the QD structure. Capping the InGaAs QDs with a GaAsSb layer prevented the QDs from collapsing because there was less In-Ga intermixing between the QDs and GaAsSb layer. In addition to maintaining the QD structure, the carrier lifetime was extended by tuning the energy band alignment of the InGaAs/GaAsSb QD structure. Inserting the AlGaAsSb layer into the spacer layer increased the band gap, which in turn increased the open-circuit voltage of the QD-IBSC. The QD-IBSC in this work shows an extension of external-quantum efficiency by up to 1200 nm (compared with a GaAs reference cell) through the absorption by QDs and increased the open-circuit voltage from 0.67 to 0.70 V by adopting the AlGaAsSb spacer layer. These results confirm that adopting a columnar InGaAs/GaAs(Sb) QD structure with a AlGaAsSb spacer layer can enhance the performance of QD-IBSC devices.

The effects of thermal annealing treatment on a vertically aligned InGaAs/GaAs(Sb)/AlGaAsSb quantum dot (QD) structure with the purpose of tailoring energy band alignment are studied. In contrast to the typical blue-shift in the emission upon annealing because of In−Ga intermixing in the typical InGaAs/GaAs QDs, thermally annealed InGaAs/ GaAs(Sb)/AlGaAsSb QDs exhibit a red-shift upon annealing at 700 °C, owing to Sb aggregation on top of the InGaAs QDs, resulting in tailoring of the band alignment and strain reduction for the reduced emission energy. Power-dependent and time-resolved photoluminescence were utilized herein to extend the carrier lifetime from 1.63 ns to 6.38 ns and to elucidate mechanisms of the aggregation of antimony that cause the energy band alignment modifying from type I to type II after rapid thermal annealing. In addition, the thermal stability of the columnar QDs was improved by capping QDs with a GaAsSb overgrown layer, because In−Ga intermixing was suppressed, helping to preserve the QD heterostructures. Therefore, the flexible modulation of energy band alignment for columnar InGaAs/GaAs(Sb)/AlGaAsSb QD structures as type I or type II by thermal annealing has potential and flexible applications to versatile QD-related optoelectronic devices.

This study grew high quality vertically-aligned InGaAs quantum dots (QDs) on a GaAs (001) substrate by using the molecular beam epitaxy system. The GaAsSb/AlGaAsSb composite overgrown layer was adopted to cap on InGaAs QDs for improving the dot size uniformity. From the experimental results, the Sb-contained overgrown layer was observed to reduce the In-Ga intermixing and contribute to the enhancement of the dot-size uniformity. Through the measurements of transmission emission microscopy, energy-dispersive X-ray spectroscopy EDX and electron-energy-loss spectrometry, vertically-aligned QDs with GaAsSb/AlGaAsSb composite structure show high dot density and dot-size uniformity. In addition, the Sb composition was observed accumulate on the top region of QDs because of the strain field distribution.