The device characteristics of semiconductor quantum dot lasers have been improved with progress in active layer structures. Self-assembly formed InAs quantum dots grown on GaAs had been intensively promoted in order to achieve quantum dot lasers with superior device performances. In the process of growing high-density InAs/GaAs quantum dots, bimodal size occurs due to large mismatch and other factors. The bimodal size in the InAs/GaAs quantum dot system is eliminated by the method of high-temperature annealing and optimized the in situ annealing temperature. The annealing temperature is taken as the key optimization parameters, and the optimal annealing temperature of 680 °C was obtained. In this process, quantum dot growth temperature, InAs deposition, and arsenic (As) pressure are optimized to improve quantum dot quality and emission wavelength. A 1.3-μm high-performance F-P quantum dot laser with a threshold current density of 110 A/cm was demonstrated. Keywords: Quantum dot (QD), Annealing, Bimodal size, Molecular beam epitaxy (MBE), Laser Introduction in InAs/GaAs quantum dot system still exists [6, 7]. The Ten years ago, the 1.3-μmquantum dot(QD)laser was de- quantum dot quality can be increased if the bimodal size veloped; however, there has been no distinct development can be eliminated. or progress on quantum dot growth since then up till now. InAs/GaAs heterostructures grown by molecular beam The 1.3-μm quantum dot laser has once again become a epitaxy (MBE) have been paid much attention in order hot topic of study. It has become one of the strong compet- to fabricate low dimensional nanostructures, such as itors for the high-speed optical communication local area self-assembled QDs due to large lattice (~ 7%) mismatch network (LAN) light source. The high density of quantum between InAs layers and GaAs substrate . The growth dots is an important factor in resulting in low power con- of InAs on GaAs (001) substrate results in the formation sumption, high-temperature stability, and high speed. As is of a three-dimensional (3D) island shape on the InAs well known, the 1.3-μmInAs/GaAs quantumdot laseris with the Stranski-Krastanov (SK) growth mode. The SK expected to exhibit excellent performance at the threshold growth technique is expected to be a convenient fabrication current, temperature stability, and modulation characteris- method of the high-density coherent QDs and is still an tics due to the three-dimensional quantum confinements open challenge [9, 10]. However, SK QDs have some prob- . In the last 10 years, a great many laboratories have lems, such as the large inhomogeneous broadening of the achieved their aim all over the world, of greatly improving QD energy levels and the bimodal size problem [11–15]. the performance of QD lasers [2–5]. However, bimodal size For MBE growing high-density quantum dots, the conven- tional way is to increase the deposition rate of InAs and lower the growth temperature. The purpose of this * Correspondence: firstname.lastname@example.org approach is to reduce the migration rate that can make the State Key Laboratory for Superlattices and Microstructures, Institute of formation of the island quickly. However, low-temperature Semiconductors, Chinese Academy of Sciences, Beijing 100083, China College of Materials Science and Opto-Electronic Technology, University of growth may reduce the lattice quality of the epitaxial mater- Chinese Academy of Sciences, Beijing 100083, China ial. On the other hand, rapid growth can increase the Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Su et al. Nanoscale Research Letters (2018) 13:59 Page 2 of 6 quantum dot density, but it also creates more dislocations. that the wavelength was less than 1300 nm; therefore, Accordingly, photoluminescence intensity of InAs QDs be- we fine-tuned the growth conditions. A 2.5 monolayer came weak when we attained a high density of InAs QDs (ML) thick InAs was grown at 520 °C and capped by a using the conventional approach. 5-nm thick In Ga As strain-reducing layer at the 0.15 0.85 In this letter, single-layer high-temperature annealing same temperature. This layer was followed by a 15-nm can effectively eliminate the defects of the cap material GaAs layer which deposited at a lower temperature (LT) and change the growth direction of dislocations. The of 520 °C. Then, we grew the final 20-nm GaAs layer at size and shape of InAs SK quantum dots show a high a higher temperature (HT) of 630 °C (as shown in degree of uniformity by single-layer annealing that Fig. 2a). grown on GaAs (001) substrates. There was an increase The PL spectrum and the atomic force microscopy in the deposition of InAs which improved each QD’s sat- (AFM) images of the surface of the QDs were measured uration at the same time. The PL spectra of the uniform for the test sample. The emission peak of 1308 nm is InAs QDs revealed a narrow linewidth of less than due to the ground-state transition, and the full width of 26 meV. A 1.3-μm InAs/GaAs QD lasers are fabricated half maximum (FWHM) of the peak is about 31 nm (as which exhibit a lasing threshold current I of 220 mA shown in Fig. 2b). We grew a layer of bare quantum dots th and a threshold current density of 110 A/cm . on the buried layer of five layers in the test sample to carry out the AFM measurement. The growth conditions Material Optimization are exactly the same as the buried quantum dots de- In this study, the quantum dot structure is grown on scribed before. The AFM image of the surface of the GaAs (001) (N+) substrates in a Veeco Gen 930 MBE QDs shows that the QD density of the annealed sample 10 − 2 system. Annealing temperature has been investigated, is about 3.2 × 10 cm (as shown in Fig. 3a). The and the annealing temperatures for these four samples quantum dot has an average height of 8 nm. On the (N170813, N170824A-N17084C) are 630, 680, 730, and contrary, the unannealed quantum dot sample’s size and 780 °C, respectively. The growth parameters of quantum distribution are not uniform. Bimodal size can be seen 10 − 2 dots of these four samples have exactly the same and QD density is about 2.9 × 10 cm . The quantum (Table 1). dot has a height of 5–7 nm (as shown in Fig. 3b). Photoluminescence (PL) measurements were con- During the epitaxial growth of a 1.3-μm quantum dot ducted for the four samples. With the increase of an- laser, the bimodal-size of InAs quantum dots can be well nealing temperature, the strongest PL intensity was eliminated through the single-layer annealing for the achieved at the annealing temperature of 680 °C (as laser active area. Compared with the sample grown with- shown in Fig. 1). This is because that arsenic (As) and out annealing (as shown in Fig. 3c), the sample grown Ga are desorbed as the annealing temperature rises with an annealing temperature at 680 C (as shown in higher. That process can create more defects, and the Fig. 3d) has a higher quantum dot density and a uniform lattice of InAs quantum dots has changed at high quantum dot size. That can be attributed to the follow- temperature. ing reasons. At first, GaAs cap layer grows immediately Quantum dot laser active area has been optimized at after the growth of InAs quantum dots, so it can only − 7 the low arsenic pressure of 4 × 10 Torr  and low grow at a low temperature, which reduces the lattice quality growth rate of 0.025 ML/s. After annealing, we found of GaAs and introduces defects. High-temperature annealing Table 1 Comparison of several types of growth parameters and annealing temperatures Growth parameters N170813 N170824A N170824B N170824C Growth T ( C) 520 520 520 520 Deposition rate (ML/s) 0.025 0.025 0.025 0.025 Deposition amount (ML) 2.3 2.3 2.3 2.3 III-V ratios (times) 25 25 25 25 Capping In GaAs thickness (nm) 5 5 5 5 0.15 In GaAs rate (ML/s) 0.67 0.67 0.67 0.67 0.15 Annealing T ( C) 630 680 730 780 Annealing position (nm) 20 20 20 20 Annealing duration (min) 5 5 5 5 PL intensity (a.u.) 23,009.2 26,309.6 18,985.9 9997.8 Su et al. Nanoscale Research Letters (2018) 13:59 Page 3 of 6 (1:1:4) after the laser epitaxial structure was completed [17, 18]. It can be seen that the PL spectrum of this sam- ple has a central wavelength of 1294 nm (as shown in Fig. 4b). The blue shift of the center wavelength com- pared to the abovementioned test sample (as shown in Fig. 2a) is due to the high-temperature growth (650 °C) during the growth step of the upper cladding with a growth time longer than 2 h. It also may be from the in- dium (In) component of the In GaAs cap layer’srock 0.15 drifts. The InAs/GaAs QD laser wafer was coated with photoresist to define the surface pattern. The first edi- tion of photolithography forms a ridge pattern of 100 μm. The ridge waveguide was fabricated by induct- Fig. 1 Comparison of photoluminescence (PL) spectra of epitaxial ively coupled plasma (ICP) etching with an etching wafers under different annealing temperature depth of 2 μm, followed by Plasma Enhanced Chemical Vapor Deposition (PECVD) in order to form SiO can eliminate defects and can grow high-quality GaAs cap insulation. In the next step, we made a contact window layer used to continue growing InAs quantum dots. In of 90 μm in width on the ridge for current injection. addition, the dislocations are generated during InAs/GaAs Then Ti/Pt/Au 51 nm/94.7 nm/1122 nm was deposited heteroepitaxy, in situ single-layer annealing can eliminate as a p-type electrode with magnetron sputtering (as dislocation or change the dislocation growth direction and shown in Fig. 5). The wafer is thinned to 120 μm, and a then improve the quality of InAs quantum dots. 50-nm thick AuGeNi (80:10:10 wt% alloy) with a 300- nm thick Au layer was deposited on the back of the Device Design and Preparation wafer, using thermal evaporation for n-type electrode The laser’s structure consisted of a GaAs layer embed- [19, 20]. The entire sample was annealed at 460 °C for ded with five layers of self-assembled InAs QD core 10 s in order to form an ohmic contact. During the layers. The 200-nm n-waveguide layer and p-waveguide whole fabrication process, the sample was cleaned se- layer were grown on top and bottom of the QD struc- quentially with acetone and isopropyl alcohol and rinsed ture. The QD active region and waveguide layer were with deionized water. sandwiched by two 1.8-μm p-type (Be: 4E18) and n-type The electrical and optical properties of the device (Si: 2E18) Al Ga As layers. A 200-nm p+ GaAs (Be: were measured when the laser was finished. Power- 0.45 0.55 3E19) layer was deposited for electrical contact (as current-voltage (P−I−V) characteristics of broad area shown in Fig. 4a). lasers were tested in the continuous wave (CW) at A small part of the wafer is etched by chemical etching RT. The threshold current density of the laser is 110 to thin the upper cladding layer with H PO -H O -H O A/cm (as shown in Fig. 6a), and the central 3 4 2 2 2 Fig. 2 The active region structure and PL spectrum. a The structure of the undoped QD laser active region. b PL spectrum of the QD laser active region at room temperature (RT). The emission peak is 1305 nm and the FWHM is about 31 nm Su et al. Nanoscale Research Letters (2018) 13:59 Page 4 of 6 Fig. 3 AFM images of the InAs/GaAs QDs. a Single layer high-temperature annealing. b No annealing. c 3D small area size distribution image with high-temperature annealing. d 3D small area size distribution image without annealing wavelength of the lasing spectrum is 1.3 μm(as system. Deeper level research will be further studied showninFig. 6b). It can be seen from the lasing based on this to further improve the density of QDs, spectrum that the central wavelength of the laser at in order to achieve a lower threshold current, lower room temperature is redshifted because of heating ef- power consumption, higher output power, and high char- fect of the laser operation. In this study, the laser can acteristic temperature. continuously lase at room temperature and reach a good threshold current density as well as a good out- Conclusions put power without facet coating and undoping in the A series of optimizations of the growth parameters of active region, which indicates the high crystal quality high-density quantum dots were investigated. The of the laser. The single-layer annealing method has a single-layer annealing method was used to successfully certain effect on the bimodal size quantum dot suppress the formation of the bimodal-size system of Fig. 4 Device structure. a 1.3-μm quantum dot F-P broad area laser's epitaxial structure. b PL spectrum of the QDs laser's epitaxial structure at RT. The central wavelength is 1294 nm Su et al. Nanoscale Research Letters (2018) 13:59 Page 5 of 6 quantum dots. We studied the annealing temperature and annealing layer position in detail. An optimized an- nealing temperature of 680 °C and a distance from the quantum dot layer of 20 nm were obtained. A threshold current density of 110 A/cm has been achieved for a 1.3-μm InAs/GaAs QD F-P laser at room temperature and continuous-wave operation with a lasing wavelength of 1.3 μm. Abbreviations AFM: Atomic Force Microscope; Annealing T: Annealing temperature; CW: Continuous wave; F-P: Fabry–Perot; FWHM: Full width at half maximum; Growth T: Growth temperature; HT: High temperature; LT: Low temperature; MBE: Molecular beam epitaxy; PL: Photoluminescence; QD: Quantum dot; RT: Room temperature; SEM: Scanning electron microscope; WPE: Wall plug efficiency Acknowledgements We appreciated Professor of Xiao-Qiang Feng and Dr. Xiang-Jun Shang for polishing the manuscript. Fig. 5 SEM image of the laser's cross section. The F-P broad area laser with Funding a standard laser fabrication process. GaAs/AlGaAs etch depth is about This work is supported by the National Natural Science Foundation of China 2-μm. The PECVD formed SiO is 260 nm (61435012, 61505196), the National 973 program (2014CB643903, 2013CB933304), the Open Fund of High Power Laser Lab, China Academy of Engineering Physics (Grant No. 2013HEL03), and Shanxi Province International Science and Technology Cooperation and Exchange Project (2016KW-040). Availability of Data and Materials We declared that materials described in the manuscript, including all relevant raw data, will be freely available to any scientist wishing to use them for non-commercial purposes, without breaching participant confidentiality. Authors’ Contributions X-BS grew the samples, carried out the alignment, took part in discussions and in the interpretation of the result, and wrote the manuscript. YD and BM participated in the design of the study and discussions of the results. K-LZ and Z-SC helped in the technical support for the characterizations and the reconstruction of the data. J-LL and X-RC co-supervised the writing of the manuscript. Y-QX, YD, H-QN, and Z-CN supervised the writing of the manuscript and the experimental part. YD edited the manuscript. All the authors have read and approved the final manuscript. Authors’ Information X-BS is a master student of Institute of Photonics and Photonic Technology, Northwest University. Ying Ding is a professor of Institute of Photonics and Photonic Technology, Northwest University. Ben Ma is a PhD student of the Institute of Semiconductors, Chinese Academy of Sciences. K-LZ and Z-SC are students of School of Physics and Nuclear Energy Engineering, Beihang Univer- sity. J-LL is a PhD student of Dept. of Missile Engineering, Shijiazhuang Campus, Army Engineering University. X-RC is a master student of Wide Bandgap Semi- conductor Technology Disciplines State Key Laboratory, Xidian University. Y-QX, H-QN, and Z-CN are professors of the Institute of Semiconductors, Chinese Academy of Sciences. X-BS, K-LZ, Z-SC, J-LL, and X-RC also work at the Institute of Semiconductors, Chinese Academy of Sciences. Competing Interests The authors declare that they have no competing interests. Author details National Key Laboratory of Photoelectric Technology and Functional Materials (Culture Base), Institute of Photonics and Photonic Technology, Northwest University, Xi’an 710069, China. State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China. College of Materials Science Fig. 6 Device measurements. a P-I-V curves of a QD laser. b The lasing and Opto-Electronic Technology, University of Chinese Academy of Sciences, wavelength is 1.3 μm Beijing 100083, China. Department of Missile Engineering, Shijiazhuang Su et al. Nanoscale Research Letters (2018) 13:59 Page 6 of 6 Campus, Army Engineering University, Shijiazhuang 050003, China. 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Nanoscale Research Letters – Springer Journals
Published: Feb 21, 2018
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