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Comparative study of photoluminescence for type-I InAs/GaAs0.89Sb0.11 and type-II InAs/GaAs0.85Sb0.15 quantum dots Chuan Zhou a, Baolai Liang a, b, *, Jingtao Liu a, Ying Wang a, Yingnan Guo a, **, Shufang Wang a, Guangsheng Fu a, Yuriy I. Mazur c, Morgan E. Ware c, Gregory J. Salamo c a b c
Hebei Key Laboratory of Optic-electronic Information and Materials, College of Physics Science & Technology, Hebei University, Baoding, 071002, PR China California NanoSystems Institute, University of California, Los Angeles, CA, 90095, USA Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, AR, 72701, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Photoluminescence Quantum dots Semiconductor compound Band alignment
InAs quantum dots (QDs) sandwiched inside a GaAsSb matrix possess advantages for achieving telecom wave length lasers and for developing high efficiency solar cells. In this work, optical properties of InAs quantum dots (QDs) capped by GaAs1-xSbx (x ¼ 0.11 and 0.15) are comparatively investigated. The photoluminescence mea surements reflect that the energy state filling, thermal activation, quenching, and lifetime of the carriers in InAs/ GaAs0.89Sb0.11 QDs are different from those in the InAs/GaAs0.85Sb0.15 QDs. These differences are attributed to the band alignment transition from type-I to type-II resulting from the Sb-composition change from x ¼ 0.11 to x ¼ 0.15 in the GaAs1-xSbx capping layer. Therefore, the emission and quenching involve excited states for type-I InAs/GaAs0.85Sb0.15 QDs, but involve InAs QDs as well as the GaAs0.85Sb0.15 QW recombination for type-II InAs/ GaAs0.85Sb0.15 QDs. So the luminescence reveals complex and distinct physics mechanisms for these two samples.
1. Introduction Self-assembled InAs/GaAs quantum dots (QDs) have attracted considerable interest due to their unique physical properties and suc cessful applications for many optoelectronic devices, such as semi conductor lasers, infrared detectors, modulators, and photovoltaic cells [1–5]. As a good example, sandwiching InAs QDs into a GaAs1-xSbx matrix has recently been extensively investigated. The InAs/GaAs1-xSbx QDs possesses several advantages for achieving telecom wavelengths in a QD laser and for developing high efficiency QD solar cells [6–10]. First, the GaAs1-xSbx layer can efficiently improve the QD uniformity and reduce the coalescence of dots [11,12]. Second, incorporation of Sb into the GaAs matrix will increase the barrier’s conduction band off-set and reduce the residual strain inside the InAs QDs, thus obtaining QD emission at the 1.3–1.55 μm telecom wavelength [13,14]. In addition, the InAs/GaAs1-xSbx QD structures will raise the barrier’s valence band toward that of the InAs QDs, achieving a transition from type-I to type-II band alignment when the Sb-composition exceeds x ¼ 0.13 [15,16]. Such type-II InAs/GaAs1-xSbx QDs are predicted to be promising active
region materials for intermediate-band solar cells due to the spatial separation of carriers, in which the electrons are confined in the InAs QDs while the holes are in the surrounding GaAs1-xSbx layer . As such, studies of the optical properties are indispensable when attempt ing to develop either lasers or photovoltaic devices based on the InAs/GaAs1-xSbx QDs. In this work, we report the comparative study of luminescence for type-I InAs/GaAs0.89Sb0.11 QDs and type-II InAs/ GaAs0.85Sb0.15 QDs regarding to their energy states filling, thermal quenching, carrier lifetime, and PLE spectrum. 2. Experiments There are two GaAs1-xSbx QD samples grown by a Veeco Gen-930 molecular beam epitaxy (MBE) reactor. First, a 100 nm GaAs buffer layer was deposited at 580 � C after the surface native oxide layer was thermally removed at 600 � C from the semi-insulating GaAs (100) sub strates. Then the substrate temperature was cooled down to 525 � C and 2 monolayers (ML) of InAs was deposited with a growth rate of 0.015 ML/ s to form the QDs. After that, the self-assembled InAs QDs were capped
* Corresponding author. Hebei Key Laboratory of Optic-electronic Information and Materials, College of Physics Science & Technology, Hebei University, Baoding, 071002, PR China. ** Corresponding author. E-mail addresses: [email protected]
(B. Liang), [email protected]
(Y. Guo). https://doi.org/10.1016/j.optmat.2019.109479 Received 18 September 2019; Accepted 21 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Chuan Zhou, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109479
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by a 10 nm GaAs1-xSbx layer with x ¼ 0.11 or x ¼ 0.15 for sample A or B, respectively. Finally, a 60 nm GaAs capping layer was deposited at the same temperature to complete the sample growth. A schematic diagram of the sample structures is shown in Fig. 1(a). The two samples were mounted in a cryostat with temperature var iable from 10 K to 300 K for luminescence measurements. For photo luminescence (PL), a continuous-wave laser operated at λ ¼ 532 nm was focused by a 50� objective lens onto the samples and the excitation intensities varied in the range from 0.001 to 104 W/cm2. The PL signal was collected by the same objective lens and sent to a 50 cm Acton spectrometer, finally detected by a liquid nitrogen cooled CCD detector array. For PL excitation (PLE) measurements, we introduced a NKT super-continuum laser as the excitation. The time-resolved PL (TRPL) are also measured with the NKT super-continuum laser as the excitation, and a PicoHarp-300 time-correlated-single-photon-counting (TCSPC) system as the detection.
for sample A to x ¼ 0.15 for sample B. To further study the energy state filling effects, excitation-intensity dependent PL spectra were obtained at 10 K as shown in Fig. 2(b) and (c) for sample A and B, respectively. For sample A, the position of the PL peak at 1186 nm is independent of excitation power before the excited state emission can be observed, indicating a typical type-I QD structure. For sample B, the Sb content (x ¼ 0.15) exceeds the critical value (x~13%) and the type-II band structure formed . So with the exci tation intensity increasing, the PL peak shows a clear blue shift, as indicated in Fig. 2(d). For sample A, in Fig. 2(b) we initially observe the PL band from recombination of excitons localized at the QD ground states at low excitation condition. Then with increasing excitation power, four excited state emission peaks appear gradually, and finally we also find the GaAs signal. For sample B, similarly it can only observe a PL band from the InAs/GaAs0.85Sb0.15 type-II QDs at low excitation power in Fig. 2(c). With the excitation power increasing, the InAs QDs and the GaAs0.85Sb0.15 QW emission peaks can be observed . Fig. 2(e) and (f) are Gaussian fitting analysis for the PL spectrum of sample A and sample B, respectively, both with an excitation intensity of 300 W/cm2. Sample A has a ground state emission peak position of 1190 nm as well as four excited states peak positions (1122 nm, 1052 nm, 986 nm and 939 nm) with the GaAs–related emission at 820 nm. Furthermore, the luminescence related to the GaAs0.89Sb0.11 layer and the wetting layer (WL) are observed at 912 nm and 870 nm, respectively. Besides the small GaAs emission at ~820 nm, the fitting result for sample B in Fig. 2(f) shows three peaks, which are proposed to correspond with the InAs/GaAs0.85Sb0.15 QDs, InAs QDs and GaAs0.85Sb0.15 QW, respectively . Therefore, sample A and sample B have different energy state filling behavior and inhabit distinct recom bination mechanics that is associated with the type-I and type-II band alignment nature. The Gaussian fitting analysis from PL spectra are coincide very well with the recombination routes in the schematic band diagram in Fig. 1(d). Fig. 3(a) and (b) are PL spectra for sample A and sample B measured as functions of temperature with the excitation intensity of 10 W/cm2. With the temperature increasing from 10 K to 295 K, the PL spectra gradually evolves from a single peak into two luminescence peaks or multiple peaks in both samples. This means that the emission involves more recombination mechanisms at higher temperature. For sample A, it is likely that the excited states are involved. But for sample B, it could be the recombination from the holes and electrons inside the InAs QDs, or recombination associated with the GaAs0.85Sb0.15 layer, i.e., the GaAs0.85Sb0.15/GaAs QW. Fig. 3(c) plots the temperature dependence for the PL integrated intensity. By using the equation . � � � � �� 1 E1 E2 IðTÞ ¼ α 1 þ C1 exp ; þ C2 exp kB T kB T
3. Results and discussion Fig. 1 (b) shows an atomic force microscope (AFM) image of the uncapped InAs QDs. The InAs QDs are measured to have an areal density of 2.6 � 1010 cm 2, an average diameter of 54.5 � 6.3 nm and an average height of 9.8 � 1.1 nm. Meanwhile, there are no incoherent islands or large defects observed through AFM, indicating good InAs QD quality. According to the literature, the InAs/GaAs0.89Sb0.11 QDs in sample A have a typical type-I band alignment with both electrons and holes inside the QDs, while the InAs/GaAs0.85Sb0.15QDs in sample B have a type-II band alignment, having electrons in the InAs QDs but holes in the GaAs0.85Sb0.15 layer [18,19]. The band alignment diagram and the possible recombination routes are plotted in Fig. 1(c) for sample A and in Fig. 1(d) for sample B. Fig. 2(a) shows the PL spectra measured at 10 K with a relatively low excitation power density of 10 mW/cm2. Sample A gives a QD emission band peaked at 1186 nm and a linewidth (Full Width at Half maximum) of 26 nm, while sample B peaks at 1250 nm with a FWHM of 51 nm. The red shift and the broadening of PL band for sample B in comparison with sample A are attributed to the band alignment change, the reduction of the residual strain and the alloy composition fluctuations as the Sbcomposition in the GaAs1-xSbx capping layer increases from x ¼ 0.11
where T is the temperature, I(T) is the integrated PL intensity, we extract the corresponding thermal activation energies, E1 and E2. For sample A, the best fit yields α ¼ 0.98, E1 ¼ 8.43 meV, and E2 ¼ 284.7 meV. For sample B, we obtain that α ¼ 1.01, E1 ¼ 3.8 meV, and E2 ¼ 238.2 meV. The relatively smaller thermal activation energies for sample B are likely attributed to its type-II band alignment for the QDs, where electrons and holes are bonded together via Columbic interaction so that they can escape from the QDs easily. For sample A, the electrons and holes both are confined in the QDs. More thermal energy is needed to active carriers to escape from the QDs. Actually, because the GaAs1-xSbx capping layer increases the confinement for the electrons, the thermal activation en ergy E1 ¼ 8.43 meV is larger than usual InAs QDs. Fig. 3(d) shows the peak energy shifts as functions of the tempera ture. Both sample A and B show a clear redshift. As the temperature increases from 10 K to 295 K, sample B has a shift of ~40 meV but sample A of 70 meV. The larger redshift is generally attributed to the thermal activation and redistribution of the carriers between
Fig. 1. (a) Diagram of the InAs/GaAs1-xSbx QD sample structure. (b) 1 μm � 1 μm AFM image of the InAs QDs. (c) and (d) are the diagram of the band alignment for sample A and sample B, respectively. 2
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Fig. 2. (a) PL spectra for sample A and sample B measured at T ¼ 10 K with excitation intensity at ¼ 10 mW/cm2 (b) PL spectra for sample A as a function of laser 2 6 2 6 excitation intensity from P– – I0¼3 mW/cm to P ¼ 10 I0. (c) PL spectra for sample B as a function of laser excitation intensity from P– – I0¼3 mW/cm to P ¼ 10 I0. (d) Peak energy E with respect to laser excitation intensity. (e) and (f) are Gaussian fitting analysis for the PL spectrum measured at P ¼ 106I0 for sample A and sample B respectively.
neighboring dots with different sizes . However, we assign the larger redshift for sample A to the appearance of recombination emis sion from the InAs QDs and from the GaAs0.89Sb0.11 QW layer. The PL linewidths as functions of temperature are shown in Fig. 3(e). In the temperature range of 10–200 K, the FWHM varies slowly for both samples. With the temperature going up, the FWHM of sample B shows a significant increase, in contrast, it changes much less for sample A. The small FWHM variation for sample A means a good size uniformity for the InAs QDs. Very interestingly, as light reduction for FWHM can be observed at ~60 K for sample B, but the FWHM shows a significant in crease at high temperature. This phenomenon is generally attributed to the thermal activation and redistribution of the carriers between neighboring dots for inhomogeneously distributed InAs QDs . Again, we attribute this to the thermal activation and redistribution of the carriers into the GaAsSb QW or InAs QDs. Because sample B has a type II band alignment, in which the electrons are confined in the QDs while the holes are confined in the GaAs0.85Sb0.15 layer. At high tem perature, both electrons and holes can be thermally activated. As a
result, the holes are activated and migrate into InAs QDs to enhance the emission from the direct recombination. But electrons can thermally escape from the QDs to the GaAs0.85Sb0.15 layer and resulting in QW emission. Therefore, we observe the abnormal behavior of the peaks energy and FWHM for sample B. It also should be mentioned that sample A also has the GaAs0.89Sb0.11 layer and the thermal activation of carriers from the QDs should enhance the GaAs0.89Sb0.11 QW emission. But this enhancement is much less than the emission intensity corresponding to the type-I recombination inside the InAs QDs. Therefore, it is impossible to observe it. From the above results, we claim that the PL emission and quenching at high temperature involves the QD excited state for sample A, but involves the InAs QDs as well as the GaAs0.85Sb0.15 QW recom bination for sample B. So the variation of the peak energy and FWHM reflect distinct and complex physical processes for these two samples. The PLE spectra shown in Fig. 4 are measured at 10 K with an NKT laser output wavelength from 760 nm to 1000 nm, while the detection are set at the peak wavelength of the QD PL band as indicated by the arrow bar in Fig. 4. Both the PLE spectra of the sample A and B show the 3
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Fig. 3. (a) and (b) are PL spectra for sample A and sample B measured as functions of temperature with the excitation intensity fixed at P ¼ 10 W/cm2. (c) Integrated PL intensity as a function of temperature. (d) PL Peak energy as a function of temperature. (e) FWHM as a function of temperature.
GaAs and the WL giving the excitation resonances at 800 nm and 860 nm. The resonance at ~930 nm for sample A is attributed to the absorption within the GaAs0.89Sb0.11 layer for sample A, and the ab sorption coming from theGaAs0.85Sb0.15layer for samples B goes to a longer wavelength at around 980 nm . The excitation resonances observed for the WL, GaAs0.89Sb0.11 layer, and the GaAs0.85Sb0.15 layer in PLE spectra agree very well with the results of the Gaussian fitting analysis of the PL spectra. Finally, we further confirm the band alignment for both samples by measuring carrier lifetime via TRPL. Fig. 5 shows the TRPL spectra for sample A and sample B measured at T ¼ 10 K with the NKT laser output fixed at 532 nm. If it is measured with a high excitation power, the measured decay is affected by not only the pure radiative recombina tion, but also by perturbing from the excited states and the electronic band bending. Herewith the excitation power is chosen at a relatively low value of 76 μW. The extracted decay times are 1.8 ns and 7.4 ns for sample A and B, respectively. Clearly, the type-II band alignment for sample B, i.e., the spatial separation of the electrons and holes results in a much longer lifetime for sample B than sample A [25,26].
4. Conclusions In conclusion, the optical properties of InAs/GaAs0.89Sb0.11 and InAs/GaAs0.85Sb0.15QDs are comparatively investigated by PL, PLE, and TRPL measurements. Due to the formation of the type-II band alignment, the InAs/GaAs0.85Sb0.15 QDs show a different energy state filling behavior, which corresponds with multiple channels for carrier recom bination as the excitation power increases. The temperature dependent PL spectra show that the carriers in the InAs/GaAs0.85Sb0.15 QDs need lower activation energy to escape, while their thermal quenching also involves different optical recombination processes. The emission and quenching involves excited states for type-I InAs/GaAs0.89Sb0.11 QDs, but involves InAs QDs as well as GaAs0.85Sb0.15 QW recombination for type-II InAs/GaAs0.85Sb0.15 QDs. Due to the type-II band alignment, i.e. the spatial separation of the electrons and holes, the InAs/GaAs0.85Sb0.15 QDs have a 4-times longer lifetime than the GaAs0.89Sb0.11 QDs. The excitation resonances observed for the WL, GaAs0.89Sb0.11 layer, and the GaAs0.85Sb0.15 layer in PLE spectra agree very well with the Gaussian fitting analysis results from the PL spectra. These luminescence 4
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Acknowledgement The authors acknowledge the financial support by the Natural Foundation of People’s Republic of China (Grant# 61774053), the Natural Science Foundation of Hebei Province of China (F2019201446), the Advanced Talents Incubation Program of the Hebei University (Grant# 8012605), and the National Science Foundation of the United States (EPSCoR Grant # OIA-1457888). References  D. Bimberg, M. Grundmann, N.N. Ledentsov, Quantum Dot Hetero-Structures, Wiley, Chichester, 1999.  H.Y. Liu, T. Wang, Q. Jiang, R. Hogg, F. Tutu, F. Pozzi, A. Seeds, Long-Wavelength InAs/GaAs quantum-dot laser diode monolithically grown on Ge substrate, Nat. Photonics 5 (2011) 416.  Z.R. Lv, Z.K. Zhang, X.G. Yang, T. Yang, Improved performance of 1.3-μm InAs/ GaAs quantum dot lasers by direct Si doping, Appl. Phys. Lett. 113 (2018), 011105.  W. Liu, B.L. Liang, D.L. Huffaker, H. Fetterman, Anisotropic performance of high speed electro-optic modulators with InGaAs quantum dot chain active region, Opt. 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Fig. 4. Intensity-normalized PLE spectra for sample A and sample B measured with a NKT laser output wavelength from 760 nm to 1000 nm are plotted with the Gaussian fitting analysis of the PL spectrum of sample A and sample B.
Fig. 5. TRPL spectra for sample A and sample B measured at T ¼ 10 K with a NKT laser output power of 76 μW at the wavelength of 532 nm.
measurements reflect more complex and distinct physical mechanism for these two samples. The results enrich our understanding of InAs/ GaAs1-xSbx QDs, which may help to develop QD lasers and high efficient QD solar cells. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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