GaAs Quantum

Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX

Development of Modulation p‑Doped 1310 nm InAs/GaAs Quantum Dot Laser Materials and Ultrashort Cavity Fabry−Perot and Distributed-Feedback Laser Diodes Qizhu Li,†,§ Xu Wang,† Ziyang Zhang,*,† Hongmei Chen,† Yuanqing Huang,† Chuncai Hou,† Jie Wang,† Ruiying Zhang,† Jiqiang Ning,*,‡ Jiahua Min,§ and Changcheng Zheng∥ †

Key Lab of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China ‡ Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China § School of Materials Science and Engineering, Shanghai University, Shanghai 200444, People’s Republic of China ∥ Mathematics and Physics Centre, Department of Mathematical Sciences, Xi’an Jiaotong-Liverpool University, Suzhou 215123, People’s Republic of China ABSTRACT: Multiple-layer InAs/GaAs quantum dot (QD) laser structures were etched to remove the p-side AlGaAs cladding layers to investigate the temperature-dependent photoluminescence (PL) characteristics. Four QD samples, including undoped as grown QDs, p-doped as grown QDs, undoped annealed QDs, and p-doped annealed QDs, were prepared by molecular beam epitaxy (MBE) and a postgrowth annealing process for comparison. Among them, modulation p-doped QD samples exhibit much less temperaturedependent characteristics of PL spectra and notable insensitivity to intermixing compared to undoped ones. This is attributed to the effects of modulation p-doping, which can inhibit holes’ thermal broadening in their closely spaced energy levels and significantly suppress In/Ga interdiffusion between QDs and their surrounding matrix. These results provide greater freedom in the choice of MBE growth for high-quality active regions and claddings of QD laser diodes. The superior features of the modulation p-doped QD materials have been transferred naturally to the laser devices. The continuous-wave ground-state (GS) lasing has been realized in both p-doped QD Fabry−Perot (F−P) and laterally coupled distributed-feedback (LC-DFB) narrow ridge lasers with very short cavity length without facet coatings, in which a 1315 nm GS lasing has been found in a F−P laser with a 400 μm cavity length, while single longitudinal mode lasing with a very large tunable range of 140 nm and side mode suppression ratio of 51 dB is achieved in an LC-DFB laser. This work demonstrates great development potential of InAs/GaAs QD lasers for applications in high-speed fiber-optic communication. KEYWORDS: quantum dots, modulation p-doping, rapid thermal annealing, ultrashort cavity, F−P laser, DFB laser Self-assembled quantum dots (QDs) of 1.3 μm InAs/GaAs have attracted great interest for the realization of high-performance laser diodes for applications in fiber-optic communication systems.1,2 A number of advantages of the GaAs-based III−V QD laser diodes have been found over the commercial laser diodes of InP-based III−V QW structures, such as low threshold current, high quantum efficiency, and high temperature stability.3−12 In addition, the high-speed performance in III−V QD lasers is another outstanding merit, which has been the main aim that both laboratories and industries are pursuing.13−17 Under this circumstance, in order to meet the increased application requirements of QD lasers, it is necessary to fully study and understand the optical and optoelectronic properties of both QD laser materials and devices. In recent years, remarkable progress has been made to analyze and optimize the optical properties of InAs/GaAs QD structures by using p-doping and rapid thermal annealing (RTA) methods.18−23 However, most of these studies were based on © XXXX American Chemical Society

solo QD structures, whose properties deviate greatly from those in the final practical laser devices,21,24−30 rather than QD laser structures. The practical InAs/GaAs QD laser structure is based on a typical p−i−n configuration, in which a thick p-AlGaAs top cladding layer is grown on the InAs QD active layers. The growth of a high-quality epitaxial p-AlGaAs layer usually requires a high temperature, which is much higher than that for InAs QD growth, and the high-temperature growth of the cladding layer is equivalent to a relatively strong RTA treatment to the QD active region, which will significantly influence the structural quality and optical properties of the QDs. For instance, a dramatic decrease of photoluminescence (PL) intensity is usually induced after the process of thermal intermixing.23 This is because the interdiffusion mechanism of In and Ga atoms modifies the composition profile from an abrupt interface to a graded one, Received: November 10, 2017 Published: December 8, 2017 A

DOI: 10.1021/acsphotonics.7b01355 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 1. Schematic structure of the QD samples (PL experiments were carried out on the samples corroded out of p+-GaAs and part of p-AlGaAs layers). Inset: Cross-sectional TEM image of the QD active layer structure.

been further proven in the laser devices. With a very short cavity of a 400 μm narrow ridge and without facet coatings, continuouswave (CW) GS lasing at 1315 nm has been realized in the QD laser diode. In addition, a 500 μm laterally coupled distributedfeedback (LC-DFB) QD laser has also been fabricated by a onestep epitaxy growth technique, and a large side mode suppression ratio (SMSR) of 51 dB and a low threshold of 30 mA have been achieved. Furthermore, a very large tunable range of 140 nm (from 1200 to 1340 nm) for this LC-DFB laser is realized by means of shortening the cavity length (L) and defining the grating periods. A shorter cavity can further reduce the photon lifetime, which is of great importance to improve the modulation bandwidths of high-speed lasers.

leading to a shallower confining profile and a reduced ground state (GS) gain. For high-speed lasers, an appropriate and optimized cavity length is very important but difficult to trace since a long cavity slows the response due to the increased photon lifetime, and a very short cavity also increases the response time due to the easily saturated gain.16,31−35 Modulation p-doping in the III−V QD structures has been proven to be an effective way to improve the device performances. Introducing p-doping allows the cavity length to be shortened, because the large built-in hole concentration can reduce the effect of gain saturation and therefore allow GS lasing in shorter cavities and at higher current densities.36−39 Moreover, it has been found that the holes by pdoping in InAs/GaAs QDs play an important role in determining carrier dynamics, leading to enhanced carrier relaxation rates, which are beneficial to the high-speed modulation of InAs/GaAs QD-based laser diodes.24,25,30,40−42 Recently, a 15 Gb/s highspeed 1.3 μm modulation p-doped QD laser has been demonstrated in a 500 μm long Fabry−Perot (F−P) QD laser by Arsenijević et al.43 Distributed-feedback (DFB) lasers are vital devices, providing longitudinal single-mode emission with a narrow line width, and they are indispensable for dense wavelength division multiplexing (WDM) systems. Besides those benefits already demonstrated for ridge-waveguide edgeemitting QD lasers, an outstanding advantage of broad distribution of target emission wavelengths and a higher insensitivity to temperature or charge carrier induced gain spectrum shifts have emerged due to the natural size distribution induced wide gain spectrum of self-assembled QD ensembles.44,45 Generally, an epitaxial regrowth step after fabricating a Bragg grating structure is necessary to complete the growth of a whole laser structure. The regrowth process is usually accompanied by the production of a number of nonradiation recombination centers (e.g., the oxidation of the Al-containing layer), which will sharply degrade the device performance.46 In this work, 1.3 μm multiple QD layer laser structures were employed to investigate the effects of modulation p-doping and postgrowth thermal intermixing, with the p-side AlGaAs cladding layer etched away for temperature-dependent PL measurements. It has been found that p-doped QD samples exhibit a high GS modal gain, very good temperature stability, and superior insensitivity to intermixing, which are mainly attributed to the improved counter to the thermal smearing of holes in closely spaced hole levels and the significantly suppressed interdiffusion between QDs and their surrounding barriers.10,26,47−49 These superior features of the modulation p-doped QD materials have

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EXPERIMENTS The InAs/InGaAs/GaAs QD laser structures were grown by a molecular beam epitaxy (MBE) system on Si-doped GaAs (100) substrates. As seen in the inset to Figure 1, a cross-sectional transmission electron microscopy (TEM) image of the QD active region of the laser structure reveals eight stacks of QD layers separated by 33 nm GaAs barriers. Each QD layer comprises 2.7 ML InAs covered with 6 nm InGaAs straining reducing layer, and the whole structure is sandwiched by ∼2800 nm lower n-Al0.3Ga0.7As and ∼1800 nm upper p-Al0.3Ga0.7As cladding layers. A schematic diagram of the QD-based laser diode structure, in a typical p−i−n configuration, is shown in Figure 1. The p-doped as grown QDs (QDP) sample was grown sequentially with identical structures, and the modulation pdoping with Be was conducted in a 6 nm layer located in the GaAs spacer layer 10 nm beneath each InAs/InGaAs QD layer to obtain a concentration of 25 acceptors per dot. The annealing process was performed in an N2 ambient at the temperature of 700 °C for 5 min. The QD samples were protected by a GaAs proximity cap during the RTA process. For the effective light excitation and PL signal collection, the upper p-side AlGaAs cladding layers were etched away by wet etching above the QD active regions. Temperature-dependent PL measurements were performed from 4 to 300 K excited with the 532 nm line of an Ar+ laser and detected with an InGaAs detector. The temperature was controlled with an accuracy of better than ±0.1 K, and the temperature was kept for 20 min for each measurement point to avoid temperature fluctuations during acquiring a spectrum. The 3.5 μm wide ridge waveguide lasers were fabricated by optical lithography and inductively coupled plasma (ICP) dry etching. For DFB lasers, first-order Bragg gratings were fabricated alongside the ridge waveguide by electron beam B


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ACS Photonics lithography (EBL) and then etched by ICP, stopping at ∼150 nm above the active range to have a good coupling with light. Ti/Au and Au/Ge/Ni/Au were deposited to form the top and bottom ohmic contacts, respectively. The substrates were thinned down to around 80 μm to minimize self-heating effects. Emitting facets were not coated, and the laser cavities of 350, 400, and 500 μm were fabricated, respectively. The laser bars were mounted on a copper heat sink and measured at room temperature (RT) under CW operation.

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RESULTS AND DISCUSSION Figure 2(a) and (b) show temperature-dependent PL spectra of undoped as grown QDs (QDU) and QDP samples, respectively,

Figure 3. (a) PL line width, (b) PL peak intensity, and (c) PL peak position as functions of temperature. Black squares and red solid circles are for the QDU and QDP samples, respectively. The inset in (b) shows the plots in the high-temperature range with an enlarged vertical scale. The blue and green lines in (c) show the linear fitting curves from 140 to 300 K for QDU and QDP samples, respectively.

Figure 2. PL spectra measured at 4−300 K from (a) the undoped and (b) p-doped eight-layer InAs/GaAs QD laser structures.

acquired at temperatures ranging from 4 to 300 K. Both the QDU and QDP samples exhibit only a GS emission peak from 4 to 300 K under an excitation power of 200 mW. No emission from the wetting layer or the InGaAs layer is observed, indicating that carriers photogenerated in these layers quickly transfer to the InAs dots and then recombine there. At 4 K, the GS emission peaks are centered at around 1.026 and 1.008 eV for the QDU and QDP samples, respectively. With the increase of temperature, they exhibit similar trends in emission energy, a gradual shift toward lower energy. Figure 3(a) shows the temperature dependence of the PL emission line width from the QDU and QDP samples. With the temperature increased from 80 to 180 K, a decrease of line width is observed for the QDU sample, reflecting that thermally activated carriers are redistributed among small and large QDs.27,50,51 Above 180 K, the line width increases rapidly as a result of increased electron−phonon scattering. On the other hand, it is clear that the line width of the QDP sample shows a broadening in comparison with that of the QDU sample, which can be mainly attributed to the state filling effect induced by modulation p-doping.19 The normalized PL intensity as a function of temperature is presented in Figure 3(b). The QDU sample shows a rising trend of the emission intensity with the temperature increased from 4 to 25 K. It is well-known that the wetting layer (WL) can work like a carrier reservoir,52 which can increase the PL luminescent intensity of the QDs by an enhanced multiphonon-assisted relaxation process for the QDU sample. However, different from the QDU sample, the QDP sample does not exhibit an increased peak intensity in this low temperature range, which implies that fast carrier capture and relaxation processes are dominant due to enhanced carrier−carrier scattering.25,30,53 With the temperature

further increased, a decrease of the PL peak intensity can be observed in both QDU and QDP samples, indicating the escape of the activated carriers from the GS to the states of nonradiative recombination centers or the GaAs barriers. In addition, it is found that the PL intensity of the GS of QDP is less than that in the QDU sample within the temperature range from 4 to 200 K, which can be attributed to the increased density of dopantrelated trap states in the p-doped GaAs.54 It is worth noting that the emission from QDP still maintains a strong intensity even at 300 K, as shown in the inset of Figure 3(b), which is around a factor of 16 reduction compared with the value at 4 K, while for the QDU, the reduction is 150 times. This obvious slowed quenching of the PL emission in QDP is mainly due to the strong compensation of the holes introduced by modulation doping,19 which can inhibit thermal broadening of holes from the GS with increasing temperature, resulting in a higher GS gain in the QDP sample. The PL peak positions are plotted with respect to temperature in Figure 3(c). The QDU and QDP samples all exhibit red-shift of the peak position with increased temperature, but the shift cannot be well fitted with Varshini’s equation in terms of band gap shrinkage,55 which suggests the carriers transfer from the small QDs over barriers to the large QDs by increasing the temperature.27,29,50,56,57 Compared with the QDU sample, the PL peak energy of the QDP sample is smaller over all the measured temperature ranges. The difference of the emission energy between QDU and QDP is caused by the prolonged higher temperature needed for the growth of the AlGaAs cladding layer, which is equivalent to an RTA process. The QDU C


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Figure 4. Schematic illustration of the RTA process of (a) undoped and (b) p-doped eight-layer InAs/InGaAs/GaAs QD structures using a GaAs proximity cap, indicating the lower concentrations of Ga vacancies in p-doped samples due to the Be dopants.

Figure 5. Normalized PL spectra measured at 10−300 K from (a) the QDAU and (b) QDAP samples, respectively. The dashed lines show the tendency of temperature dependence of GS and ES emission peaks.

equivalent to a relatively strong RTA treatment for the QD active region. Moreover, RTA can also be utilized to get rid of the point defects that are produced during the epitaxy growth. Therefore, it is necessary to investigate the effect of the RTA process on the optical properties of QDU and QDP samples. To illustrate the different situations of QDU and QDP samples subjected to RTA treatment, the structural and compositional details are depicted in Figure 4, with the Ga vacancies and doped Be atoms included. Temperature-dependent PL spectra were also measured on the undoped and p-doped InAs/GaAs QDs, with the same excitation power of 200 mW as in measuring the unannealed samples. Figure 5(a) and (b) show the temperature-dependent PL spectra of the annealed undoped QD (QDAU) and the annealed p-doped QD (QDAP) samples, respectively. The PL spectra of both QDAU and QDAP samples exhibit a distinctive doublepeaked feature within the measured temperature range from 10 to 300 K, corresponding to the GS and excited state (ES) emission of the QDs. The appearance of the ES peak is mainly due to the interdiffusion mechanism of In and Ga atoms, which results in a shallower confining profile and consequently a reduced GS gain.23,26 From 10 to 300 K, a red-shift of ∼68.9 meV (from 1.024 to 0.955 eV) of the GS emission from the QDAP was observed, which is smaller than that of ∼88.0 meV (from 1.115 to 1.027 eV) in the QDAU sample. The smaller red-shift suggests the less temperature-dependent behavior of the PL emission in the QDAP sample.

sample suffered an annealing effect at the higher growth temperature, and a strong interdiffusion between QDs and surrounding barrier layers occurred as intermixing, resulting in a remarkable blue-shift of the peak position. In the case of the QDP sample, the modulation p-doping can significantly inhibit the Ga vacancy propagation, which leads to smaller interdiffusion and a reduced intermixing effect.21,23,26,58−60 As a result, the QDP sample experienced less change in the emission behavior due to the reduced intermixing. The fast red-shift of the peak position over the range from 140 to 300 K can be linear fitted. The slopes of the fitting lines are 0.35 and 0.29 meV/K with respect to the QDU and QDP. The smaller slope of the QDP sample suggests that the p-doping has effectively modified its emission properties, making its PL peak position less temperature dependent. As we can see from spectral results discussed above, the intermixing effect is significant in determining the emission behaviors of the self-assembled InAs/GaAs QD system. Actually, it has been found that well-controlled QD intermixing allows tuning of their emission wavelength, making it an attractive tool for the manufacturing of multiple-section photonic integrated circuits.22,61,62 However, a desirable large blue-shift of the emission wavelength is always accompanied by a dramatic decrease of PL intensity during the intermixing process.23,26,58−60,63 In addition, the high-quality AlGaAs cladding layer usually is grown at much higher temperatures compared with that for the growth of the QD active medium, which can be D


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Figure 6. P−I characteristics of the QD lasers with different L, measured under CW operation at RT: (a, c, e) for QDU lasers with 500, 400, and 350 μm; (b, d, f) for QDP lasers with 500, 400, and 350 μm, respectively. Inset: Corresponding lasing spectra of the device under various injection currents.

Further inspection reveals that the GS peak positions of QDAP and QDAU samples are 0.955 and 1.027 eV, respectively, and the QDAP sample shows a much stronger GS emission. The QDAP sample shows a ∼22.3 meV blue-shift of GS emission wavelength upon annealing, which is much smaller than that of ∼80.4 meV in the QDAU sample. Furthermore, the energy separation between the GS and ES of QDAP (∼71 meV) is much larger than that (∼45 meV) of QDAU. All these spectral characteristics lead to the fact that modulation p-doping has reduced the effect of intermixing. As previously reported,26 the degree of intermixing is mainly determined by the number of Ga vacancies. The concentrations of Ga vacancies are lower in p-type materials,23 leading to the reduction of In−Ga intermixing. This is consistent with the smaller blue-shift in peak position and intensity reduction in the p-doped samples. The effect of group III intermixing on QDs increases the lateral size, rather than the vertical height, of the QD,23 and the intersubband energy can be considered to be mainly governed by the lateral dimension of the QDs. This explains the observed energy separation difference between GS and ES in the annealed samples, and the results discussed above confirm RTA treatment is an effective method to

tailor the band gap energy and energy separation between quantized states in modulation p-doped InAs/GaAs QDs. Besides the RTA effects on samples revealed by those spectral characteristics, the p-doping effects on carrier dynamics can be also discovered by comparing temperature-dependent spectral evolutions in Figure 5(a) and (b). As illustrated by Figure 5(a), the GS and ES emissions are present in all the spectral lines of sample QDAU from 10 to 300 K, but their relative intensity changes remarkably; that is, the intensity of the GS emission increases largely relative to the ES intensity. Figure 5(b) exhibits the same intensity evolution for the QDAP sample except that GS gets much stronger than the ES band at RT. These results suggest that we have to take into account the carrier relaxation process in understanding the spectral evolution. Obviously, the relative increase of the GS intensity along with the increased temperature leads to a decrease of intensity of the ES emission, which indicates the evolution of the carrier distribution between the GS and ES bands according to temperature. The thermal distribution of the carriers is governed by the carrier relaxation mechanism, which induces more carriers relaxing from the ES band to the GS band with increased temperature. The wellE


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a cavity length L of 500 μm, the GS lasing is observed with a threshold current of 34 mA (1.94 kA/cm2) and a slope efficiency of 0.30 W/A. Compared with QDU devices, the QDP devices show a higher threshold current density and higher slope efficiency of the GS lasing, which can be attributed to an increase in nonradiative recombination64 and improved gain characteristics of the QD active layer caused by p-doping. As shown by the inset of Figure 6(b), illustrating three lasing spectra measured at different injection currents, the device exhibits only the GS lasing at around 1330 nm, with an emission intensity lager than 52 dB by subtracting the maximum power of the background noise. This indicates that p-type doping reduces the effect of gain saturation and leads to a significant inhibition of the ES lasing. In contrast to the QDU laser result shown in Figure 6(c), the QDP laser, with a very short cavity of 400 μm, still maintains a good GS lasing characteristic, as shown in Figure 6(d), lasing at the GS band of around 1315 nm at the injection current of 45 mA (3.21 kA/cm2). Simultaneous two-state lasing behavior is observed with increased injection current up to 64 mA (4.57 kA/cm2). It is worth noting that the GS emission still maintains a large intensity with further increased injection current, even at >110 mA. It is well known that the strong competition for the holes between the GS and ES electrons is the main reason for suppressing GS lasing of QD lasers. The hole concentration in a QD structure can be significantly increased by modulation pdoping, which effectively reduces this competition and consequently facilities the GS lasing. Moreover, as shown in ref 65, the increased hole capture rate is favorable for GS lasing in the two-state lasing regime. The holes introduced by modulation pdoping play an important role in the dominant carrier relaxation from the GaAs barriers to the GS of the dots due to the enhanced Auger scattering. So far we have seen the effects of p-doping on both material properties and device performances. To be more specific, we employ the scheme depicted in Figure 7 to illustrate the role of pdoping in a microscopic view. The modulation p-doping exerts influence on both material structure of the QDs and carrier dynamics in the complex InAs/GaAs system. As having been revealed by the PL characterization, the p-doping in the selfassembled InAs/GaAs QD system effectively reduces the intrinsic intermixing effects, which modifies the composition profile across the QD interface and therefore results in a shallower confining potential profile. As depicted by the dashed red lines in Figure 7, as the result of intermixing, the potential profile of the undoped sample is severely altered, while that of the doped sample almost maintains its original profile due to the intermixing inhibition by p-doping. Besides the effects on the QD structure as well as the confining potentials, the modulation pdoping influences the carrier behaviors in a variety of means. As shown in Figure 7, along with the distribution of the carriers in the conduction and valence bands, the typical dynamical behaviors of the major carriers are depicted, including the drift (process 1), capture (process 2), relaxation (process 3), thermal excitation (process 4), and diffusion (process 5) of the holes upon p-doping. The modulation doping of Be dopants in GaAs barriers produces the major carrier of holes which spread via the diffusion mechanism to the WL and the strain-reducing layer. The excessive hole carriers around the QDs lead to two results: more holes residing in the QDs and enhanced capture of holes in the carrier dynamics. The slower quenching of PL intensity revealed in Figure 2 with the doped sample is direct evidence of the enhanced capture behavior which effectively compensates the carrier escape from the QDs originating from the thermal

known phonon-assisted carrier relaxation explains the observed relative intensity evolution revealed by Figure 5. However, compared with the QDAU sample, the QDAP sample exhibits a much stronger GS emission than the ES. We attribute this result to the carrier scattering induced relaxation effect owing to pdoping, which further relaxes more carriers to the GS band from the ES at increased temperatures in addition to the phononassisted relaxation. It should therefore be pointed out that, when we observe a stronger emission of the GS band at room temperature with the InAs/GaAs QDs, the carrier relaxation from the ES band as well as the state density of the GS band should be considered. From the spectral results discussed above, we have seen the superior emission properties of the modulation p-doped QD sample compared to the others. To further confirm the optical superiority, as well as to demonstrate the advantageous device applications, we also fabricated laser devices by using these QD materials, and notable lasing performance has been achieved in both F−P and DFB lasers of ultrashort cavity lengths. High-speed lasers are fascinating for their applications in highspeed data computation and communication, and the high-speed performance can be realized by shortening the laser L to reduce photon lifetime. The shortening of the laser cavity, therefore, imposes strict requirements on active materials, demanding higher gain characteristics. As will be seen in Figure 6, the very short cavity laser of modulation p-doped InAs/GaAs QDs exhibits superior lasing performance. Figure 6(a), (c), and (e) show the power−current (P−I) characteristics of QDU lasers with a very short L of 500, 400, and 350 μm, respectively. Two obvious thresholds are observed in Figure 6(a), the first at 23 mA (1.31 kA/cm2) corresponding to the GS lasing at ∼1318 nm, followed by the GS saturation profile starting from 35 mA to ∼50 mA, and the second at 50 mA (2.86 kA/cm2) corresponding to the ES lasing at ∼1223 nm. The GS slope efficiency is calculated to be ∼0.24 W/A, while the ES slope efficiency is as high as ∼0.41 W/A, which can be attributed to the double degeneracy and high gain of the ES band. The detailed lasing profile of the device is shown in the inset of Figure 6(a), which reveals the transition from GS to ES lasing, the only GS lasing at low injection current, the simultaneous GS and ES lasing at increased current, and finally the only ES lasing at a further increased current. This is a result of the saturation of the GS gain and increased population of the ES due to Pauli blocking. As shown in Figure 6(c) and (e), threshold currents for ES lasing are 53 (3.79 kA/cm2) and 57 mA (4.65 kA/cm2) for 400 and 350 μm long QDU lasers, respectively. Compared with the 500 μm long device, the threshold current is increased, which can be attributed to the increased mirror loss due to shortened L. Moreover, no GS lasing can be found in the insets of Figure 6(c) and (e). The inhibition of the GS lasing can be attributed to the reduced GS density in the undoped QD materials and the shortened cavity length L. As we have seen in the spectral analysis presented above, the intermixing effect is remarkable in the undoped QD samples, which greatly reduced the confined states in the GS band. As a result, when the cavity length is shortened to 400 and 350 μm, the reduced state density in the GS band cannot fulfill the gain requirement for lasing. On the contrary, besides the higher GS gain by p-doping, the intermixing effect is greatly reduced, and the high state density of the GS band in the modulation p-doped QD sample can be well maintained. We therefore expect that GS lasing in a shorter cavity will be observed in QDP lasers. The P−I characteristics of QDP lasers with different L are shown in Figure 6(b), (d), and (f). As shown in Figure 6(b), with F


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is indispensable for long-distance optical transmission and WDM systems. Since the superior material properties and device performances have been observed in the modulation p-doped QD system, we further demonstrate its application in DFB lasers. In order to avoid detrimental nonradiation recombination centers by the regrowth process, in our work, an LC-DFB laser structure, with gratings located alongside the ridge waveguide, is fabricated by a one-step epitaxy growth technique. A scanning electron microscopy (SEM) image of the 500 μm long LC-DFB laser structure is shown in Figure 8(a), revealing a high-quality grating with very sharp sidewalls tightly wound alongside the laser ridge waveguide. The detailed information for the grating is presented in Figure 8(b), the cross-sectional SEM image for the shallow-etched gratings, indicating an etching depth of ∼135 nm and an aspect ratio of ∼1.4:1. Figure 9 shows the P−I characteristics of the p-doped QDs LC-DFB laser obtained under CW operation at RT. The

Figure 7. Energy band diagram in the active region of (a) the QDU and (b) QDP samples. The dashed red lines are the potential profile as the result of intermixing. The typical dynamical carrier behaviors include the carrier drift (process 1), capture (process 2), relaxation (process 3), thermal excitation (process 4), and diffusion (process 5).

excitation process. Because more holes can reside in individual QDs, the carrier−carrier scattering mechanism is expected to be greatly enhanced. The enhanced carrier−carrier scattering facilitates the relaxation of carriers from the ES band to the GS band, which is manifested as the stronger GS emission intensity relative the ES band, as discussed in interpreting the spectral characteristics in Figure 5(b). We believe that the enhanced relaxation of the ES carriers also plays an important role in the device performance, such as the reduction of the GS gain saturation revealed by Figure 6. We can then conclude that the pdoping is essential to achieve the GS lasing in QD devices, especially with short cavities, as is required for high bit rate telecommunication systems. We will therefore demonstrate the application of the p-doped InAs/GaAs QDs in fabricated DFB laser devices. DFB lasers are key devices for modern optical data communication due to their narrow emission spectrum, which

Figure 9. P−I characteristic of a p-doped QD DFB laser under CW operation at RT. Inset: Lasing spectrum of a p-doped QD DFB laser measured at 1.8Ith.

threshold current is 30 mA, corresponding to 1.71 kA/cm2 threshold current densities, which is less than the value of the 1.3 μm QD DFB laser with facet coatings in ref 67. When the injection current reaches 150 mA, the output power of the laser can be up to 23 mW without the obvious appearance of heating saturation phenomena. The inset of Figure 9 shows an output spectrum of the LC-DFB working at RT under CW operation. At a drive current of 55 mA, the laser shows a single longitudinal mode operation with an SMSR as high as 51 dB, which is much higher than the value of a similar-type laser by recent reports.2,46,66 Stubenrauch et al. realized a 10 Gb/s data transmission across 30 km of a single-mode fiber by using an

Figure 8. (a) Top view of the SEM image of the LC-DFB laser structure with first-order grating. (b) Cross-sectional SEM image of grating after dry etching. G


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ACS Photonics 800 μm long p-doped InGaAs QD DFB laser.67 It is worth noting that the LC-DFB laser we fabricated has a much shorter cavity of 500 μm and as-cleaved facets. The high-speed performance of the DFB laser can be further improved by shortening L due to the decreasing of the cavity’s photon lifetime,33 and superior gain materials are, therefore, highly demanded. As we have demonstrated above, the modulation p-doped InAs/GaAs QDs possess a great potential for further improving high-speed performance of DFB lasers. There is an increasing demand for widely tunable single-mode lasers for applications such as WDM, gas sensing, or spectroscopy. Due to the shallow-etched lateral gratings fabricated independently, this LC-DFB structure allows high flexibility in defining the designed Bragg wavelength. Furthermore, compared with a QW, self-assembled QD ensembles have a wider gain spectrum because of the natural size distribution and the low states density. So, a large tuning range of the lasing wavelength for this QD LC-DFB laser can be realized. Figure 10 shows output spectra of the LC-DFB lasers with different grating periods working at RT under CW operation.

which shows the different lasing wavelengths corresponding to the different SMSR and Bragg periods.

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CONCLUSION In summary, multiple InAs/GaAs QD layer laser structures, with the p-side AlGaAs cladding layers etched away, have been investigated with the technique of temperature-dependent PL spectroscopy. The comparative analysis of the PL characteristics of four QD samples reveals that the modulation p-doped QD sample is much more temperature-stable and has notable insensitivity to intermixing, which is mainly attributed to pdoping-induced inhibition of the holes’ thermal broadening in closely spaced hole subbands and the suppression of In/Ga interdiffusion between QDs and their surrounding matrix. These superior features of the modulation p-doped QD materials have been further proven by their laser devices, in which the lasing onset at the ES transition is inhibited and the GS lasing is achieved in very short cavities. With the p-doped QD materials, CW GS lasing has been achieved in a 400 μm long narrow-ridge F−P laser, and a 500 μm long LC-DFB laser has been fabricated, exhibiting a large SMSR as high as 51 dB. By means of shortening the cavity length (L) and defining the grating periods, a very large tunable range of 140 nm (from 1200 to 1340 nm) for this LCDFB laser is realized. These results demonstrate the promising application of modulation p-doped QD materials for approaching ultra-high-speed QD lasers for fiber-optic communication systems.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 10. Lasing spectra of modulation p-doped QD DFB lasers with different grating periods. Inset: Detailed information on the LC-DFB lasers, including the lasing wavelengths, SMSR, and Bragg periods.

ACKNOWLEDGMENTS We would like to acknowledge the financial support from the Natural Science Foundation of China (61575215) and the Thousand Youth Talents Plan, the Key Research and Development Plan of the Ministry of Science and Technology (2016YFB0402303). J.Q.N. acknowledges the financial support from National Key Technologies R&D Program of China (2016YFA0201101), Cutting-edge Key Research Program of Chinese Academy of Sciences (QYZDB-SSW-JSC014), Natural Science Foundation of China (61674166), and Hundred Talents Program of Chinese Academy of Sciences. C.C.Z. acknowledges the support from the Natural Science Foundation of China (11504299) and Jiangsu University Natural Science Research Program (16KJB140015) and English language proofreading by Jessie Cannady from the Language Centre. We are thankful for the technical support from the Nanofabrication facility for device fabrication and Platform for Characterization and Analysis in Suzhou Institute of Nano-Tech and Nano-Bionics (CAS).

Different wavelengths of single longitudinal mode lasing operation with a high SMSR are achieved by means of defining grating with different Bragg periods. For devices with a grating period of 194 nm, the lasing wavelength is 1292 nm. By tuning the grating periods to 202 nm, the lasing wavelength is extended to 1340 nm. To get a larger tuning range, the L of LC-DFB lasers is shortened to 400 μm. The ES emission is obtained due to the effect of GS gain saturation and increased population of the ES. The shorter cavity LC-DFB lasers with grating periods of 179, 182, and 184 nm show a single longitudinal mode operation at 1200, 1218, and 1230 nm, respectively. By implementing different period gratings laterally to a ridge waveguide and shortening the L carefully for getting ES lasing, laser diodes based on InAs/GaAs QDs could be realized that are tunable over a spectral range of 140 nm (from 1200 to 1340 nm). Very recently, Cantú et al.68 reported a tunable range from 1291 to 1326 nm with five detuned wavelengths, which is achieved by the identical method of modifying the laser grating period gradually in order to detune the lasing wavelength from the peak optical gain centered at 1310 nm of the multiquantum wells (MQWs). In contrast, we tuned the peak optical gain to shorter wavelength centered at 1218 nm by shortening L to 400 μm, so a much wider tunable range is realized in QD LC-DFB laser than conventional QW DFB laser. More detailed information on the emission properties of the LC-DFB laser is in the inset table of Figure 10,

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