Open Access
Add to BibSonomy
Abstract
With the development of dry fiber over the past two decades, the E-band has become a new telecommunication wavelength. However, owing to material constraints, an effective high-performance semiconductor light source has not yet been realized. InAs quantum dot (QD) lasers on GaAs substrates are in the spotlight as O-band light sources because of their excellent thermal properties and high efficiency. The introduction of a very thick InGaAs metamorphic buffer layer is essential for realizing an E-band InAs QD laser, but it can cause degradation in laser performance. In this study, we fabricate an E-band InAs/GaAs QD laser on a GaAs substrate with an AlInGaAs multifunctional metamorphic buffer layer that realizes the function of the bottom cladding layer of normal thickness in addition to the functions of a metamorphic buffer layer and a dislocation filter layer. The lasing oscillation at a wavelength of 1428 nm is demonstrated at room temperature under continuous-wave operation. This result paves the way toward the realization of highly efficient light sources suitable for E-band telecommunications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Corrections
Jinkwan Kwoen, Takaya Imoto, and Yasuhiko Arakawa, "InAs/InGaAs quantum dot lasers on multi-functional metamorphic buffer layers: erratum," Opt. Express 30, 6617-6617 (2022)https://opg.optica.org/oe/abstract.cfm?uri=oe-30-5-6617
1. Introduction
Optical communication wavelength bands range from 1260 to 1675 nm, which is in the low-loss wavelength region of the optical fiber [1,2]. Among them, the O-band (original band: 1260-1360 nm) and C-band (conventional band: 1530-1565 nm), for which high-efficiency semiconductor lasers have been developed with a small loss under 1 dB/km in the fiber, are frequently used wavelength bands. With the introduction of optical fibers (ITU-T G.652.D) [2] that use the dehydration process and proposals for coarse wavelength division multiplexing (CWDM) communication [3], utilization of all of the optical communication wavelength bands is likely for technological development. In particular, the E-band, which had a large loss at approximately 1380 nm due to the presence of the water absorption peak in early optical fibers, has since been improved to reduce losses and is now attracting attention as a fifth window. However, the E-band has been difficult to commercialize because it has not been adopted by several CWDM systems. One of the reasons for this is the unavailability of a high-efficiency light source for the E-band.
Although several attempts such as InGaAsP multi quantum well (MQW) lasers on InP substrate, GaInNAs MQW lasers on GaAs [4–6], QD lasers which can potentially realize high temperature-stability as well as high temperature operation in E-band have not yet been developed. InAs quantum dot (QD) lasers on GaAs substrates have been in the spotlight as O-band light sources because of their excellent thermal properties and high efficiency [7–9]. In particular, the insensitivity of the emission intensity to crystal defects has been reported even in high-density threading dislocation environments, such as GaAs on a Si substrate [10–12]. To obtain an E-band InAs QD laser, it is necessary to introduce thick metamorphic buffer layers, which cause device performance degradation [13–17]. In the metamorphic layer, crystal defects occur because of lattice mismatch lattice mismatch; these reduce device performance and lifetime. Introducing a metamorphic buffer layer also increases the time and material costs of production because a metamorphic layer is required in addition to a general lattice-matched layer.
A multifunctional metamorphic buffer (MFMB) layer has been recently proposed as a layer that functions as the bottom cladding layer of the quantum well (QW) lasers while modulating the lattice constant [18,19]. In these reports, the compositionally graded InAsP MFMB layer grown on an InP substrate increased the lattice constant from 5.87 Å to 5.95 Å, while maintaining a low refractive index of ∼3.2 on average. The thickness of the MFMB was about 1.5 µm which is about half the total thickness of the conventional metamorphic buffer layer and the bottom cladding layer that play the same role [20].
In this study, we fabricated an E-band QD laser on a GaAs substrate, incorporating an AlInGaAs MFMB layer that serves as the bottom cladding layer of normal thickness, a metamorphic buffer layer, and a dislocation filter layer. Lasing oscillation under continuous wave (CW) operation at a wavelength of 1428 nm was demonstrated at RT.
2. Experiments
2.1 Crystal growth
N-type doped GaAs (001) substrates were used in this study. The wafers were placed into a standard solid-source molecular beam epitaxy (MBE) chamber. After the normal outgassing process in the preparation chamber at 450 °C, the wafers were transferred into the deposition chamber and then heated to 620 °C to remove the native oxide layer. Subsequently, three groups of multifunctional metamorphic buffer layers, including strained layer superlattice (SLS) filter layers, were grown. The indium contents (x) of the MFMB layers were 0.15, 0.20, and 0.23, respectively. Each SLS filter layer with a total thickness of 100 nm was sandwiched between the top and bottom AlInGaAs with a thickness of 100-nm. The SLS layers consisted of five pairs of 10 nm InxAl0.35Ga(0.65-x)As/10 nm In(x+0.1)Al0.35Ga(0.55-x)As [12,17]. A thin AlAs layer was grown on each group of MFMB as a protective layer to avoid indium desorption during the high-temperature (700 °C) thermal annealing [15,21]. Figure 1 shows a schematic cross-section of epitaxially grown MFMB layers on a GaAs substrate. Above the MFMB layers, a 600-nm-thick In0.23Al0.35Ga0.42As n-type doped clad layer was grown. Then, eight layers consisting of InAs/InGaAs QD and 1500 nm p-In0.23Al0.35Ga0.42As cladding layers were grown. Finally, a 400 nm p-type doped In0.23Ga0.77As contacting layer was grown (see Fig. 2).
Fig. 1. Schematic diagram of the multifunctional metamorphic buffer layer structure grown on the GaAs substrate.
Fig. 2. Schematic diagram of the InAs/InGaAs QD laser structure grown on the multifunctional metamorphic buffer layer on the GaAs substrate.
2.2 Device fabrication
Figure 3 shows a schematic illustration of the structure of the fabricated InAs/InGaAs QD laser device. To reduce the scattering loss because of the higher RMS roughness in the $[{110} ]$ direction, the laser mesa were formed parallel to $[{1\bar{1}0} ]$ direction [22]. Laser mesa with widths of 6 µm and 8 µm were fabricated using photolithography and wet etching. A 200-nm-thick SiO2 layer was deposited by sputtering. After etching the SiO2 with HF, AuGeNi/Au was deposited. Then, p- and n-electrodes were separated by a lift-off process. After annealing both electrodes under an argon (Ar) atmosphere at 450 °C, the structures were cleaved to produce 1.5-mm and 2.0-mm-long laser bars. Note that no high-reflection coatings were applied to the cleaved facets. Figure 4 shows a bird’s eye view of the fabricated laser structure.
Fig. 3. (a) Schematic illustration and (b) a scanning electron microscope image of the structure of InAs/InGaAs QD laser device on the MFMB layer.
Fig. 4. (a) Lattice constant profile and (b) refractive index profile in the InAs/InGaAs QD laser diode on MFMB layer.
2.3 Measurements
Cross-sectional images of the laser structure were obtained using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The QD and surface morphologies were obtained using atomic force microscopy (AFM). The emission wavelength and intensity were observed by photoluminescence (PL). Laser device characteristics were measured under pulsed current injection (pulse width: 1 µs, pulse repetition time: 1 ms) and DC injection. The heat sink was equipped with a temperature control unit consisting of a Peltier element and a thermocouple feedback circuit.
3. Results and discussion
3.1 Layer design
Figure 4 shows the lattice constant profile and refractive index profile based on the epitaxial structure shown in Fig. 2. It can be seen that the lattice constant of MFMB in Fig. 4(a) increased from 0.56 nm for GaAs to 0.575 nm for In0.23Al0.35Ga0.42As. In0.23Ga0.77As. A lattice match of In0.23Al0.35Ga0.42As was adopted as a barrier material for InAs QDs, enabling longer emission wavelength. At the same time, as shown in Fig. 4(b), the refractive index decreased from 3.4 for GaAs to 2.92 for In0.23Al0.35Ga0.42As. In particular, the In0.15Al0.35Ga0.50As layer and the In0.20Al0.35Ga0.45As layer had smaller refractive indexes than the In0.23Al0.35Ga0.42As layer, and thus, they could be used as cladding layers.
Figure 5 shows the optical TE mode intensity profile and refractive index. The optical confinement factor (i.e., the geometric overlap between the TE-mode and the QD structures. The cuboidal QD model was used for calculation.) of 1.7% was calculated. This value is almost same as the optical confinement factor (1.7%) in the QD laser using a constant Al0.23In0.35Ga0.42As bottom cladding layer without MFMB.
Fig. 5. Optical TE mode intensity profile (blue) and refractive index (gray) for the InAs/InGaAs QD laser diode on the MFMB layer. Blue-filled area indicates the QD active layer.
3.2 Crystal growth
Figure 6 shows a bright-field HRTEM image of the FIB-processed InAs/InGaAs QD laser structure grown on the MFMB layers. From this picture, the layer structure can be grasped, including the generation and disappearance of crystal defects in the sample. Figure 6(a) shows a region near the GaAs buffer layer and the first metamorphic layer, the In0.15Ga0.85As layer. The lattice constant of In0.15Ga0.85As is approximately 1.1% larger than that of GaAs; thus, misfit dislocations and the threading dislocations caused by them can be found at the interfaces of metamorphic layers having thicknesses greater than the critical thickness in the People and Bean model [23]. It can be seen in Fig. 6(b) that the resulting threading dislocation proceeds to the SLS filter layer composed of In0.15Ga0.85As and In0.15Ga0.85As and then disappears at the interface. Owing to the InGaAs SLS filter layer, defect-free high-quality 8-layer stacked InAs/InGaAs QD layers were grown on the MFMB layer and the bottom clad layer. The InAs/InGaAs QD layers grown here were adjusted to a constant height using partial capping and the indium flush method [24] (see Fig. 6(c)). Figure 7 shows an AFM image of a sample grown to InAs QD in the same way as the above laser structure. This shows that InAs QDs formed on the cross-hatch pattern formed along the $[{1\bar{1}0} ]$ direction. The average lateral size of the uncapped QDs was approximately 30 nm, and the densities of the QDs were approximately 2×1010 cm−2. Figure 8 shows the PL spectra of a single QD layer of structures grown on the MFMB layer at RT. At RT, we observed a PL peak at 1440 nm with a full width at half maximum (FWHM) of 44 meV. Note that absorption of water-vapor occurs around 1.4 µm.
Fig. 6. Bright-field cross-sectional HRTEM images of InAs/InGaAs QD laser structure grown on MFMB layer. (a) Near the interface of GaAs buffer layer and InGaAs metamorphic layer; (b) the lowest MFMB layer; (c) 8-layer stacked InAs/InGaAs QD
Fig. 8. RT PL spectra of InAs/InGaAs QD grown on MFMB layer. Blue region indicates telecommunication E-band.
3.3 Device properties
Figure 9 shows the temperature dependence of the light-current (L-I) characteristics under pulse operation mode. The threshold current at 20 °C was 72 mA, and the corresponding threshold current density was only 380 A/cm2. Additionally, lasing was observed up to 80 °C in the fabricated device. The slope efficiencies were 92.3 mW/A at 20 °C, 70.4 mW/A at 50 °C, and 23.2 mW/A at 80 °C. The characteristic temperature (T0) was nearly constant at approximately 44 K between the operating temperatures of 20 °C and 70 °C.
Fig. 9. Temperature dependence of L-I characteristics in pulse mode for the InAs/InGaAs QD laser on MFMB layer. (Curves correspond to 10-°C increments in heat sink temperature from 10 °C to 80 °C.)
Figure 10 shows L-I-V characteristics under CW operation. The threshold current and the threshold current density at 25 °C were 69.2 mA and 360 A/cm2, respectively. The resistance of the device was 2.4 Ω. The threshold current density of the laser device obtained in this study was 360 A/cm2. This is much smaller than the threshold current density of ∼1 kA/cm2 reported from InGaAsP MQW lasers and GaInNAs MQW lasers. [4,5]. Figure 11 shows the laser spectrum of the InAs/InGaAs QD laser on the MFMB layer at RT (25 °C). Multi-mode lasing was observed in the sample with the ground state lasing wavelength being 1428 nm, which is within the optical telecommunication E-band.
Fig. 10. L-I-V characteristics under CW operation at 25 °C for the InAs/InGaAs QD laser on MFMB layer.
4. Conclusion
In summary, we have successfully grown AlInGaAs MFMB layers on GaAs substrates. QD lasers fabricated by incorporating the MFMB layer enable the MFMB layers to simultaneously function as metamorphic buffer layers, dislocation filtering layers, and even the bottom cladding layers. The QD lasers not only showed pulsed operation at the E-band wavelength up to 80°C but also succeeded in demonstrating RT CW operation. These results suggest that the emission wavelength of the QD lasers can be controlled over a wide range of values, even on heterogeneous substrates using an MFMB layer. Therefore, QD lasers on MFMB layers are promising light sources not only for conventional CWDM systems but also for Si photonics.
Funding
New Energy and Industrial Technology Development Organization (JPNP13004).
Acknowledgments
The authors thank Prof. Satoshi Iwamoto, Dr. Natália Morais, Dr. Katsuyuki Watanabe, and Mr. Pholsen Natthajuks for the constructive discussions.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. G. P. Agrawal, Fiber-optic communication systems (Wiley, 2010).
2. ITU-T Recommendation G.652, “Characteristics of a Single-Mode Optical Fibre and Cable” (International Telecommunication Union, 2003).
3. H.-J. Thiele and M. Nebeling, Coarse wavelength division multiplexing: technologies and applications (CRC Press, 2007).
4. W. Ha, V. Gambin, M. Wistey, S. Bank, S. Kim, and J. S. Harris, “Multiple-quantum-well GaInNAs-GaNAs ridge-waveguide laser diodes operating out to 1.4 µm,” IEEE Photonics Technol. Lett. 14(5), 591–593 (2002). [CrossRef]
5. X. Zhu, D. T. Cassidy, M. J. Hamp, D. A. Thompson, B. J. Robinson, Q. C. Zhao, and M. Davies, “1.4-µ m InGaAsP–InP Strained Multiple-Quantum-Well Laser for Broad-Wavelength Tunability,” IEEE Photonics Technol. Lett. 9(9), 1202–1204 (1997). [CrossRef]
6. C.-L. Tsai, Y.-L. Chou, Y. S. Wang, S. J. Chang, M.-C. Wu, and W. Lin, “1.3 µm Strain-Compensated InGaAsP Planar Buried Heterostructure Laser Diodes with a TO-Can Package for Optical Fiber Communications,” J. Electrochem. Soc. 156(12), H903 (2009). [CrossRef]
7. Y. Arakawa and H. Sakaki, “Multidimensional quantum well laser and temperature dependence of its threshold current,” Appl. Phys. Lett. 40(11), 939–941 (1982). [CrossRef]
8. T. Kageyama, K. Nishi, M. Yamaguchi, R. Mochida, Y. Maeda, K. Takemasa, Y. Tanaka, T. Yamamoto, M. Sugawara, and Y. Arakawa, “Extremely high temperature (220°C) continuous-wave operation of 1300-nm-range quantum-dot lasers,” in (IEEE, 2011), p. 1.
9. K. Nishi, T. Kageyama, M. Yamaguchi, Y. Maeda, K. Takemasa, T. Yamamoto, M. Sugawara, and Y. Arakawa, “Molecular beam epitaxial growths of high-optical-gain InAs quantum dots on GaAs for long-wavelength emission,” J. Cryst. Growth 378, 459–462 (2013). [CrossRef]
10. A. Y. Liu, S. Srinivasan, J. Norman, A. C. Gossard, and J. E. Bowers, “Quantum dot lasers for silicon photonics [Invited],” Photonics Res. 3(5), B1 (2015). [CrossRef]
11. S. Chen, W. Li, J. Wu, Q. Jiang, M. Tang, S. Shutts, S. N. Elliott, A. Sobiesierski, A. J. Seeds, I. Ross, P. M. Smowton, and H. Liu, “Electrically pumped continuous-wave III–V quantum dot lasers on silicon,” Nat. Photonics 10(5), 307–311 (2016). [CrossRef]
12. J. Kwoen, B. Jang, J. Lee, T. Kageyama, K. Watanabe, and Y. Arakawa, “All MBE grown InAs/GaAs quantum dot lasers on on-axis Si (001),” Opt. Express 26(9), 11568 (2018). [CrossRef]
13. Y.-C. Xin, L. G. Vaughn, L. R. Dawson, A. Stintz, Y. Lin, L. F. Lester, and D. L. Huffaker, “InAs quantum-dot GaAs-based lasers grown on AlGaAsSb metamorphic buffers,” J. Appl. Phys. 94(3), 2133–2135 (2003). [CrossRef]
14. A. Zhukov, A. Vasilyev, E. Semenova, N. Kryzhanovskaya, A. Gladyshev, M. Maximov, V. Ustinov, and N. Ledentsov, “Epitaxial growth of quantum-dot heterostructures on metamorphic buffers,” Proc. SPIE 5946, 594616–594616-10 (2005). [CrossRef]
15. Z. Mi, P. Bhattacharya, and J. Yang, “Growth and characteristics of ultralow threshold 1.45µm metamorphic InAs tunnel injection quantum dot lasers on GaAs,” Appl. Phys. Lett. 89(15), 153109 (2006). [CrossRef]
16. Z. Mi, C. Wu, J. Yang, and P. Bhattacharya, “Molecular beam epitaxial growth and characteristics of 1.52 µm metamorphic InAs quantum dot lasers on GaAs,” J. Vac. Sci. Technol., A 26(1), 5–7 (2008). [CrossRef]
17. J. Kwoen, W. Zhan, and Y. Arakawa, “E-band InAs Quantum Dot Laser on InGaAs metamorphic buffer layer with filter layer,” Electron. Lett. 57(14), 567–568 (2021). [CrossRef]
18. D. Jung, L. Yu, S. Dev, D. Wasserman, and M. L. Lee, “Room-temperature mid-infrared quantum well lasers on multi-functional metamorphic buffers,” Appl. Phys. Lett. 109(21), 211101 (2016). [CrossRef]
19. D. Jung, L. Yu, S. Dev, D. Wasserman, and M. L. Lee, “Design and growth of multi-functional InAsP metamorphic buffers for mid-infrared quantum well lasers on InP,” J. Appl. Phys. 125(8), 082537 (2019). [CrossRef]
20. Y. Gu, Y. G. Zhang, Y. J. Ma, L. Zhou, X. Y. Chen, S. P. Xi, and B. Du, “InP-based type-I quantum well lasers up to 2.9 µm at 230 K in pulsed mode on a metamorphic buffer,” Appl. Phys. Lett. 106(12), 121102 (2015). [CrossRef]
21. H. Ji-Fang, W. Hai-Li, S. Xiang-Jun, L. Mi-Feng, Z. Yan, W. Li-Juan, Y. Ying, N. Hai-Qiao, X. Ying-Qiang, and N. Zhi-Chuan, “GaAs-based long-wavelength InAs quantum dots on multi-step-graded InGaAs metamorphic buffer grown by molecular beam epitaxy,” J. Phys. D: Appl. Phys. 44(33), 335102 (2011). [CrossRef]
22. I. Tångring, S. Wang, M. Sadeghi, and A. Larsson, “1.27 µm metamorphic InGaAs quantum well lasers on GaAs substrates,” Electron. Lett. 42(12), 691 (2006). [CrossRef]
23. R. People and J. C. Bean, “Calculation of critical layer thickness versus lattice mismatch for GexSi1-x/Si strained-layer heterostructures,” Appl. Phys. Lett. 47(3), 322–324 (1985). [CrossRef]
24. Z. R. Wasilewski, S. Fafard, and J. P. McCaffrey, “Size and shape engineering of vertically stacked self-assembled quantum dots,” J. Cryst. Growth 201-202, 1131–1135 (1999). [CrossRef]
