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This study presents an band-alignment tailoring of a vertically aligned InAs/GaAs(Sb) quantum dot (QD) structure and the extension of the carrier lifetime therein by rapid thermal annealing (RTA). Arrhenius analysis indicates a larger activation energy and thermal stability that results from the suppression of In-Ga intermixing and preservation of the QD heterostructure in an annealed vertically aligned InAs/GaAsSb QD structure. Power-dependent and time-resolved photoluminescence were utilized to demonstrate the extended carrier lifetime from 4.7 to 9.4 ns and elucidate the mechanisms of the antimony aggregation resulting in a band-alignment tailoring from straddling to staggered gap after the RTA process. The significant extension in the carrier lifetime of the columnar InAs/GaAsSb dot structure make the great potential in improving QD intermediate-band solar cell application.
© 2014 Optical Society of America
1. Introduction
The self-assembled InAs/GaAs quantum dots (QDs) with the Stranski-Krastanow (S-K) growth mode were comprehensively investigated during the past few decades because of their discrete energy levels and particular three-dimensional carrier confinement. These particular properties cause InAs/GaAs QDs to perform effectively in various QD optoelectronic devices, including semiconductor lasers [1–3], infrared detectors [4], optical amplifiers [5], and solar cells [6–8]. Although S-K growth easily yields dense QDs, which provide a sufficient modal gain for operating the QD laser in the ground state (GS) [9–11], self-assembled QDs with dot-size fluctuations show large spectral inhomogeneous broadening, which can degrade the operating characteristics of QD lasers. Therefore, improving the dot-size uniformity in the InAs QD heterostructure is necessary [12].
Solomon et al. presented a vertically aligned InAs/GaAs QD structure with highly uniform QDs. The coupling effect between each QD stack in this structure improves dot-size uniformityand was shown to be beneficial for laser diode applications because of its increasing modal gain [13,14]. Moreover, a vertically aligned QD structure has a greater localization energy and capture efficiency compared to a single dot layer because of the electronic coupling of each dot-layer, which is beneficial in photodetector applications [15]. Additionally, the vertically aligned QD structure is superior for broad-band amplifiers, which is advanced with improved gain saturation, modulation bandwidth, and a lower threshold current [16]. Furthermore, vertically aligned QD heterostructures feature an intermediate band that is generated by the electronic coupling effect, which is potentially useful in the fabrication of quantum dot intermediate band solar cells (QD-IBSC) [17, 18].
Martí et al. indicated the presence of a closed relationship between the performance of a QD-IBSC and the carrier lifetime. The decreased carrier lifetime from 3.5 to 0.005 ns, revealing the worsened short-circuit current density and open-circuit voltage of the QD-IBSC with increased crystal dislocation [19]. The extension of the carrier lifetime is thus important and necessary for improving the current-voltage characteristics in solar cells; thus, a stack of defect-free vertically aligned QD layers is essential for intermediate band formation and high-performance QD device operation. These layers can apparently extend the carrier lifetime and additional photon absorption below the GaAs bandgap and increase the photocurrent in a QD-IBSC [19, 20].
This study has indicated that the vertically aligned QD structure has potential applications in various optoelectronic devices; however, the stacking in the vertically aligned InAs QD structure may cause large strain accumulation and the formation of dislocations [19–22]. Improving the quality of a vertically aligned QD structure is important for the development of optoelectronic devices with columnar stacked QD structures.The optical and material properties of the post-annealed multi-stack InAs/GaAsSb QD structures are investigated by PL and transmission electron microscope (TEM) images. Extended carrier lifetime from 4.7 to 9.4 ns is demonstrated in the dislocation free QD heterostructures and the mechanism is elucidated by the band-alignment tailoring. These significant band alignment tailoring and extension of the carrier lifetime of the columnar InAs/GaAsSb dot structure make the great potential in improving QD intermediate-band solar cell application.
Recently, the InAs QDs that are overgrown by a thin GaAsSb layer have attracted substantial interests owing to the extended emission wavelength and type-II band alignment of the InAs/GaAsSb QD system. Sb atoms behave as a surfactant in reducing the surface energy of the InAs/GaAs QD hetero-epitaxial layer, which improves the crystal quality, dot density, and reduces the formation of coalescent dots [23–26]. Further improvements in the material quality involve using rapid thermal annealing (RTA) for enhancing the optical properties of the InAs/GaAs(Sb) QD structures through the reduction of the crystal defect density. Although examinations of the effects of the RTA process on a single-layer InAs/GaAs(Sb) QD structure have been completely investigated [27,28], studies regarding the effects of thermal annealing on the vertically-aligned InAs/GaAsSb QD structure are scarce. In this study, the columnar dot structure was combined with GaAsSb strain-reducing layers (SRLs) as a vertically aligned InAs/GaAsSb columnar QD structure with ten dot layers.
Typically, the energy band alignment of single InAs/GaAsSb QD layer is tailored from staggered (type-II) to straddling (type-I) gap by In-Ga intermixing after high-temperature annealing [27, 28]. However, the time-resolved photoluminescence (TRPL) results in this study showed a noticeable extension of the carrier lifetime from 4.7 to 9.4 ns, and the power-dependent PL (PDPL) results reveal a band bending behavior in the annealed columnar InAs/GaAsSb QD structure with an Sb content of 10%, indicating the tailoring of the energy band alignment from type-I to type-II by RTA, which was less discussed previously.
2. Experimental details
In this study, two different columnar QD structures were grown using a Riber 32P solid source molecular beam epitaxy system. Following thermal treatment of the GaAs substrate at 630 °C, a 300 nm-thick GaAs buffer layer was grown at 580 °C. The substrate temperature was lowered to 500 °C for subsequent QD growth. Figure 1(a) and 1(b) schematically depict the structure of the vertically aligned QD samples. In these samples, ten layers of InAs QDs were grown by depositing 2.7 monolayers (MLs) at a rate of 0.1 ML/s. Each QD layer was separated by a 10 nm-thick thin spacer layer, which was a pure GaAs layer in sample A, and a combination of a 5.5 nm-thick GaAs layer and a 4.5 nm-thick GaAs1-xSbx SRL for sample B with x = 10%.
Fig. 1 Schematic diagram of vertically aligned InAs quantum dot structure with ten stacked dot layers in (a) sample A: InAs/GaAs, and (b) sample B: InAs/GaAs1-xSbx (x = 10%). TEM image of (c) sample A and (d) sample B.
The material quality and optical properties of the columnar dot structures (i.e., Samples A and B) was comprehensively analyzed through atomic-force microscopy (AFM) and transmission electron microscopy (TEM) in the previous studies [6, 26]. Both of the ten-layer QD structures were free of dislocations and exhibited columnar growth of QDs along the growth direction, as shown by the TEM results in Fig. 1(c) and 1(d).
A single-layered InAs/GaAs QD sample that was grown using an identical procedure was utilized as a reference sample. Annealing treatment was performed on all studied samples for 30s at annealing temperatures from 650 to 900 °C with GaAs wafer proximity capping in pure nitrogen ambient.
The 661nm line of a laser diode was used as the excitation source for the low-temperature PL and PDPL measurements in a helium-cooled cryogenic system. A cooled InGaAs detector was used to measure the signal that was dispersed by a 0.5m monochromator (iHR 550) by the lock-in technique. The TRPL system utilized a Ti:sapphire laser with an emission wavelength of 635nm and a frequency of 80MHz as the excitation source, and the carrier decay signal was recorded by the time-correlated single photon counting technique with an overall time resolution of 28 ps.
3. Results and discussion
Figure 2(a) and 2(b) present the PL spectra that were measured at a temperature of 10 K with an excitation power of 100 mW for samples A and B from as-grown sample up to annealed at 900 °C. The results reveal that the GS peak intensity of the as-grown sample B was stronger than that of the as-grown sample A because the surfactant effect of the incorporated Sb increased the dot density [23–26].The emission wavelength was considerably extended from 1116 nm (as-grown sample A) to 1186 nm (as-grown sample B), which is attributable to the reduction of compressive strain and In-Ga material intermixing.
Fig. 2 Photoluminescence spectra, obtained at low temperature of 10 K, of (a) sample A and (b) sample B following annealing at various temperatures.
After RTA process, samples A and B yielded the highest PL intensities at annealing temperatures of 750 and 650 °C, respectively. This improvement in the PL intensity indicates that thermal annealing process reduced the density of the nonradiative recombination centers [29, 30]. Consequently, the PL intensities of samples A and B decreased as the annealing temperature increased. A significant drop in the PL intensity of sample B was observed upon annealing at temperature higher than 650 °C. Since the redistribution of Sb element and compositional fluctuation becomes pronounced after thermal annealing process, the tailoring of the band alignment toward to the type-II configuration for reducing the carrier wavefunction overlap was considered in this study. The induced Sb accumulation on top of the QDs by lattice strain and thermal energy was accounted for the type-II band alignment formation, hence dramatically reduced PL intensity with an inhomogeneous broadening of the PL linewidth after high-temperature thermal annealing process [31].
Figure 3(a) and 3(b) show the normalized PL spectra that are obtained from Fig. 2(a) and 2(b). In the figures, the energy separation between the GS and first excited state (ES) of sample B is larger than that of sample A. Since the presence of Sb in the GaAsSb SRL reduces In-Ga intermixing, the high potential barrier in columnar InAs/GaAsSb QD is responsible for a large energy separation [32–34].
Fig. 3 Photoluminescence spectra, obtained at low temperature of 10 K, of (a) sample A and (b) sample B following annealing at various temperatures. Power-dependent photoluminescence spectra of samples A and B following annealing at 800 °C are shown in (c) and (d), respectively. For convenience of comparison, the emission intensity of the spectral lines in the figures is normalized. The insets of Fig. 3 (c) and (d) show the TEM images of samples A and B with post-growth annealing process at 800°C.
As the annealing temperature increases, the energy separation for both samples is reduced, indicating compositional intermixing at the InAs/GaAs(Sb) interface, which reduces the height of the potential barrier [35, 36]. Notably, the GS and the ES emission peaks of sample A merged into a primarily Gaussian distribution following annealing at 800 °C; however, the individual energy states of sample B were still observed, revealing the preservation of the QD heterostructure upon annealing at this high temperature.
To further understand the optical properties on the energy-band structure of samples A and B after annealing at 800 °C, the PDPL of both samples was measured at 10K, and presented in Fig. 3(c) and 3(d), in which spectral lines are normalized and offset for convenient comparison. Solid and dashed lines show the positions of the GS and ES peaks, marked as E0 and E1, respectively. At low excitation power, samples A and B yielded GS emission peaks at 1.15 and 1.08 eV, respectively. Sample B yielded an ES emission peak at 1.12 eV as the excitation power increased from 10 to 100 mW. The particular feature of three-dimensional carrier confinement in a QD and the consequent δ-like density of states, which are responsible for the state filling effect that is evident in the PDPL results from sample B [37–39]. However, sample A following annealing at 800 °C does not exhibit this filling effect. This finding indicates the transition of the QD to a quantum-well-like structure in sample A while sample B retains its QD structure owing to reduced In-Ga intermixing and the preservation of the QD heterostructure by capping with a GaAsSb SRL. This outcome demonstrates unambiguously that sample B has a higher dot-to-well transition temperature than sample A because the reduced In-Ga intermixing behavior by capping with a GaAsSb SRL.
The insets of Fig. 3(c) and 3(d) show the TEM images of samples A and B with post-growth annealing process at 800°C. In the inset of Fig. 3(c), the shape of quantum dots was degraded by strong In-Ga intermixing between QD and GaAs capping layer during annealing process. However, the inset of Fig. 3 (d) performed clear quantum dot structure because the suppression of In-Ga intermixing by Sb element in the QD/GaAsSb interface during annealing. Although the spectral blue-shift was observed for 800 °C-annealed sample B, indicating the slight intermixing behavior, the QD structure preserved by the GaAsSb capping layer in sample B contribute to enhanced carrier confinement than the quantum-well-like structure of 800 °C-annealed sample A.
Figure 4(a) and 4(b) display Arrhenius plots for fitting the activation energies of samples A and B from as-grown sample up to annealing temperature at 900 °C [36]. At the elevated measurement temperature, carriers can overcome the potential barrier and escape from the QDs followed by undergoing a nonradiative recombination process, reducing the integrated PL intensity. Figure 4(c) shows the summarized activation energies of annealed samples A and B. The activation energy of as-grown sample A is 192 meV, which agrees with the reported results in [40].
Fig. 4 Arrhenius plot for temperature-dependent integrated PL intensity from samples (a) A and (b) B at excitation power of 100 mW. Figure 4(c) summarizes the activation energies of samples A and B as functions of annealing temperature, respectively. The insets represent the band alignment of QD heterostructures of the as-grown samples A and B. The GS transition energies and carrier activation energies of both samples in the inset were conducted by the temperature-dependent PL measurement in this work.
The activation energy of annealed sample A reduces from 164 to 162, 135, 123, and 100 meV as the annealing temperature is increased from 650 to 750, 800, 850, and 900 °C, respectively. The activation energy declines as the annealing temperature increases, indicating that the In-Ga intermixing that is caused by thermal annealing results in a shallow potential barrier of the InAs QD structure [35, 36]. The activation energy of as-grown sample B is 303 meV, which is higher than that of sample A, because of the improving carrier confinement by Sb incorporation. The activation energy of annealed sample B decreases from 214 to 198, 178, 117,and 99 meV as the annealing temperature increases from 650, 750, 800, 850,and 900 °C, respectively. Typically, capping the InAs QDs with GaAsSb SRL in InAs/GaAs QD system suppresses the In-Ga intermixing, as evidenced by the study of Ulloa et al. [31]. Therefore, the increased activation energy as well as the carrier confinement was observed in sample B. The schematic band diagrams of as-grown samples A and B which referred from the band edge diagram in [41] were represented as inset in Fig. 4(c) for carrier-thermal-escape clarification. The GS transition energies and carrier activation energies of both samples in the inset were conducted by the temperature-dependent PL measurement in this work. Since the reduced decomposition as well as the mass transport process was observed in GaAsSb capped InAs QDs [32], the deeper confinement of sample B was observed than that of sample A, which was in accordance with the activation energy results in Arrhenius study.
In Fig. 4(c), the activation energies for samples A and B following annealing at 800 °C are 135 and 178 meV, respectively. Comparing the PL spectra in Fig. 3(c) and 3(d) reveals that the quantum dots in sample B still exhibit strong carrier confinement, corresponding to its higher activation energy than that of sample A at 800 °C. Nevertheless, both samples exhibit a quantum-well-like spectral emission and similar activation energies after they are annealed at a temperature higher than 850 °C. The activation energy studies demonstrate that sample B with Sb incorporated in the SRL has a better thermal stability than sample A at temperatures below 800 °C.
To gain more insight into the dynamic behavior of carriers, TRPL measurements of annealed samples A and B are made at 10 K, and the decay traces are shown in Fig. 5(a) and 5(b), respectively. Figure 5(c) plots the carrier lifetime, obtained by fitting the decay traces with a single exponential decay. Sample A had a slightly longer carrier lifetime compared to its as-grown sample due to improved crystal quality by thermal annealing at 650 and 750 °C. However, a dramatically increased carrier lifetime to 9.4 ns was found in sample B at annealing temperature of 800 °C, suggesting that thermal energy in sample B not only enhanced the material quality, but also the energy bandgap tailoring from the increased Sb content in the vicinity of InAs QDs.
Fig. 5 Time-resolved PL decay traces, measured at low temperature of 10 K, of (a) sample A and (b) sample B following annealing at temperatures from as-grown to 900 °C. The carrier lifetimes of all investigated samples are summarized in (c). The inset in the Fig. 5(c) represents the schematic illustrations of type-I and type II carrier transitions.
This result corresponds to the findings of a previous investigation of the effects of increasing the Sb content in single GaAsSb-capped QD layer [42]. Increasing the Sb content can tailor the band alignment toward to the type-II configuration, extending carrier lifetime. The resulting type-II band alignment also exhibits a blueshift of the GS emission peak in the PDPL spectrum at a temperature of 10 K.
Figure 6(a)-6(c) plot the position of the GS emission peak from each examined samples as a function of (excitation power)1/3 for a reference single-layered InAs/GaAs QDs and samples A and B, respectively. The degree of GS energy blueshift (ΔE) are summarized in Fig. 6(d), which indicates the GS energy difference between different excitation powers of 10 and 100 mW. Meanwhile, the dash line at 0 meV represents the GS energy without spectral blueshift.
Fig. 6 PL ground-state peak position of (a) reference single-layer InAs/GaAs QDs, (b) sample A, and (c) sample B following annealing at various temperatures as a function of (excitation power)1/3. The degrees of GS energy blueshift (ΔE) are summarized in (d), which indicates the GS energy difference between different excitation powers of 10 and 100 mW. The dash line at 0 meV represents the GS energy without spectral blueshift.
The GS emission peak of the columnar QD structures is blueshifted by the formation of intermediate bands as observed by Sugaya et al. [43]. Besides, the GS emission peaks from as-grown sample A up to annealed at temperature of 900 °C were all identically blueshifted by 2 meV. However, the blueshift of the GS emission peak position of the sample B show distinct behavior than sample A, and increased from 2.1 to 8.3 meV from the as-grown sample up to annealing temperature at 900 °C, respectively.
The spectral blueshift of the peaks from sample B exceeds significantly than those from sample A that are highly consistent with the increase in Sb composition around InAs QD hetero-interface. Therefore, a band bending of the type-II band alignment is observed through the spectral blueshift in PDPL measurement [42].
For clarity, Fig. 7 schematically depicts the redistribution of Sb atom in sample B after RTA. In the stacking process in a columnar dot structure, accumulated compressive strain is caused by InAs QDs, which results in the selective growth of successive dots on top of the lower QDs [44]. The thermal energy that is provided by RTA contributes to the aggregation of the Sb atoms toward to the InAs QDs with large strain field because of energy favorable (approached lattice constant), increasing the Sb content around the QDs. Consequently, RTA process at adequate annealing temperature is found contributing to the improvement of crystal quality and the particular band alignment tailoring from type-I to type-II in annealed vertically aligned InAs/GaAsSb QD structure. Hence, the superior results in this study lead to a long carrier lifetime in Fig. 5(c).
Fig. 7 Schematic illustration vertically aligned InAs/GaAsSb QD structure, showing aggregation of Sb atoms upon rapid thermal annealing. Top part presents strain field of columnar QD structure.
4. Conclusion
The extension of carrier lifetime and band alignment tailoring in an annealed vertically aligned InAs/GaAsSb QD structure by RTA process was investigated. PL measurements at a temperature of 10 K demonstrated that RTA at adequate temperatures increased the PL intensity for both samples, revealing that the thermal energy improved the crystal quality and reduced the defect density. The PL measurements revealed the larger activation energy and a state-filling effect of the sample B annealed at 800 °C than that of sample A at the same annealing temperature, indicating that the capping layer of GaAsSb SRL on QDs can suppress In-Ga intermixing in the InAs QD system and ensure that the QD heterostructure is preserved during annealing at high temperature. Therefore, sample B exhibited a high dot-to-well transition temperature and activation energy. According to TRPL measurements, incorporating Sb into the SRL yields a long carrier lifetime in columnar dot structure. The aggregation of Sb toward InAs QDs is caused by strain-field selectivity upon RTA process, which further increases the carrier lifetime to 9.4 ns. The PDPL results show the blueshift of the GS emission peak increased with annealing temperature, providing evidence of a transition of the band alignment from type-I to type-II that is associated with band-bending behavior.
Acknowledgments
The authors are grateful to Prof. Chyi in Natl. Cent. Univ. for instrument support, and the National Science Council, Taiwan for its financial support under contracts NSC102-2221-E-155-083. The provision of research equipment by the Optical Sciences Center and Center for Nano Science and Technology at National Central University is greatly appreciated.
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