High efficient luminescence in type-II GaAsSb-capped InAs quantum dots upon annealing (original) (raw)
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Type-II InAs/GaAsSb Quantum Dot Solar Cells With GaAs Interlayer
—One of the primary challenges facing quantum dot (QD)-based intermediate band solar cells is the short lifetime of charge carriers (∼1 ns). To investigate this, InAs QD/GaAs 1–x Sb x quantum well (QW) solar cells (SCs) with a 2-nm GaAs interlayer between the QDs and QW were fabricated for x = 0, 0.08, 0.14, and 0.17, respectively. Time-resolved photo-luminescence measurements demonstrated prolonged carrier lifetimes up to 480 ns for the type-II SCs with x 14%. This improvement in carrier lifetime is assigned to the GaAs interlayer that reduces the wavefunction overlap between the electrons accumulated in the QDs and holes in the QW, and hence limits the possible emission pathways. External quantum efficiency measurements were performed to analyze the SC performance. An order of magnitude improvement was observed in the QD region (900– 1200 nm) for the type-II SCs and is linked to the prolonged carrier lifetime. Index Terms—Intermediate band solar cells (IBSCs), molecular beam epitaxy, quantum dot (QD) solar cells.
Optical investigation of type II GaSb∕GaAs self-assembled quantum dots
Applied Physics Letters, 2007
We have studied the emission and absorption properties of type II GaSb/ GaAs quantum dots embedded in a p-i-n photodiode. The excitation power evolution provides clear signatures of the spatially separated confinement of electrons and holes in these nanostructures. We have estimated the confinement potential for the holes to be ϳ500 meV, leading to an intense room temperature emission assisted by recapture processes from the wetting layer. Photocurrent measurements show strong absorption in the wetting layer and in the quantum dots at room temperature which are important for photodetection applications based in this system.
Effects of thermal annealing on the emission properties of type-II InAs/GaAsSb quantum dots
Applied Physics Letters, 2009
We report the effects of thermal annealing on the emission properties of type-II InAs quantum dots ͑QDs͒ covered by a thin GaAs 1−x Sb x layer. Apart from large blueshifts and a pronounced narrowing of the QD emission peak, the annealing induced alloy intermixing also leads to enhanced radiative recombination rates and reduced localized states in the GaAsSb layer. Evidences of the evolution from type-II to type-I band alignments are obtained from time-resolved and power-dependent photoluminescence measurements. We demonstrate that postgrowth thermal annealing can be used to tailor the band alignment, the wave function overlaps, and hence the recombination dynamics in the InAs/GaAsSb type-II QDs.
InAs/GaAsSb quantum dot solar cells
The hybrid structure of GaAs/GaAsSb quantum well (QW)/InAs quantum dots solar cells (QDSCs) is analyzed using power-dependent and temperature-dependent photoluminescence. We demonstrate that placing the GaAsSb QW beneath the QDs forms type-II characteristics that initiate at 12% Sb composition. Current density-voltage measurements demonstrate a decrease in power efficiency with increasing Sb composition. This could be attributed to increased valence band potential in the GaAsSb QW that subsequently limits hole transportation in the QD region. To reduce the confinement energy barrier, a 2 nm GaAs wall is inserted between GaAsSb QW and InAs QDs, leading to a 23% improvement in power efficiency for QDSCs.
Journal of Luminescence, 2020
In this study, the optical properties of InAs quantum dots (QDs) were characterized using photoluminescence (PL) measurements. The QDs, capped with GaAs and GaAs 1À x Sb x (x ¼ 6%) strain-reducing layer (SRL), were grown by Molecular Beam Epitaxy. Temperature-dependent photoluminescence (TDPL) of both ground state (GS) and first excited state (ES) was carried out through the analysis of the PL peak position as well as the integrated PL intensity. The temperature dependence of the integrated PL intensity shows the carrier trapping in the potential barrier at the interface between the capping layer and QDs in both samples at low temperatures for the GS and ES. The Excitation density dependent photoluminescence (EDPL) showed a redshift of the GS and ES PL peak energies with increasing excitation density. We attribute this variation to the bandgap renormalization (BGR) effect. The potential barrier reduction for the GaAsSb-capped QDs increases carrier injection efficiency inside the QDs, giving rise to a larger BGR effect compared to the QDs capped with GaAs. With increasing temperature, BGR redshift varies considerably for the GaAs-capped QDs but less for the Q with GaAsSb SRL. This effect was explained using the population rate of carriers inside the QDs while taking into account the nonradiative recombination process for the two samples. Furthermore, the variation of the integrated intensity with the excitation power density grows superlinearly for the two samples for a temperature range from 10 K to 220 K. This behavior was explained by the random capture of carriers in the dots. The sample with GaAsSb SRL having a smaller potential barrier has reduced superlinear dependence at low temperatures. Losses mechanism has a significant impact on increasing the superlinear dependence at high temperatures. The ES showed a stronger superlinearity compared to the GS. This study helps to understand the optical mechanisms in some devices, such as QD lasers.
Carrier dynamics of type-II InAs∕GaAs quantum dots covered by a thin GaAs1−xSbx layer
Applied Physics Letters, 2008
Carrier dynamics of InAs/ GaAs quantum dots ͑QDs͒ covered by a thin GaAs 1−x Sb x layer were investigated by time-resolved photoluminescence ͑PL͒. Both the power dependence of PL peak shift and the long decay time constants confirm the type-II band alignment at the GaAsSb-InAs interface. Different recombination paths have been clarified by temperature dependent measurements. At lower temperatures, the long-range recombination between the QD electrons and the holes trapped by localized states in the GaAsSb layer is important, resulting in a non-single-exponential decay. At higher temperatures, optical transitions are dominated by the short-range recombination with the holes confined to the band-bending region surrounding the QDs.