As an essential structure of infrared semiconductor lasers, the optical properties of InGaAsSb/AlGaAsSb multiple quantum wells need to be fully investigated. In this paper, the temperature and excitation power-dependent photoluminescence (PL) spectra of the InGaAsSb/AlGaAsSb MQWs are measured. A strong free exciton emission with a photon energy of 0.631 eV was observed at room temperature. Besides the main peak, an obvious shoulder peak, located at the low photon energy position under low temperature range (T≤90 K), was confirmed to be an emission of defect related localized carriers by power-dependent PL spectra. The power-dependent PL spectra were dominated by the localized carriers emission under low excitation power (Iex≤20 mW), while the free-exciton emission dominated the PL spectra gradually when excitation power higher than 40 mW. This phenomenon is ascribed to the dissociation of localized states. Our work is of great significance for the device applications of InGaAsSb/AlGaAsSb multiple quantum wells.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
GaSb-based multinary semiconductor alloys have received great interest for application of optoelectronic devices in recent years. The InGaAsSb/AlGaAsSb quantum well (QW) as active materials can be used for mid-infrared lasers in the wavelength range of 2-3 μm [1–5]. By using quinternary AlGaInAsSb barrier in quantum well structures, the emission wavelength of Sb-based semiconductor lasers has been extended above 3 μm [6,7]. Such lasers have been widely used in medical diagnosis and surgery, molecular spectroscopy, gas chemical sensing, etc. However, problems also exist on effective laser diodes operating in mid-infrared range. Generally, good optical properties of active material lead to high performance of the laser devices. The device lasing performance can be improved by controlling epitaxial growth techniques combined with material characterization [8,9]. Therefore, it is necessary to study the optical properties of the grown QW structure, which act as the active region of the laser.
Indeed, the optical properties of InGaAsSb/AlGaAsSb multiple quantum wells (MQWs) have been studied by photoluminescence (PL) measurements in past years [10–13]. Some researchers also have investigated the effect of annealing on PL properties of InGaAsSb/AlGaAsSb MQWs . However, the phenomenon of carrier localization in quaternary InGaAsSb/AlGaAsSb MQWs is rarely studied. Localized states, a common phenomenon in semiconductor materials caused by alloy composition fluctuation and defects, have been investigated in detail on binary and ternary alloy systems [15–18]. In our previous studies , localized states had been found in GaAsSb materials and GaAsSb/AlGaAs QWs and studied by temperature- and excitation power- dependent PL measurements. Meanwhile, the phenomenon of localization and delocalization of localized states have been observed in ZnO/ZnS core-shell structure. In addition, the PL properties of ZnO/ZnS core-shell structure and electroluminescence properties of ZnO/GaN heterojunction have been improved by utilizing localized states. Therefore, the study of the localized states in quaternary InGaAsSb/AlGaAsSb MQWs is of great benefits to improve the properties of quantum well materials and devices.
In this work, the InGaAsSb/AlGaAsSb MQWs was grown on semi-insulating (100) GaAs substrates followed by a 500 nm GaSb buffer layer. We present an analysis of optical properties for MQWs structure using temperature dependent and excitation power dependent PL measurement. The 10 K PL spectra with excitation power of 300 mW for the InGaAsSb/AlGaAsSb MQWs were fitted by Gaussian functions to confirm the origins of emission peak. Temperature dependent emission peaks and temperature dependent integrated intensity curves were employed to analyze the localized states characteristics in MQWs. The investigation of PL for MQWs structure is meaningful for the fabrication of high-performance lasers.
The quaternary InGaAsSb/AlGaAsSb MQWs were grown on semi-insulating (100) GaAs substrates by DCA P600 solid-source molecular beam epitaxy (MBE) system. A 500 nm GaSb buffer layer was first grown on the GaAs substrate. Then 3 periods InGaAsSb/AlGaAsSb MQWs with 20 nm In0.1Ga0.9As0.08Sb0.92 wells separated by 30nm Al0.3Ga0.7As0.13Sb0.87 barriers was grown. The growth temperature of In0.1Ga0.9As0.08Sb0.92 and Al0.3Ga0.7As0.13Sb0.87 were 530 ℃ and 680 ℃, respectively. The beam ratios of V/III for In0.1Ga0.9As0.08Sb0.92 and Al0.3Ga0.7As0.13Sb0.87 were 6.28 and 6.01, respectively. All the layers, during the growth of MQWs structure, were unintentionally doped. The schematic diagram of MQWs is shown in the inset of Fig. 1.
Temperature and excitation power dependent PL spectra were carried out for the optical properties investigation of InGaAsSb/AlGaAsSb MQWs. The sample was mounted on a closed-cycle helium cryostat with CaF2 windows, which provide the measurement temperature range from 10 K to 300 K for PL measurements. A 655 nm continuous wave semiconductor lasers, excitation power changed from 1 mW to 300 mW with a 0.4 cm2 light spot, was used as the excitation light source. The PL signal from the sample was dispersed by a HORIBA iHR550 monochromator, detected by an electric-cooled InGaAs detector and amplified by the standard lock-in amplifier technique. During the measurement of temperature-dependent PL spectra, the excitation power was fixed at 300 mW. The temperature was fixed at 10K when excitation power dependent PL spectra were measured.
3. Result and discussion
In order to confirm the quality of InGaAsSb/AlGaAsSb MQWs, double crystal X-ray diffraction (DCXRD) measurements were carried out by Bruker D8 system (as shown in Fig. 1). Five satellite peaks are observed in the XRD spectrum, which indicates InGaAsSb/AlGaAsSb MQWs have been grown on GaAs substrate successfully. In XRD experimental results, the spacing between -3 and -4, -4 and -5 satellite peaks both are 0.11°. Hence, the thickness of one QW period (the sum of well and barrier) L = tw+tb=λ/(2Δθ·cosθB) = 46.6 nm, which is consistent with the designed thickness. Here, Δθ is 0.11°, θB is 30.45°.
Figure 2 shows the temperature-dependent PL spectra of InGaAsSb/AlGaAsSb MQWs from 10 K to 300 K under the excitation power of 300 mW. At 10 K, the dominated PL peak located at 0.702 eV and a distinct shoulder peak located at 0.685 eV are found. As the temperature increases from 10 K to 300 K, the main peak shifts to long wavelength due to temperature-dependent energy shrinkage. The PL intensity reduced by about 25 times. However, the shoulder peak only exists at temperature from 10 K to 90 K, and disappeared when the temperature higher than 100 K. A distinct change of PL line-shape induced by the dependent of temperature is observed. The PL line-shape appears asymmetrical as a shoulder peak located at the low energy side when temperature below 90 K, but it appears Gaussian-like at T≥100 K. Through the above analysis, there must be more than two types of emission mechanisms for InGaAsSb/AlGaAsSb MQWs.
As low temperatures PL spectra can give us more information about the recombination channels in MQWs, detailed PL line-shape analysis by fitting curves based on Gaussian functions has been carried out. Figure 3 shows the fitting result, where open cycles are the measured PL spectrum of the InGaAsSb/AlGaAsSb MQWs at 10 K and solid lines are fitting curves labeled as P1, P2, P3 and P4, respectively. The four peaks of P1, P2, P3 and P4 located at 0.672 eV, 0.685 eV, 0.702 eV and 0.722 eV, respectively. Combined with the evolvement characteristic of Fig. 2, the peak of P1 may be associated with defect recombination , the peak P3 may originate from free exciton emission [12,21–22]. The lattice constant of GaAs and GaSb are 5.65325 Å and 6.09593 Å. While the lattice constant of In0.1Ga0.9As0.08Sb0.92 and Al0.3Ga0.7As0.13Sb0.87 in this research are 6.09904 Å and 6.04903 Å. It can be seen that the lattice constant values of GaAs and GaSb exist small lattice mismatch. While the InGaAsSb/AlGaAsSb MQWs structure was grown on GaSb buffer layer which had been grown on the GaAs substrate first. Therefore, stress was generated by lattice mismatch from GaAs and GaSb will transfer to MQWs, and then small compressive stress will exist in QW which closed to GaAs substrate side. Considering this situation and the peak energy difference between P3 and P4 is about 20 meV, the peak P4 may be determined as free exciton emission related to light hole in the valence band, which caused by separation of light and heavy hole .
Figure 4 plots the PL peak energy of P2 and P3 as a function of temperature for MQWs sample to further analyze the evolvement of the peak positions. The P2 and P3 peak positions change with temperature can be described by the band gap shrinkage model :
Where E0 is the band gap of InGaAsSb/AlGaAsSb MQWs at T=0 K; β is related to the Debye temperature; α is a constant. In Fig. 4(a), the solid lines are fitting curves based on Eq. (1), the solid scatter symbol plot is peak energy from temperature dependent PL spectrum fitted by Gaussian functions. The fitting curves of peak P2 and P3 show similar characteristic and the peak P3 can be well fitted by the traditional Varshni formula over a wide range of temperatures (10 K - 300 K), which suggest peak P3 is in agreement with the band edge related emission. The parameters of peak P3 fitted by Varshni’s equation are E0 = 0.703 eV, α = 5.229 ×10−4 eV K−1, and β = 344 K. Hence, the peak P3 could be conformed to the emitting characteristics of free exciton emission. When the excitation power for PL measurement is large enough, the localization effect will disappear, and the behavior of the peak energy with temperature will closely follow Varshni law. While the experimental data of peak P2 showed an unusual trend at higher temperature, it may be related to localized carriers in the InGaAsSb/AlGaAsSb MQWs.
Figure 4 (b) shows the temperature-dependent integrated intensity of peak P2 and P3 under excitation power of 300 mW, and the insert is the intensity ratio of P2 and P3. Both the P2 and P3 integrated intensity decrease with temperature increasing. The integral intensity ratio also decreases when temperature increases from 10 K to 90 K. This result indicates the P2 is localized states at low temperature, when temperature higher than 100 K the localized states will be delocalized, and the localized carriers will transfer to free exciton. Therefore, the origins of P2 and P3 are localized carriers and the free exciton, respectively.
In order to further analyze the origins of P2 and P3 emission at different temperatures from the InGaAsSb/AlGaAsSb MQW structure, excitation power- dependent PL measurements were performed as the incident laser power range from 1 mW to 300 mW, the corresponding power density was 2.5 mW/cm−2 to 750 mW/cm−2. Figure 5 shows the PL spectra of the InGaAsSb/AlGaAsSb MQW measured at a temperature of 10 K under different excitation power. Under lower excitation power, the localized states-related emission dominated the recombination process. However, with increasing of the excitation power, the localized states were saturated. And then, the free exciton radiative recombination dominated the recombination process gradually. So, the intensity ratio of P3 and P2 increases with the excitation power. Finally, the peak P3 became the dominate peak under excitation power of 300 mW with a slight blueshift about 1 meV. This shift is negligible and it may be related with band-filling effect in QWs.
The excitation power dependent PL spectra were fitted based on Gaussian functions for four peaks as showing in Fig. 3. The integrated intensity of the peaks IPL can be expressed as the following equation [10,25]:
Where Iex is the excitation power of laser radiation, η is the PL efficiency and that the exponent α describes the radiative recombination mechanism . It is known that the value of α is in the region of 1<α<2 for exciton-like transition, while for free electron-hole pairs recombination the value of α is 2, for free-to-bound and donor-acceptor transitions α is in the region of α<1 [10,25–27].
Figure 6 shows the integrated intensity as a function of excitation power along with the solid fitted curve. The symbol lines are integrated intensity data fitted by Gaussian functions for peak P2 and P3. According to the fitting results, the parameter of α for peak P2 is 0.69. This result indicates the P2 is not exciton-like transition but is related with localized carriers. Whereas, the parameter of α for peak P3 is 1.15, which means P3 is related to free excitons recombination.
The intensities of peak P2 exhibit exponential trend and saturated effect, and that further proved this peak comes from emission of localized carriers. This may be carriers localized by indium cluster located at the interface of the barrier in quantum well structure, which was coming from the segregation of indium in InGaAsSb layer during the growth of AlGaAsSb barrier layers. Depending on the previous analysis, the emission of peak P3 is confirmed to the emission from free excitons.
In summary, we have grown quaternary InGaAsSb/AlGaAsSb MQWs by MBE, and the optical properties have been studied by temperature-dependent and excitation power-dependent PL measurements. The InGaAsSb/AlGaAsSb MQW structure PL peak was dominated by free-exciton recombination under excitation power of 300 mW during temperature range of 10 K to 300 K. Strong and highly efficient free-exciton recombination up to room temperature with a photon energy of 0.631 eV were observed. At lower temperature range from 10 K to 90 K a shoulder located at low energy position beside the sharp PL peak, above 100 K the shoulder peak became narrower and disappeared gradually. The PL spectra at 10 K fitted by Gaussian functions shows four peaks named P1, P2, P3, P4 located at 0.672 eV, 0.685 eV, 0.702 eV and 0.722 eV, respectively. Through the analysis of temperature dependent PL spectra combined with excitation power dependent PL measurements, the origin of PL emission peak are identified. The emission of peak P2 is localized carriers related recombination. Peak P3 is the recombination of free exciton come from quantum wells. This research provides valuable information for definite the recombination mechanism in the system of InGaAsSb/AlGaAsSb MQWs which grown on GaAs substrate by MBE. It is of great significance to further optimize the quality of quantum well materials, which will helpful to obtain high performance laser devices.
Developing Project of Science and Technology of Jilin Province (20200301052RQ); Project of Education Department of Jilin Province (JJKH20200763KJ); Youth Foundation of Changchun University of Science and Technology (XQNJJ-2018-18); National Natural Science Foundation of China (11674038, 61674021, 61704011, 61904017).
The authors declare no conflicts of interest.
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