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In this study, the optical properties of InAs quantum dots (QDs) with various strain-reducing layers (SRLs) of GaAsSb and InGaAsSb are characterized using photoluminescence (PL) and time-resolved PL (TRPL) measurements. The room-temperature PL results for the InAs/InGaAsSb QDs revealed stronger emission intensities than InAs QDs capped with an GaAs1-xSbx (x = 20%)SRL, although both samples were grown under the same Sb flux during the molecular beam epitaxy process. The InAs/InGaAsSb QDs showed a significant elongation of emission wavelengths to 1450 and 1310 nm for the ground and first-excited state at room temperature. The energy band alignment of the InAs QD heterostructures was found tailoring from type II to type I as the GaAsSb SRL was replaced by InGaAsSb layer, which improved the radiative efficiency and was verified by power-dependent PL and TRPL measurements. Post-growth rapid thermal annealing was applied on the InAs/InGaAsSb QDs to further enhance the QD quality and PL emission efficiency. The greatly improved PL intensity, reduced linewidth, shortened radiative lifetime, with increasing annealing temperature were demonstrated, and InAs/InGaAsSb QDs exhibited enhanced optical characteristics for long-wavelength emission applications.
© 2014 Optical Society of America
1. Introduction
Over the last decade, substantial efforts have been made to develop In(Ga)As/GaAs 1.3 μm quantum dot (QD) lasers with ultra-low threshold current density, high thermal stability, enhanced quantum efficiency, and rapid bandwidth modulation [1–3].Recently, the development of a 1.3 μm QD lasers has progressed rapidly, and the pursuit of GaAs-based long-wavelength emission QD lasers for optical-fiber telecommunication have become important, particularly for use in the InAs/GaAs systems. To further enhance the laser performance in long-wavelength applications, QD laser with excited-state (ES) operation was also studied and demonstrated superior characteristics to that with ground-state (GS) operation because of the advantages of high saturation gain, reduced carrier scattering time, and large modulation bandwidth in ES QD lasers [4–7].
Although the development of long-wavelength QD laser on a GaAs substrate is proceeding rapidly, extending the QD emission wavelength remains a challenge because of the degraded material and optical properties of InAs/GaAs QD heterostructures [8, 9]. Numerous growth techniques have been proposed to extend the emission wavelength of InAs QDs, including the growth of QDs at low temperature [10], use of a metamorphic buffer layer for QD growth [11], growth of larger QDs [12],and covering strain-reduced InAs QDs by an indium-contained overgrown layer [13–15]. However, several of these techniques mentioned above are all associated with difficulties that are related to complicated fabrication processes and strain-induced crystal defects, limiting their application range.
To improve the epitaxial quality of QDs, antimony (Sb) was incorporated into InAs/GaAs QDs as the GaAsSb strain-reducing layers (SRLs) [16–18]. Antimony was shown to be a surfactant that reduces the surface energy in heteroepitaxy to increase dot density [19,20]. A GaAsSb capping layer was also reported to reduce the In-Ga intermixing and lattice mismatches during capping processes on InAs QDs, thereby preserving the dot height compared with GaAs-capped InAs QDs [21,22].Furthermore, the use of GaAsSb SRL in InAs QD laser fabrication improves the optical properties of QDs, extending the emission wavelength. Liu et al. proposed high-performance InAs QD lasers with GaAsSb SRL that had emission wavelengths to 1.31 μm [23].
However, further incorporating Sb into InAs/GaAsSb QDs to elongate the QD emission wavelength can degrade the heterostructural material quality. The significantly reduced luminescent intensity and enlarged inhomogeneous QD spectral broadening in the long-wavelength emission region were observed because of structural defects, which induced from crystal lattice mismatch and can affect the operation characteristics of the QD optoelectronic devices [9,24,25].In addition, InAs/GaAsSb QDs exhibit a staggered (type-II) band alignment with a longer carrier lifetime that has less radiative recombination probability than that of InAs/GaAs capped QDs, and it is unsuitable for laser application with Sb composition exceeds over 14% [26,27].
To improve the optical properties of InAs QDs, InGaAsSb SRL has been proposed to extend their emission wavelengths with enhanced thermal stability and the luminescent properties in the previous research [28,29].Nevertheless, the time-resolved photoluminescence and energy band alignment study on the InAs/InGaAsSb QDs are scant, and the thoroughly elucidation is important and still necessary.
This study utilizes photoluminescence (PL) and time-resolved PL (TRPL) to study the mechanisms of the improved optical properties of InGaAsSb-capped InAs QDs. The optical characteristics of InAs/GaAsSb QDs with a type-II band alignment were also compared with those of InAs/InGaAsSb QDs. The type-II band alignment of the InAs/GaAsSb QD structure was found to be transformed into a straddling (type-I) band alignment as the GaAsSb SRL was replaced by InGaAsSb SRL, which can improve the QD radiative efficiency, and this band alignment tailoring was verified by power-dependent PL and TRPL measurement. Therefore, the PL intensity of InAs/InGaAsSb QDs is stronger than that of InAs/GaAsSb QDs because of the increase in the probability of overlapping between the electron and hole wave functions. In addition, extended QD emission wavelengths were observed in this study to 1450 and 1310 nm of the ground-state and first-excited state emissions, respectively, by indiumin corporation as InGaAsSb SRL for reducing the compressive strain in InAs QDs.
To further improve the material and optical properties of the QD heterostructure, post-growth rapid thermal annealing (RTA) was applied to the InAs/InGaAsSb QDs. The band alignment and energy levels were tailored by the RTA-induced intermixing of In and Ga in the QDs and barrier layer [30]. This annealing process induced a blueshift in the emission wavelength, substantially enhanced the emission intensity, narrowed the PL spectral linewidth of the QD emission, and reduced the inter-sublevel spacing energies in the annealed InAs/InGaAsSb QD structure. This study demonstrates the superior optical characteristics in InAs/InGaAsSb QD structure, which is highly desirable for long-wavelength optoelectronic devices such as semiconductor lasers, single-photon sources, and optical amplifiers.
2. Experiment
The samples under investigation were grown by Riber 32P solid-source molecular-beam epitaxy (SSMBE) on GaAs substrates. After a 300 nm-thick GaAs buffer layer was grown at 580 °C, the substrate temperature was rapidly cooled to 500 °C for subsequent QD growth. Three samples were designed with the purpose of studying their optical properties. All QD samples were grown with V/III ratio of 65 under arsenic flux of 4 × 10−6 Torr with nominal InAs thickness and deposition rate of 2.7 monolayers (MLs), and 0.1 ML/s. The QD samples were overgrown by 8 nm-thick GaAs, GaAs1-xSbx (x = 20%), and InyGa1-yAs1-xSbx (x = 20%, y = 18%) SRLs on InAs QDs as samples A, B, and C, respectively. All samples were finally capped by a 100 nm-thick GaAs layer. Figure 1 presents the layer structures of the samples in this study.
Post-growth rapid thermal annealing treatment for sample C was performed with GaAs wafer proximity capping in the nitrogen ambient for 30s at 650, 750, 800, 850 and 900 °C,respectively. Finally, the samples were cooled to 350 °C and then down to RT. The 661 nm line of a laser diode was used as the excitation source for low-temperature and temperature-dependent photoluminescence (PL) measurements in a helium-cooled cryogenic system. A cooled InGaAs detector was utilized to measure the signal that was dispersed by a 0.5 m monochromator (iHR-550) using the lock-in technique. The time-resolved photoluminescence (TRPL) system employed a semiconductor laser with an emission wavelength of 635 nm/80 MHz as the excitation source, and the carrier decay signal was recorded by time-correlated single photon counting technique with an overall time resolution of 28 ps.
3. Results and discussion
Figure 2 shows the PL spectra of samples A, B and C at temperature of 10 K. The QD ground-state emission intensities of samples A and C are similar, while that of sample B is weaker. The energy band diagrams of InAs/GaAs QDs (sample A) and InAs/GaAsSb QDs (sample B) are type-I and type-II band alignments of the QD heterostructure, respectively, while the Sb content in the GaAsSb capping layer exceed 14%. Since the type-II system in sample B, electrons and holes are confined separately in the InAs QDs and GaAsSb SRL, and lead to the reduction of carrier recombination probability as well as a poor luminescent intensity. However, sample C with an InGaAsSb SRL exhibits stronger emission intensity than sample B, although both samples were grown under the same Sb flux during the molecular beam epitaxy process. A tailored energy band alignment of sample C is suggested and analyzed in detail by power-dependent PL and TRPL, as shown below. Capping the QDs by GaAsSb SRL red-shifts the ground-state emission wavelength of InAs QDs from 1100 nm (sample A) to 1186 nm (sample B), which is attributable to the reduction of compressive strain and In-Ga material intermixing. The emission wavelength is further extended to 1300 nm (sample C) through incorporating indium (In) into the capping layer to further reduce the strain by increasing the lattice constant of InGaAsSb SRL [28].
Fig. 2 Low-temperature photoluminescence spectra of samples A, B and C at temperature of 10 K. Inset shows the room-temperature photoluminescence spectra of samples A, B and C.
The inset in Fig. 2 displays the room-temperature PL spectra of samples A, B and C, which include GS emission peaks at 1196, 1302 and 1400nm, respectively. Notably, InAs QDs capped by InGaAsSb SRL exhibited extended first excited state emission peak to 1310nm. Since the bandwidth modulation of the QD laser with ES operation were studied to be superior than those of GS operation, the long-wavelength emission of ES in this work demonstrates the feasibility of using an InAs/InGaAsSb QD structure for 1310nm ES QD laser applications [4,5].
Figure 3 presents the TRPL measurements of samples A, B and C that were made at 10 K. The inset summarizes the QD ground-state energy trace of the power-dependent PL (PDPL) spectra of samples A, B and C, respectively. The traces in the inset reveal that the behavior of the ground-state spectral emissions of samples A and C remain a constant function of (excitation power)1/3. This result is believed to demonstrate that the QD heterostructures in samples A and C exhibit a type-I band alignment, even though the high Sb content in sample C. However, the behavior of the trace of sample B depends linearly on (excitation power)1/3, corresponding closely to the band bending behavior of a type-II band structure [31–33]. Since the photo-excited electrons and holes are separately confined in InAs QDs and the GaAsSb SRL, establishing an electronic field, resulting in the bending of the triangular potential well in the type-II band alignment. Therefore, increasing the PL exciting power can raise the ground-state energy, causing a significant blueshift in the GS energy of sample B in the power-dependent PL experiment.
Fig. 3 Time-resolved photoluminescence (TRPL) decay traces of samples A, B and C at a temperature of 10 K. Inset plots QD ground-state energy as a function of (exciting power)1/3 for samples A, B and C.
To elucidate thoroughly the band alignment transition in the power-dependent PL spectra, a series of time-resolved PL measurements were made on GaAs(Sb) and InGaAsSb SRL-capped InAs QDs. The carrier lifetimes of samples A, B and C are fitted with the PL decay curves. From Fig. 3, sample B has a longer carrier lifetime of 5.1 ns, than that of sample A (1.1 ns) or C (2.0 ns), verifying the type-II band alignment in Sample B, and the carrier lifetime associated with this alignment is much longer than that of the type-I band structure. Owing to the two distinct carrier recombination mechanisms in the type-II QD structure, a double exponential function: I(t) = A1exp(-t/τ1) + A2exp(-t/τ2) is utilized to specify the lifetime accurately. The short initial decay time constant τ1 and the slow-tail recombination term τ2 for sample B are determine by fitting to be 5.1 and 12.3 ns, respectively. Since the different spatial distribution of the carrier wavefunction in the GaAsSb-capped InAs QD heterostructure, a lesser overlap of the wave functions causes the carrier lifetime to be longer in sample B than in other samples [34]. It worthy notices that sample C represents a short carrier lifetime and type-I band alignment with only a single recombination carrier lifetime, therefore, the radiative recombination efficiency in the InAs/InGaAsSb QDs exceeds that in the InAs/GaAsSb QDs. The results of PDPL and TRPL reveal the type-I band alignment in sample C, which is responsible for the reduction in the carrier lifetime because of the increase in the probability of carrier wavefunction overlapping.
Figures 4(a)–4(c) display schematic band diagrams of the InAs/GaAs (sample A), InAs/GaAsSb (sample B) and InAs/InGaAsSb (sample C) QD structures, respectively. The band structures are determined by referencing from the literatures [35,36].The spectral transition energies of samples A and C are extracted from the PL results in Fig. 3 as 1.13 and 0.95 eV for the ground-state emission transition (E0H0), and 1.20 and 1.01 eV for the first-excited state emission peak (E1H1). Sample B has a ground state emission peak around approximately 1.05 eV (E0H0) with two higher energy peaks at about 1.11 and 1.15 eV. These two higher peaks from sample B correspond to the carrier recombination between electrons in the first-excited state, and holes in the ground-state (E1H0) and first excited state (E1H1), respectively. The E1H0 state is observed only in sample B which has the type-II band alignment, because the selection rule does not apply owing to the spatial separation of carriers [27,37].
Fig. 4 Schematic energy band structures of InAs QDs overgrown by (a)GaAs, (b)GaAsSb, and (c) InGaAsSb SRL. (Inset: PL spectra of samples A, B and C, fitted by Gaussian distribution curves.) The energy band structure is determined from equations taken from the literatures in [35,36].
The valence band offset (VBO) of InAs/GaAsSb QDs decreases with increasing Sb content in GaAsSb, becoming negligible at around 14%. Further increasing Sb content causes a type-II band alignment between InAs and GaAsSb SRL. However, the band alignment between InAs QDs and InGaAsSb SRL in sample C is verified as a type-I structure in Fig. 3, although the high Sb content, and it indicate a distinct carrier dynamic from that of InAs/GaAsSb QDs. In this study, the mechanism of indium aggregation from InGaAsSb SRL to InAs QDs is proposed due to low-surface energy selection of indium adatom. Hence, the increased dot size with reduced energy band gap corresponds to a reduced energy-state separation between the GS and ES in sample C. These results indicate the elevated valence band energy of the InAsQDs and thus a negligible VBO between InAs QDs and InGaAsSb SRL, as shown in Fig. 4(c). With respect to the type-I band alignment in sample C, the GS PL emission remains constant as the increased exciting power, and the increased overlapping of the carrier wavefunctions contribute to the improved PL intensity of InAs/InGaAsSb QDs to a value over that of the InAs/GaAsSb QDs (sample B).
Since sample C exhibits a particular tailoring from type-II to type-I band alignment at the same Sb flux within sample B, further investigation of thermal annealing on InAs/InGaAsSb QDs is also studied. Recently, several studies of the effect of annealing on GaAsSb and GaAs-capped InAs QDs have been extensively performed [38,39]; however, the discussion of annealing effect on InGaAsSb-capped InAs QDs is much less discussed. Therefore, the optical properties of the annealed InAs/InGaAsSb QD structure are studied herein and compared with those of the as-grown QDs.
Figure 5 shows the PL spectra of InAs/InGaAsSb QDs (sample C) that were obtained at a temperature of 10 K under an excitation power of 100 mW, and the QD samples were post-growth annealed for 30s with annealing temperatures of 650, 750, 800, 850 and 900 °C,respectively. The as-grown sample was also compared in the discussion.
Fig. 5 PL spectra of as-grown and the annealed QD samples C (as-grown, 650, 750, 800, 850 and 900 °C), measured at temperature of 10 K under a excitation power of 100mW. Inset shows state separation for the InAs/InGaAsSb QDs following annealing at various temperatures.
The PL emission intensity increases significantly with annealing temperature because thermal annealing improves the quality of the QD structural material [40]. The FWHM decreases from 56 to 29 meV with emission wavelength blueshifted as the annealing temperature increases from as-grow up to 900 °C, because thermally induced In-Ga intermixing reduces the fluctuation in relative dot sizes, as presented in Fig. 5 [41]. The energy-state separation in annealed sample C was fitted with Gaussian distribution, as presents in the inset. The inset shows the decrease in the separation of the energies between the ground and the first excited states of the InGaAsSb SRL-capped InAs QDs as the increased annealing temperatures. The reduced state separation indicates that thermally induced In-Ga intermixing reduces the carrier confinement in the QD heterostructures [42]. To examine the effects of thermal annealing on the rate of radiative recombination in the InGaAsSb-capped QDs, TRPL measurements are performed, and the results are presented in Fig. 6.The PL decay time is found to decrease from 2.0 to 0.9 ns as the as-grown sample up to annealing temperature of 900 °C. The reduced decay time corresponds to the reduced radiative recombination time at low temperature, and thus is consistent with the improvement in luminescent intensity in Fig. 5.
Fig. 6 TRPL decay traces of as-grown and annealed InAs/InGaAsSb QDs. Inset schematically depicts the transformation of QD band alignment after thermal annealing.
The insets in Fig. 6 displays band diagrams of the as-grown and annealed InAs/InGaAsSbQDs. Since the aggregation of indium adatom, therefore, a type-I band alignment is formed in InAs/InGaAsSb QDs, whereas InAs/GaAsSb is type-II. Upon thermal annealing, the material intermixing in the InGaAsSb SRL is pronounced, tending to shallow the energy potential and increase the valence-band offset between InAs QD and InGaAsSb SRL [43]. Therefore, the distribution of electron-hole pairs are more energy confined in the InAs QD than spreading of hole wavefunction to the InGaAsSb SRL in the as-grown sample. The increase in the overlapping probability of the electron-hole pair wavefunctions with increasing annealing temperature, and the reduced recombination lifetime in the TRPL study, are therefore verified.
4. Conclusion
The TRPL results show the carrier lifetime in sample B (5.1 ns) exceeds those in sample A (1.1 ns) and sample C (2.0 ns). The extended carrier lifetime in sample B is attributable to the different spatial separation of the paired electrons holes in a type-II band alignment. InAs QDs that are covered with an InGaAsSb SRL exhibit a significant redshift and enhanced PL intensity relative to the type-II InAs/GaAsSb QD structure. InAs/InGaAsSb QDs (sample C) exhibit extended ground and first excited state emission wavelengths to 1450 and 1310nmat room temperature, favoring the application of the ES quantum dot laser in optical-fiber communications. Furthermore, the shorter carrier lifetime in sample C reflects a type-I band structure with enhanced probability of wavefunction overlapping. Based on the PDPL and TRPL measurements, the type-I band alignment of InAs/InGaAsSb QD structure is verified and this structure is observed to exhibit a stronger emission intensity than InAs/GaAsSb QDs (sample B), which were grown under an identical Sb flux during the epitaxy process. This study demonstrates that the advantages of InAs/InGaAsSb QDs - an elongated emission wavelength and improved luminescent intensity - that are associated with the reduced lattice strain and the type-I band alignment. The much improvement in PL intensity, spectral linewidth, and reduction of the radiative lifetime, with increasing annealing temperature are caused by improved material quality and carrier recombination. Therefore, the remarkable results in this study make InAs/InGaAsSb QDs the most promising candidates for long-wavelength emission QD optoelectronic device applications.
Acknowledgments
The authors are grateful to Prof. Chyi in Natl. Cent. Univ. for instrument support, and the National Science Council, Taiwan, R.O.C., for its financial support under contracts NSC 102-2221-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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