February 1, 2014 / Vol. 39, No. 3 / OPTICS LETTERS
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Reduced efficiency droop in blue InGaN light-emitting diodes by thin AlGaN barriers Jih-Yuan Chang,1 Yi-An Chang,1 Tsun-Hsin Wang,1 Fang-Ming Chen,2 Bo-Ting Liou,3 and Yen-Kuang Kuo1,* 1
Department of Physics, National Changhua University of Education, Changhua 500, Taiwan Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan
2 3
Department of Mechanical Engineering, Hsiuping University of Science and Technology, Taichung 412, Taiwan *Corresponding author: [email protected] Received October 9, 2013; revised December 16, 2013; accepted December 20, 2013; posted December 20, 2013 (Doc. ID 199196); published January 21, 2014
The phenomenon of efficiency droop in blue InGaN light-emitting diodes (LEDs) is studied numerically. Simulation results indicate that the severe Auger recombination is one critical mechanism corresponding to the degraded efficiency under high current injection. To solve this issue, LED structure with thin AlGaN barriers and without the use of an AlGaN EBL is proposed. The purpose of the strain-compensation AlGaN barriers is to mitigate the strain accumulation in a multiquantum well (MQW) active region in this thin-barrier structure. With the proposed LED structure, the hole injection and transportation of the MQW active region are largely improved. The carriers can thus distribute/disperse much more uniformly in QWs, and the Auger recombination is suppressed accordingly. The internal quantum efficiency and the efficiency droop are therefore efficiently improved. © 2014 Optical Society of America OCIS codes: (230.0250) Optoelectronics; (230.3670) Light-emitting diodes; (230.5590) Quantum-well, -wire and -dot devices. http://dx.doi.org/10.1364/OL.39.000497
In blue InGaN-based light-emitting diodes (LEDs), the nonuniform carrier distribution in quantum wells (QWs) is a critical issue limiting the illumination efficiency. For III-nitride LEDs, the built-in electric polarization and the corresponding polarization-induced electric field modifies the band profile of the multiquantum well (MQW) active region and forms sloped triangular wells and barriers, which seriously influences the carrier confinement and transportation [1]. On the other hand, the holes in GaN-based materials have a relatively high effective mass and therefore a low mobility. Furthermore, the widebandgap electron-blocking layer (EBL) creates a potential barrier in the valance band as well and thus obstructs the holes from injecting into the active region. Note that this hole obstruction is even more severe due to the fact that the energy band of EBL is pulled down by the polarization-induced electric field [2,3]. As a result, it has been found that holes were majorly distributed in the QWs close to the p-type region, and only these few QWs contributed to light emission no matter how many QWs were grown [4,5]. The nonuniform carrier distribution increases the local carrier density and thus enhances the Auger recombination loss in the dominant QWs. Auger recombinations in InGaN-based LEDs were originally believed to be negligibly small due to their wide bandgap energy [6]. However, large Auger coefficients, i.e., much larger than the original theoretical prediction, have been measured [7–9]. The Auger recombination thus must be considered as an important nonradiative loss mechanism in blue LEDs. One solution to mitigate the severe carrier loss from Auger recombination is to reduce the carrier density since it scales with the cubic power of the free carrier concentration. In this study, the optical and electrical characteristics of a typical blue InGaN LED structure are investigated with a finite element analysis simulator [10]. Specifically, modified structures with appropriate band engineering are proposed to 0146-9592/14/030497-04$15.00/0
pursue for the reduced Auger losses by efficiently uniformizing the carrier distribution in QWs. The original blue LED structure used as a reference is composed of 2 μm thick n-GaN layer (2 × 1018 cm−3 ), MQW active region, 20 nm thick p-Al0.1 Ga0.9 N EBL (5 × 1017 cm−3 ), and 100 nm thick p-GaN layer (7 × 1017 cm−3 ). The active region consisted of six 2.5 nm thick In0.17 Ga0.83 N QWs, embedded in seven 12 nm thick GaN barriers. The carrier densities considered in the simulation represent actual densities of free carriers. In order to eliminate the issue of current crowding, ideal ohmic contacts are set to cover the full top and bottom surfaces of the simulated vertical LED structure. Consequently, internal quantum efficiency (IQE) is utilized to judge the optical performance of LEDs. The Shockley– Read–Hall and Auger recombination coefficients are set to be 1 × 107 l∕s and 1 × 10−30 cm6 ∕s, respectively [8,11]. Details of the material parameters employed in the simulation are described elsewhere [12]. Figures 1(a)–1(c) show the simulated energy band diagram, carrier concentrations, and recombination rates near the active region of the original LED structure at 100 A∕cm2 , respectively. In Figs. 1(a) and 1(b), simulation results indicate that the polarization-induced electric field causes a strong deformation of the band profiles in the active region, and a nonuniform carrier distribution in QWs is observed. Under this circumstance, the illumination efficiency is severely deteriorated by the spatial separation of electron and hole wavefunctions and the Auger recombination. Note that the impact of Auger recombination becomes more severe when the driving current is high. It therefore reveals an obvious droop phenomenon in IQE-I characteristic, as shown in Fig. 1(d). In order to mitigate such high Auger loss in blue LEDs, the thickness of QW barriers is varied to explore its impact on carrier injection and transportation. Figure 2 shows the carrier concentrations and recombination rates of the blue LED with 6 and 3 nm thick barriers © 2014 Optical Society of America
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Fig. 1. (a) Energy band diagram, (b) carrier concentrations, and (c) recombination rates near the active region of the original LED at 100 A∕cm2 . Inset of (c), enlarged plot of last QW. The gray areas represent the location of QWs. (d) IQE-I–V characteristics of the original LED.
at 100 A∕cm2 . In comparison with the original LED with 12 nm thick barriers, it is obvious that the hole injection into the QWs near the n-GaN layer is enhanced with the decrease of barrier thickness. A much more uniform carrier distribution of the MQW active region is thus observed when the barrier thickness is reduced to 3 nm. Similar results of the barrier effect have also been reported in [5]. In this case, the Auger recombination is consequently suppressed due to the reduced carrier density of the dominant QWs. In this simulation, compared to the original structure, the integrated radiative recombination rate of the active region is increased by 30.5% while the integrated Auger recombination rate is decreased by 27.0% in the structure with 3 nm thick barriers. Although the above theoretical approach has gained achievements in enhancing the efficiency of blue InGaN
Fig. 2. (a), (b) Carrier concentrations and (c), (d) recombination rates of the blue LED with 6 and 3 nm thick barriers at 100 A∕cm2 .
LEDs, questions exist in maintaining the crystalline quality of the active region and the following p-type layers in real epitaxy. In blue LEDs, the InGaN wells exhibit compressive strain based on the foundation of GaN. When the barrier thickness is reduced, the average indium content of the MQW active region increases and the associated increased strain accumulation thus makes it a great challenge to grow high-crystalline-quality devices. To solve this issue, an LED structure with thin AlGaN barriers and without the use of EBL is proposed. In contrast to the InGaN wells, AlGaN barriers exhibit tensile strain on the basis of GaN. The tensile strain in barriers forms a compensation for a compressive one in wells. It can thus prevent lattice relaxation, which originates from the reduced barrier thickness, and is quite feasible for epitaxy growth. In addition, since the carrier confinement of the QWs is promoted when the wider-bandgap AlGaN barriers are employed, the p-AlGaN EBL is therefore removed. Note that the enlarged polarization mismatch between the InGaN wells and AlGaN barriers will cause severer band deformation and thus degrade the hole transportation between QWs, while the removal of EBL is beneficial for the hole injection. These two factors form a trade-off for carrier distribution. Figure 3 shows the electrical characteristics of the blue LED with 3 nm thick Al0.1 Ga0.9 N barriers and without the use of an AlGaN EBL at 100 A∕cm2 . In this structure, the indium content in InGaN wells is slightly modified to be 18% in order to keep the peak emission wavelength unchanged. In Fig. 3(b), due mainly to the removal of EBL, it is found that the injection and distribution of holes are even further enhanced in comparison with the results revealed in Fig. 2(b). Under this circumstance, the Auger recombination is further suppressed and a better light output performance can therefore be expected. It is noteworthy that, as shown in Fig. 3(d), the proposed structure does not suffer from severe electron leakage even though the AlGaN EBL is removed.
Fig. 3. (a) Energy band diagram, (b) carrier concentrations, (c) recombination rates, and (d) electron current density near the active region of the blue LED with 3 nm thick Al0.1 Ga0.9 N barriers and without the use of an AlGaN EBL at 100 A∕cm2 .
February 1, 2014 / Vol. 39, No. 3 / OPTICS LETTERS
Fig. 4. (a) Carrier concentrations and (b) recombination rates of the blue LED with increased 12 pairs MQWs, 3 nm thick Al0.1 Ga0.9 N barriers, and without the use of an AlGaN EBL at 100 A∕cm2 .
In traditional blue LED structure with nonuniform carrier distribution, the LED does not benefit much from the reduced Auger recombination by increasing the number of QWs [12]. However, in the proposed structure, it is possible to gain this benefit from the additional QWs since the carriers distribute in QWs much more uniformly. Figure 4 shows the carrier concentrations and recombination rates of the blue LED with increased 12-pair MQWs, 3 nm thick Al0.1 Ga0.9 N barriers, and without the use of an AlGaN EBL at 100 A∕cm2 . It is evident that the carriers can efficiently disperse into the added QWs and thus the goal of lowering the carrier density is accomplished. In this case, further suppression of Auger recombination is therefore achieved. Figure 5 shows the IQE and power efficiency of the original LED structure and the proposed structures with six- and 12-pairs MQWs. For the calculation of power efficiency, 50% light extraction efficiency is assumed. In Fig. 5(a), comparing to the original LED, the IQEs of both proposed structures are largely enhanced in almost all the current densities studied. In addition, the phenomena of efficiency droop are much improved as well. The promoted light output performance of the proposed structure is ascribed to the
Fig. 5. (a) IQE and (b) power efficiency of the original LED structure and the proposed structures with six- and 12-pairs MQWs.
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increased hole injection efficiency and the uniformlydispersed carriers. It is noteworthy that, as shown in Fig. 5(b), the enhancements in power efficiency are even more marked due to the reduced forward voltages of proposed structures, e.g., the increments in IQE and power efficiency are 41% and 67%, respectively, for the structure with 12-pair MQWs at 100 A∕cm2 . Since the use of AlGaN barrier should degrade the carrier transport, the promoted electrical properties are attributed to the use of thin barriers and the removal of EBL [13,14]. In general, InGaN quantum barriers are employed to obtain more uniform carrier distribution and better output performance [15,16]. Consequently, as shown in Fig. 6, the carrier distribution using InGaN barriers are also compared to further elucidate the actual mechanism to improve the carrier distribution in InGaN/AlGaN MQWs. In this simulation, the structure and parameters are identical to the original blue LED structure except for the QW and barrier materials. The indium composition in QWs is slightly modified in order to keep the peak emission wavelength unchanged. In Fig. 6, it is obvious that the employment of InGaN barriers is beneficial in dispersing the carriers more uniformly due to the smaller potential height of InGaN barriers and the slighter polarization effect, especially when the indium composition of InGaN barriers is high. Note that there remain challenges in epitaxy needing to be overcome, e.g., the increased average indium content and the associated increased strain accumulation in active region. As a result, both methods are worth trying, while the decisive factor might be that which one can be used to fabricate a highcrystalline-quality LED device in practice. As stated previously, the values reported for the Auger coefficient in nitride materials reveal large variances and remain an open issue till now. Therefore, to investigate the effect of the proposed designs, it is better to examine the LED performance under other Auger coefficients adjacent to the popular one of 1.0 × 10−30 cm6 ∕s. Figure 7 shows the IQE characteristics of the original LED
Fig. 6. (a) (b) Energy band diagram and (c) (d) carrier concentrations of the blue LED with In0.05 Ga0.95 N and In0.1 Ga0.9 N barriers at 100 A∕cm2 .
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This work is supported by the National Science Council of Taiwan under grant NSC-102-2112-M-018-004-MY3.
Fig. 7. IQE characteristics of the original LED structure and the proposed structures with six- and 12-pairs MQWs under an Auger coefficient of (a) 0.5 × 10−30 and (b) 5.0 × 10−30 cm6 ∕s.
structure and the proposed structures with six- and 12pairs MQWs under an Auger coefficient of 0.5 × 10−30 and 5.0 × 10−30 cm6 ∕s, respectively. It is observed that the proposed structures with 3 nm thick Al0.1 Ga0.9 N barriers and without the use of an AlGaN EBL can obtain better IQE characteristics than the original LED in both conditions, which demonstrates the effectiveness of the proposed designs. In conclusion, a blue InGaN LED structure with thin AlGaN barriers and without the use of an AlGaN EBL is proposed to improve the phenomenon of efficiency droop. The strain-compensation AlGaN barriers are designed to mitigate the strain accumulation in MQW active region in this thin-barrier structure. In this case, the Auger recombination is largely suppressed due to the enhanced hole injection/transportation and the uniformlydispersed carriers. The IQE and the efficiency droop are efficiently improved accordingly. Furthermore, in contrast to the traditional blue LED structure with nonuniform carrier distribution, the IQE characteristics can be even upgraded when the proposed structure is with more QWs because the carrier density and the corresponding Auger recombination are further reduced.
References 1. M. H. Kim, M. F. Schubert, Q. Dai, J. K. Kim, E. F. Schubert, J. Piprek, and Y. Park, Appl. Phys. Lett. 91, 183507 (2007). 2. M. F. Schubert, J. Xu, J. K. Kim, E. F. Schubert, M. H. Kim, S. Yoon, S. M. Lee, C. Sone, T. Sakong, and Y. Park, Appl. Phys. Lett. 93, 041102 (2008). 3. Y.-K. Kuo, J.-Y. Chang, M.-C. Tsai, and S.-H. Yen, Opt. Lett. 35, 3285 (2010). 4. A. David, M. J. Grundmann, J. F. Kaeding, N. F. Gardner, T. G. Mihopoulos, M. R. Krames, T. G. Mihopoulos, and M. R. Krames, Appl. Phys. Lett. 92, 053502 (2008). 5. J. P. Liu, J.-H. Ryou, R. D. Dupuis, J. Han, G. D. Shen, and H. B. Wang, Appl. Phys. Lett. 93, 021102 (2008). 6. J. Hader, J. V. Moloney, B. Pasenow, S. W. Koch, M. Sabathil, N. Linder, and S. Lutgen, Appl. Phys. Lett. 92, 261103 (2008). 7. Y. C. Shen, G. O. Müller, S. Watanabe, N. F. Gardner, A. Munkholm, and M. R. Krames, Appl. Phys. Lett. 91, 141101 (2007). 8. M. Meneghini, N. Trivellin, G. Meneghesso, E. Zanoni, U. Zehnder, and B. Hahn, J. Appl. Phys. 106, 114508 (2009). 9. M. Zhang, P. Bhattacharya, J. Singh, and J. Hinckley, Appl. Phys. Lett. 95, 201108 (2009). 10. APSYS by Crosslight Software Inc., Burnaby, Canada, http://www.crosslight.com. 11. J. Piprek and Z. M. Simon Li, Appl. Phys. Lett. 102, 023510 (2013). 12. J.-Y. Chang, F.-M. Chen, Y.-K. Kuo, Y.-H. Shih, J.-K. Sheu, W.-C. Lai, and H. Liu, Opt. Lett. 38, 3158 (2013). 13. S.-H. Han, D.-Y. Lee, S.-J. Lee, C.-Y. Cho, M.-K. Kwon, S. P. Lee, D. Y. Noh, D.-J. Kim, Y. C. Kim, and S.-J. Park, Appl. Phys. Lett. 94, 231123 (2009). 14. D.-Y. Lee, S.-H. Han, D.-J. Lee, J. W. Lee, D.-J. Kim, Y. S. Kim, and S.-T. Kim, Appl. Phys. Lett. 100, 041119 (2012). 15. Y.-K. Kuo, J.-Y. Chang, M.-C. Tsai, and S.-H. Yen, Appl. Phys. Lett. 95, 011116 (2009). 16. Y.-K. Kuo, M.-C. Tsai, S.-H. Yen, T.-C. Hsu, and Y.-J. Shen, IEEE J. Sel. Top. Quantum Electron. 15, 1115 (2009).
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Reduced efficiency droop in blue InGaN light-emitting diodes by thin AlGaN barriers Jih-Yuan Chang,1 Yi-An Chang,1 Tsun-Hsin Wang,1 Fang-Ming Chen,2 Bo-Ting Liou,3 and Yen-Kuang Kuo1,* 1
Department of Physics, National Changhua University of Education, Changhua 500, Taiwan Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan
2 3
Department of Mechanical Engineering, Hsiuping University of Science and Technology, Taichung 412, Taiwan *Corresponding author: [email protected] Received October 9, 2013; revised December 16, 2013; accepted December 20, 2013; posted December 20, 2013 (Doc. ID 199196); published January 21, 2014
The phenomenon of efficiency droop in blue InGaN light-emitting diodes (LEDs) is studied numerically. Simulation results indicate that the severe Auger recombination is one critical mechanism corresponding to the degraded efficiency under high current injection. To solve this issue, LED structure with thin AlGaN barriers and without the use of an AlGaN EBL is proposed. The purpose of the strain-compensation AlGaN barriers is to mitigate the strain accumulation in a multiquantum well (MQW) active region in this thin-barrier structure. With the proposed LED structure, the hole injection and transportation of the MQW active region are largely improved. The carriers can thus distribute/disperse much more uniformly in QWs, and the Auger recombination is suppressed accordingly. The internal quantum efficiency and the efficiency droop are therefore efficiently improved. © 2014 Optical Society of America OCIS codes: (230.0250) Optoelectronics; (230.3670) Light-emitting diodes; (230.5590) Quantum-well, -wire and -dot devices. http://dx.doi.org/10.1364/OL.39.000497
In blue InGaN-based light-emitting diodes (LEDs), the nonuniform carrier distribution in quantum wells (QWs) is a critical issue limiting the illumination efficiency. For III-nitride LEDs, the built-in electric polarization and the corresponding polarization-induced electric field modifies the band profile of the multiquantum well (MQW) active region and forms sloped triangular wells and barriers, which seriously influences the carrier confinement and transportation [1]. On the other hand, the holes in GaN-based materials have a relatively high effective mass and therefore a low mobility. Furthermore, the widebandgap electron-blocking layer (EBL) creates a potential barrier in the valance band as well and thus obstructs the holes from injecting into the active region. Note that this hole obstruction is even more severe due to the fact that the energy band of EBL is pulled down by the polarization-induced electric field [2,3]. As a result, it has been found that holes were majorly distributed in the QWs close to the p-type region, and only these few QWs contributed to light emission no matter how many QWs were grown [4,5]. The nonuniform carrier distribution increases the local carrier density and thus enhances the Auger recombination loss in the dominant QWs. Auger recombinations in InGaN-based LEDs were originally believed to be negligibly small due to their wide bandgap energy [6]. However, large Auger coefficients, i.e., much larger than the original theoretical prediction, have been measured [7–9]. The Auger recombination thus must be considered as an important nonradiative loss mechanism in blue LEDs. One solution to mitigate the severe carrier loss from Auger recombination is to reduce the carrier density since it scales with the cubic power of the free carrier concentration. In this study, the optical and electrical characteristics of a typical blue InGaN LED structure are investigated with a finite element analysis simulator [10]. Specifically, modified structures with appropriate band engineering are proposed to 0146-9592/14/030497-04$15.00/0
pursue for the reduced Auger losses by efficiently uniformizing the carrier distribution in QWs. The original blue LED structure used as a reference is composed of 2 μm thick n-GaN layer (2 × 1018 cm−3 ), MQW active region, 20 nm thick p-Al0.1 Ga0.9 N EBL (5 × 1017 cm−3 ), and 100 nm thick p-GaN layer (7 × 1017 cm−3 ). The active region consisted of six 2.5 nm thick In0.17 Ga0.83 N QWs, embedded in seven 12 nm thick GaN barriers. The carrier densities considered in the simulation represent actual densities of free carriers. In order to eliminate the issue of current crowding, ideal ohmic contacts are set to cover the full top and bottom surfaces of the simulated vertical LED structure. Consequently, internal quantum efficiency (IQE) is utilized to judge the optical performance of LEDs. The Shockley– Read–Hall and Auger recombination coefficients are set to be 1 × 107 l∕s and 1 × 10−30 cm6 ∕s, respectively [8,11]. Details of the material parameters employed in the simulation are described elsewhere [12]. Figures 1(a)–1(c) show the simulated energy band diagram, carrier concentrations, and recombination rates near the active region of the original LED structure at 100 A∕cm2 , respectively. In Figs. 1(a) and 1(b), simulation results indicate that the polarization-induced electric field causes a strong deformation of the band profiles in the active region, and a nonuniform carrier distribution in QWs is observed. Under this circumstance, the illumination efficiency is severely deteriorated by the spatial separation of electron and hole wavefunctions and the Auger recombination. Note that the impact of Auger recombination becomes more severe when the driving current is high. It therefore reveals an obvious droop phenomenon in IQE-I characteristic, as shown in Fig. 1(d). In order to mitigate such high Auger loss in blue LEDs, the thickness of QW barriers is varied to explore its impact on carrier injection and transportation. Figure 2 shows the carrier concentrations and recombination rates of the blue LED with 6 and 3 nm thick barriers © 2014 Optical Society of America
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Fig. 1. (a) Energy band diagram, (b) carrier concentrations, and (c) recombination rates near the active region of the original LED at 100 A∕cm2 . Inset of (c), enlarged plot of last QW. The gray areas represent the location of QWs. (d) IQE-I–V characteristics of the original LED.
at 100 A∕cm2 . In comparison with the original LED with 12 nm thick barriers, it is obvious that the hole injection into the QWs near the n-GaN layer is enhanced with the decrease of barrier thickness. A much more uniform carrier distribution of the MQW active region is thus observed when the barrier thickness is reduced to 3 nm. Similar results of the barrier effect have also been reported in [5]. In this case, the Auger recombination is consequently suppressed due to the reduced carrier density of the dominant QWs. In this simulation, compared to the original structure, the integrated radiative recombination rate of the active region is increased by 30.5% while the integrated Auger recombination rate is decreased by 27.0% in the structure with 3 nm thick barriers. Although the above theoretical approach has gained achievements in enhancing the efficiency of blue InGaN
Fig. 2. (a), (b) Carrier concentrations and (c), (d) recombination rates of the blue LED with 6 and 3 nm thick barriers at 100 A∕cm2 .
LEDs, questions exist in maintaining the crystalline quality of the active region and the following p-type layers in real epitaxy. In blue LEDs, the InGaN wells exhibit compressive strain based on the foundation of GaN. When the barrier thickness is reduced, the average indium content of the MQW active region increases and the associated increased strain accumulation thus makes it a great challenge to grow high-crystalline-quality devices. To solve this issue, an LED structure with thin AlGaN barriers and without the use of EBL is proposed. In contrast to the InGaN wells, AlGaN barriers exhibit tensile strain on the basis of GaN. The tensile strain in barriers forms a compensation for a compressive one in wells. It can thus prevent lattice relaxation, which originates from the reduced barrier thickness, and is quite feasible for epitaxy growth. In addition, since the carrier confinement of the QWs is promoted when the wider-bandgap AlGaN barriers are employed, the p-AlGaN EBL is therefore removed. Note that the enlarged polarization mismatch between the InGaN wells and AlGaN barriers will cause severer band deformation and thus degrade the hole transportation between QWs, while the removal of EBL is beneficial for the hole injection. These two factors form a trade-off for carrier distribution. Figure 3 shows the electrical characteristics of the blue LED with 3 nm thick Al0.1 Ga0.9 N barriers and without the use of an AlGaN EBL at 100 A∕cm2 . In this structure, the indium content in InGaN wells is slightly modified to be 18% in order to keep the peak emission wavelength unchanged. In Fig. 3(b), due mainly to the removal of EBL, it is found that the injection and distribution of holes are even further enhanced in comparison with the results revealed in Fig. 2(b). Under this circumstance, the Auger recombination is further suppressed and a better light output performance can therefore be expected. It is noteworthy that, as shown in Fig. 3(d), the proposed structure does not suffer from severe electron leakage even though the AlGaN EBL is removed.
Fig. 3. (a) Energy band diagram, (b) carrier concentrations, (c) recombination rates, and (d) electron current density near the active region of the blue LED with 3 nm thick Al0.1 Ga0.9 N barriers and without the use of an AlGaN EBL at 100 A∕cm2 .
February 1, 2014 / Vol. 39, No. 3 / OPTICS LETTERS
Fig. 4. (a) Carrier concentrations and (b) recombination rates of the blue LED with increased 12 pairs MQWs, 3 nm thick Al0.1 Ga0.9 N barriers, and without the use of an AlGaN EBL at 100 A∕cm2 .
In traditional blue LED structure with nonuniform carrier distribution, the LED does not benefit much from the reduced Auger recombination by increasing the number of QWs [12]. However, in the proposed structure, it is possible to gain this benefit from the additional QWs since the carriers distribute in QWs much more uniformly. Figure 4 shows the carrier concentrations and recombination rates of the blue LED with increased 12-pair MQWs, 3 nm thick Al0.1 Ga0.9 N barriers, and without the use of an AlGaN EBL at 100 A∕cm2 . It is evident that the carriers can efficiently disperse into the added QWs and thus the goal of lowering the carrier density is accomplished. In this case, further suppression of Auger recombination is therefore achieved. Figure 5 shows the IQE and power efficiency of the original LED structure and the proposed structures with six- and 12-pairs MQWs. For the calculation of power efficiency, 50% light extraction efficiency is assumed. In Fig. 5(a), comparing to the original LED, the IQEs of both proposed structures are largely enhanced in almost all the current densities studied. In addition, the phenomena of efficiency droop are much improved as well. The promoted light output performance of the proposed structure is ascribed to the
Fig. 5. (a) IQE and (b) power efficiency of the original LED structure and the proposed structures with six- and 12-pairs MQWs.
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increased hole injection efficiency and the uniformlydispersed carriers. It is noteworthy that, as shown in Fig. 5(b), the enhancements in power efficiency are even more marked due to the reduced forward voltages of proposed structures, e.g., the increments in IQE and power efficiency are 41% and 67%, respectively, for the structure with 12-pair MQWs at 100 A∕cm2 . Since the use of AlGaN barrier should degrade the carrier transport, the promoted electrical properties are attributed to the use of thin barriers and the removal of EBL [13,14]. In general, InGaN quantum barriers are employed to obtain more uniform carrier distribution and better output performance [15,16]. Consequently, as shown in Fig. 6, the carrier distribution using InGaN barriers are also compared to further elucidate the actual mechanism to improve the carrier distribution in InGaN/AlGaN MQWs. In this simulation, the structure and parameters are identical to the original blue LED structure except for the QW and barrier materials. The indium composition in QWs is slightly modified in order to keep the peak emission wavelength unchanged. In Fig. 6, it is obvious that the employment of InGaN barriers is beneficial in dispersing the carriers more uniformly due to the smaller potential height of InGaN barriers and the slighter polarization effect, especially when the indium composition of InGaN barriers is high. Note that there remain challenges in epitaxy needing to be overcome, e.g., the increased average indium content and the associated increased strain accumulation in active region. As a result, both methods are worth trying, while the decisive factor might be that which one can be used to fabricate a highcrystalline-quality LED device in practice. As stated previously, the values reported for the Auger coefficient in nitride materials reveal large variances and remain an open issue till now. Therefore, to investigate the effect of the proposed designs, it is better to examine the LED performance under other Auger coefficients adjacent to the popular one of 1.0 × 10−30 cm6 ∕s. Figure 7 shows the IQE characteristics of the original LED
Fig. 6. (a) (b) Energy band diagram and (c) (d) carrier concentrations of the blue LED with In0.05 Ga0.95 N and In0.1 Ga0.9 N barriers at 100 A∕cm2 .
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This work is supported by the National Science Council of Taiwan under grant NSC-102-2112-M-018-004-MY3.
Fig. 7. IQE characteristics of the original LED structure and the proposed structures with six- and 12-pairs MQWs under an Auger coefficient of (a) 0.5 × 10−30 and (b) 5.0 × 10−30 cm6 ∕s.
structure and the proposed structures with six- and 12pairs MQWs under an Auger coefficient of 0.5 × 10−30 and 5.0 × 10−30 cm6 ∕s, respectively. It is observed that the proposed structures with 3 nm thick Al0.1 Ga0.9 N barriers and without the use of an AlGaN EBL can obtain better IQE characteristics than the original LED in both conditions, which demonstrates the effectiveness of the proposed designs. In conclusion, a blue InGaN LED structure with thin AlGaN barriers and without the use of an AlGaN EBL is proposed to improve the phenomenon of efficiency droop. The strain-compensation AlGaN barriers are designed to mitigate the strain accumulation in MQW active region in this thin-barrier structure. In this case, the Auger recombination is largely suppressed due to the enhanced hole injection/transportation and the uniformlydispersed carriers. The IQE and the efficiency droop are efficiently improved accordingly. Furthermore, in contrast to the traditional blue LED structure with nonuniform carrier distribution, the IQE characteristics can be even upgraded when the proposed structure is with more QWs because the carrier density and the corresponding Auger recombination are further reduced.
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