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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.
© 2014 Optical Society of America
1. Introduction
Intermediate band solar cells (IBSCs) are predicted to exceed the Shockley-Queisser limit and achieve efficiencies up to 63% [1]. However, the use of QDs to form the IB in III-V solar cells (SCs) has encountered a number of difficulties. These include thermal coupling between the GaAs conduction band (CB) and the discrete QD energy levels that are to form the IB, which leads to a significant reduction in open-circuit voltage (VOC) [2,3]. Secondly, in attempt to maximize the absorption from QDs, numerous stacks of QD layers are needed [4–6]. This leads to the accumulation of strain and results in crystal dislocations that are observed to thread through the layers of QDs [4,7]. These strain-induced defects act as carrier recombination sites and significantly reduce the short circuit current density (JSC) of the QDSC [4,5]. In addition, a type-I band arrangement is formed between the GaAs and InAs QDs. This corresponds to photo-excited carriers exhibiting shorter lifetimes [8], which can limit the carrier collection from the IB to the CB.
Recent experimental studies have shown that GaAsSb acts as a strain-reducing layer on InAs QDs, which improves the crystal quality and dot density, and reduces the formation of coalescent dots [8–10]. This leads to enhanced optical properties of the QD region and reduced non-radiative recombination. Secondly, GaAsSb was shown to form a type-II band structure with the InAs QDs, which spatially separates the photo-excited charge carriers. This is expected to reduce carrier recombination and improve the carrier separation [11–13]. Type-II behavior between the QDs and QW is dependent on the Sb composition in the GaAsSb layer, and was previously reported to occur when the Sb composition reached 14% [14,15]. However, a GaAsSb strain-reducing layer grown on the top of InAs QDs leads to difficulty in employing the high-growth-temperature step for the GaAs spacer layer, which is used to eliminate the defected dots covered by the GaAsSb layer [16]. Here, the hybrid structure with InAs QDs grown above the GaAsSb/GaAs QW has been investigating to achieve type-II band alignment. The Sb composition in GaAsSb QWs between 0 and 21% were investigated by temperature and power-dependent photoluminescence (PL) measurements. SCs were characterized using current density-voltage (J-V) and external quantum efficiency (EQE) measurements. Although it is widely accepted that the increase of electron lifetime in type-II QDs is desired for high efficiency QDSCs, the change of hole dynamics by GaAsSb QWs becomes a limiting factor to fulfill the promise of type-II InAs/GaAsSb QDSCs. We have demonstrated that by inserting a 2 nm GaAs wall between the InAs QD and GaAsSb QW, the hole confinement can be engineered to improve the hole transportation, and hence the efficiency of QDSCs.
2. Experimental methods
The QDSC epitaxy structures were grown by solid-source molecular beam epitaxy (MBE) on n+-GaAs (100) substrates. All QDSCs had a p-i-n structure that consists of a 200 nm GaAs buffer layer with Si doping density of 5 × 1018 cm−3, 1000 nm GaAs base with Si doping density of 1 × 1017 cm−3, 420 nm intrinsic region, 200 nm GaAs emitter with Be doping density of 2 × 1018 cm−3, 30 nm Al0.79Ga0.21As window layer, and 50 nm GaAs contact layer with Be doping density of 1 × 1019 cm−3. The intrinsic region of the QDSCs contained 20 stacks of GaAsSb QW/InAs QDs layers separated by a 45 nm GaAs spacer. The Sb content in the GaAs1-xSbx QW was changed for six devices, where x = 0, 0.1, 0.12, 0.14, 0.17 and 0.21. The InAs QDs and GaAsSb QWs were grown by the Stranski–Krastanov mode at a substrate temperature of ~490 °C measured by a pyrometer. Any further increase in growth temperature led to desorption of Sb from the GaAsSb QW layer. At the same time, the quality of InAs QDs grown at temperatures below 490 °C were found to diminish as evidenced by the large incoherent relaxation islands that formed and the reduced PL intensity. High-growth-temperature GaAs spacer layers were applied during growth of the QD region to eliminate the defected dots and suppress the formation of dislocations for multiple-QD-layer devices [4,7,16]. A final QDSC was constructed with a 2 nm GaAs wall inserted between the InAs QD and GaAs1-xSbx for x = 0.17. A gold-zinc alloy (~90 nm) was thermally evaporated to form a p-type grid-pattern electrode with the use of a metal mask. A thermally evaporated n-type electrode coats the whole back surface and consists of nickel/gold-germanium/nickel/gold (5 nm/150 nm/50 nm/200 nm thicknesses, respectively).
The morphology of uncapped QDs surface was characterized by a Veeco Nanoscope V atomic force microscope (AFM). Temperature-dependent and power-dependent photoluminescence spectra were obtained using continuous-wave PL measurements, which were performed by using 532 nm excitation from a diode pumped solid-state laser. A cryostat was used to maintain the sample temperature between 20 and 300 K. J-V measurements were performed under one sun (AM 1.5G) illumination using a LOT calibrated solar simulator with a Xeon lamp. Photocurrent measurements were obtained with a Halogen lamp chopped to a frequency of 188 Hz through a Newport monochromator; a 4-point probe in connection with a lock-in amplifier was used to collect data. The monochromatic beam was calibrated using a Silicon photodiode and the data was analyzed with Photor 3.1 software to provide EQE.
3. Results and discussion
The growth of InAs QDs on a QW layer of GaAs1-xSbx was investigated for different Sb compositions (x = 10%, 12%, 14%, 17%, and 21%). Figure 1 shows the morphology of the InAs QDs on GaAsSb QWs with different Sb composition. The average width of the QDs is 40 nm, with an average height between 5 and 8 nm. The absence of large defected dots indicates a high structural quality with few dislocations [17]. In addition, unlike Sb mediated QD growth [18], the change in Sb composition does not have any observable influence on the size and shape of InAs QDs.
Fig. 1 AFM images (1 μm x 1 μm) of InAs QDs grown on a GaAs1-xSbx QW with different Sb composition (a) x = 10%, (b) x = 14%, (c) x = 17%, and (d) x = 21%.
Figure 2 plots the integrated PL intensity of the QDSCs as a function of temperature. For the QDSCs with low Sb composition in the QW, the PL quenches rapidly in the high temperature region. For example, the QDSCs with 0%, 10%, and 12% Sb shows a decrease in integrated PL intensity of two orders of magnitude from 20 K to 300 K. In contrast, the 14%, 17%, and 21% Sb only drop by one order of magnitude. This demonstrates that by varying the Sb composition in the GaAsSb QW, the carrier dynamics can be largely altered. The reduced PL quenching of the QDSCs with high Sb composition in the QW indicate a suppressed carrier escape rate and thus larger thermal activation energy. This is of great interest because the hindrance of thermal coupling between the IB and CB previously mentioned could be reduced at room temperature.
Figure 3(a) shows that at 10 K the QDSC absent of Sb (x = 0) has its PL peak energy at 1.27 eV (976 nm). As the Sb composition increases in the GaAsSb QW, the PL peak energy is shown to distinctly shift to lower energies. Furthermore, the rate at which the redshift occurs with increasing temperature is slower for QDSCs that have higher Sb composition. The similar InAs QDs morphologies shown in Fig. 1 indicate the QD size has a negligible contribution to the peak energy shift. Instead, the peak shift must therefore be attributed to the change of energy band alignment between the QD and QW with different amounts of Sb composition.
Fig. 3 Plot showing the shift of photoluminescence (PL) peak energy with increasing a) temperature and b) laser excitation power for GaAs/GaAs1-xSbx QW/InAs QDs.
To confirm the PL redshift with increasing Sb composition in the GaAsSb QW, excitation power-dependent PL measurements were performed at 10 K as shown in Fig. 3(b). When the Sb composition in the GaAsSb QW is below 10%, no blueshift occurs with increasing laser excitation power. For the sample with 12% and 14% Sb composition, a slight blueshift as well as a second PL peak can be observed as the laser excitation power increases. This can be attributed to the crossover between a type-I and type-II band structure. For higher compositions of Sb (17% and 21%), the blueshift becomes more significant. The clear correlation between the PL peak shift and cube root law provide strong evidence for the formation of type-II band alignment between the InAs QDs and GaAsSb QW [19].
Figure 4 shows the J-V plot of the InAs QD/GaAs1-xSbx QW QDSCs. The fill factor and power conversion efficiency were derived from the data in Fig. 4 and are presented in Table 1.As the Sb composition increases in the GaAsSb QW, both the VOC and JSC decrease. The reduction in VOC is typical for QDSCs and is attributed to thermal coupling between the QD ground state and GaAs CB. The VOC is linked to the quasi-Fermi level (Eq-F) split between the InAs QD and GaAsSb QW, and is dependent on the electron and hole carrier concentrations of the QD and QW, respectively. In this case, the formation of type-II band alignment between the QDs and QW is expected to result in a slight reduction in VOC. This is evidenced by the redshift in PL peak energy from 1.27 eV to 1.09 eV, which represents a decrease in band-gap energy. However, this does not explain the large drop in VOC from 0.65 V (17% Sb) to 0.60 V (21% Sb). This may be due to the contribution from non-radiative recombination related strain relaxation for QDSCs with higher Sb composition [20–22].
Table 1. Short-circuit Current Density (JSC), Open-circuit Voltage (VOC), Fill Factor (FF) and Efficiency (η) Values for GaAs/GaAs1-xSbx QW/InAs QD SCs Extracted from Fig. 4
The decreasing JSC for QDSCs with less than 17% Sb composition might be attributed to increased hole confinement. Type-II band alignment increases hole confinement in the GaAsSb QW, which restricts the hole transport across the QD region and increases carrier recombination in the QW. This is supported by the EQE measurements shown in Fig. 5, where decreasing EQE contribution between 700 and 1100 nm with increasing Sb composition (10 to 17% Sb) can be observed. The exception of the sample with 21% Sb composition shows significant improvement in EQE above 870 nm compared with lower Sb composition QDSCs. We attribute this behavior to enhanced carrier extraction from the GaAsSb QW through the presence of defects that formed as a result of the high (21%) Sb composition. These defects act as a lower potential barrier for carrier escape from the QW [22]. This demonstrates that, although type-II band alignment separates photo-excited carriers between the QDs and QW, and potentially prolongs carrier lifetime in the QD, it comes at the cost of carrier lifetime across the depletion region.
To resolve this, we implemented a 2 nm GaAs wall between the QW and QDs that would assist in increasing the confinement level in the GaAsSb QW and lower the potential barrier for holes to transition to the valence band edge. At the same time, the GaAs wall will increase the carrier separation between InAs QDs and GaAsSb QWs. A significant comparative improvement of 23% in power efficiency is observed between the sample with 17% Sb composition with and without the GaAs wall insertion. This originates mainly from the increase in JSC (Table 1), as the VOC remains relatively constant. The advantage of inserting a GaAs wall between the QDs and QW is also presented in the EQE spectra, see Fig. 5. The 17% Sb composition with the wall insert shows a peak QE of 62% and achieved the maximum QE between 900 and 1100 nm for InAs QDs compared with a peak QE of 57% for the 17% Sb composition without the wall.
4. Conclusion
The hybrid structure between GaAsSb/GaAs QW and InAs QDs were investigated with InAs QDs grown above GaAsSb QWs and various compositions of Sb in GaAsSb QWs for QDSCs. Type-II behaviors for this hybrid structure was achieved when Sb composition reached ~12%. Improved thermal stability with increasing Sb composition was observed. Power-dependent PL provided evidence of the gradual crossover from type-I to type-II band alignment. J-V and EQE measurements indicate increased carrier recombination for the samples with higher Sb composition and could be linked to the confinement of holes in the GaAsSb QW. The efficiency of type-II InAs/GaAsSb QDSCs was increased by inserting a thin GaAs layer between InAs QDs and GaAsSb QW, as evidenced by the significant improvement in EQE.
Acknowledgments
The authors acknowledge the financial support of EPSRC Grant EP/K029118/1 and US Army International Technology Center-Atlantic. H. Liu would like to thank The Royal Society for funding his University Research Fellowship.
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