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GaN-Based Light-Emitting Diodes With Staircase Electron Injector Structure Shoou-Jinn Chang, Fellow, IEEE, and Yu-Yao Lin
Abstract—The authors experimentally studied GaN-based lightemitting diodes (LEDs) with both an staircase electron injector (SEI) structure and a conventional electron blocking layer (EBL). With the EBL, it was found that we could enhance LED output power, reduce forward voltage, and mitigate efficiency droop by inserting the SEI structure. These improvements could all be attributed to the effective cooling of the injected hot electrons. However, it was also found that some of the injected electrons could still leak into the p-GaN layer in the LED with SEI structure but without the EBL. Index Terms—Efficiency droop, electron blocking layer (EBL), GaN-based light-emitting diodes (LEDs), staircase electron injector (SEI) structure.
I. INTRODUCTION GaN and its related compounds, such as AlGaN and InGaN, are interesting direct bandgap materials. The bandgap energy of varies from 0.7 eV of InN to 6.3 eV of AlN. Therefore, one can adjust the emitting wavelength of these III-nitride materials from infrared to ultraviolet by changing the composition ratio between aluminium, gallium and indium. Over the past two decades, extraordinary efforts have been focused on the developments of GaN-based light emitters, especially for GaN-based light-emitting diodes (LEDs) [1], [2]. These developments have propelled the group III-nitride materials to the forefront of semiconductor research worldwide. These efforts have continuously enhanced the performances of GaN-based light emitters over the years. Indeed, GaN-based LEDs, prepared on c-plane sapphire substrate by metalorganic chemical vapour deposition (MOCVD), have already been extensively used in traffic light lamps, outdoor full-colour displays, backlight of liquid crystal display (LCD) panels and automobile taillights [3]–[6]. These LEDs are also potentially useful for solid-state lighting. Compared with other applications, it Manuscript received October 13, 2013; revised November 15, 2013; accepted November 15, 2013. Date of publication November 20, 2013; date of current version February 10, 2014. This work was supported in part by the Advanced Optoelectronic Technology Center, NCKU, the Research Center for Energy Technology and Strategy, NCKU under projects from the Ministry of Education, Taiwan, and the Bureau of Energy, Ministry of Economic Affairs of Taiwan, R.O.C. under Contract 102-E0603. S.-J. Chang is with the Institute of Microelectronics and the Department of Electrical Engineering, Advanced Optoelectronic Technology Center, Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan (e-mail:
[email protected]). Y.-Y. Lin is with Epistar Corporation, Tainan 744, Taiwan. Color versions of one or more of the figures are available online at http:// ieeexplore.ieee.org. Digital Object Identifier 10.1109/JDT.2013.2291776
should be noted that one needs to inject a significantly larger current into these LEDs for lighting application. Typical high-brightness LEDs exhibit a multiquantum well (MQW) active region. The MQW LED is a kind of heterostructure LED in which the thickness of well layers is less than the deBroglie wavelength of the carriers in the semiconductor material. Thus, electrons and holes are confined in the well layers. However, it has been shown that recombination efficiency of such MQW LEDs is high at low injection currents. The efficiency decreases gradually as the injection current increases [7]. The exact origin of such “efficiency droop” is still not fully understood. It has been reported previously that charge separation could be one of the possible reasons for the “efficiency droop” of GaN-based LEDs. To suppress band-bending induced charge separation, one can adopt non-polar/semi-polar QWs [8], [9], and polar QW with large overlap designs [10]–[12]. It has also been reported that Auger effect is related to the “efficiency droop” [13], [14]. This Auger related “efficiency droop” could be mitigated by using new materials based on dilute-As GaNAs with low Auger recombination [14]. However, it is generally believed that carrier overflow should be one of the main causes of efficiency droop [15]. The most commonly used method to reduce electron current leakage is to insert a p-AlGaN electron blocking layer (EBL) between the MQW active region and the p-GaN cap layer. With larger bandgap, this EBL provides a potential barrier which can reduce the probability for the injected electrons to overflow from the MQW active region into the p-GaN cap layer. However, EBL might also block the holes from entering the MQW active region. It is also possible to use barrier engineering to suppress carrier leakages in InGaN QW LEDs [16], [17]. Previously, Ni et al. proposed the use of an InGaN staircase electron injector (SEI) structure to mitigate the efficiency droop [18]. By inserting a SEI with step-like increased indium composition prior to the active region, they could effectively “cool” the injected hot electrons and thus nearly eliminate the overflow current even for the LEDs without EBL. However, their LEDs were prepared on the expensive freestanding m-plane GaN substrate and used only one quantum well in their active region. Very recently, Zhang et al. reported the insertion of SEI structure onto GaN-based MQW LEDs prepared on c-plane sapphire substrate [19]. They also found that SEI thickness plays an important role in quantum efficiency of the LEDs through reduction of electron overflow. However, no EBL was used in their SEI LEDs. In this work, we report a detailed study on the EBL LEDs. SEI LEDs with and without EBL were also fabricated and compared. Detailed fabrication procedures and the electro-optical properties of the fabricated LEDs will also be discussed.
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Fig. 1. Schematic conduction band diagram of the SEI LEDs.
II. EXPERIMENTS Samples used in this study were all prepared on c-plane sapphire substrate by MOCVD. Details of the growth can be found elsewhere [20], [21]. Structure of the reference samples used in this study consists of a 50-nm-thick GaN nucleation layer, a 2- -thick un-doped GaN buffer layer, a 3- -thick Si-doped GaN layer , a Si-doped strain releasing MQW (SRMQW) grown at 890 [22], an undoped light emitting MQW active region grown at 770 , a 45-nm-thick Mg-doped electron blocking layer (EBL), a Mg-doped GaN layer , a Mg-doped GaN contact layer and a Si-doped -InGaN/GaN short period superlattice (SPS) tunneling contact structure [23]. The SRMQW consists of 3 periods of 3.3-nm-thick well layers and 30-nm-thick GaN barrier layers. The InGaN/GaN MQW active region consists of 15 periods of 2-nm-thick well layers and 15-nm-thick GaN barrier layers. On the other hand, the SPS structure consists of four pairs of 5-A-thick layers and 5-A-thick GaN layers. Here, we call the last GaN layer in SRMQW as the charge layer. Instead of the charge layer, a SEI structure was used for the SEI LEDs. In this study, we prepared three different SEI LEDs (i.e., 3-layer SEI, 6-layer SEI and 9-layer SEI). Schematic conduction band diagram of the SEI LEDs is plotted in Fig. 1. In this figure, Z equals 3, 6 and 9 for the 3-layer SEI LED, 6-layer SEI LED and 9-layer SEI LED, respectively. It should be noted that the indium composition (x) increased monotonically while the thickness of each layer was identical for each sample. It should also be noted that total thickness of the SEI structure was the same as that of the charge layer (i.e., 30 nm). The parameters used to grow these SEI structures were kept the same as those
Fig. 2. (a) Pulsed L-I and (b) I-V characteristics measured from the fabricated SEI LEDs. For comparison, pulsed L-I and I-V characteristics of the reference LED were also plotted.
used to grow the well layers in the MQW active region. The as-grown samples were then annealed in the furnace at 750 in ambient to activate Mg in the p-layers. Standard procedures were then used to fabricate the LED chips with indium-tin-oxide (ITO) p-contact and Ti-Al-Ti-Au n-contact. It should be noted that these LEDs were all fabricated using exactly the same process parameters. The current-voltage (I-V) characteristics of the fabricated devices were then measured at room temperature using an HP4156 semiconductor parameter analyzer. These chips were subsequently packaged into LED lamps. The intensity-current (L-I) characteristics of the packaged lamps were then measured with the molded LEDs attached to an integrating sphere detector. In order to minimize the joule heating effect, properties of the fabricated LEDs were all measured with pulse current injection with 20 width and 1% duty cycle. III. RESULTS AND DISCUSSION Fig. 2(a) shows pulsed L-I characteristics measured from the fabricated SEI LEDs. For comparison, L-I characteristic with pulsed current injection of the reference LED was also plotted. With the same injection current, it was found that output power measured from LED with 9-layer SEI was the largest, followed
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Fig. 3. Normalized EQE as a function of injection current for the SEI LEDs and the reference LED.
by the 6-layer SEI LED. On the other hand, output powers measured from the 3-layer SEI LED and the reference LED were about the same and were much smaller. The larger output powers observed from 9-layer SEI LED and 6-layer SEI LED should be attributed to the loss of energy for the electrons moving through the SEI region [18], [19]. Although the 3-layer SEI LED could also cool the injected electrons, it is possible that the thicker layers in the SEI structure could result in partial relaxation and defect generation in the sample. As a result, the output power enhancement was insignificant for the 3-layer SEI LED. Fig. 2(b) shows room temperature I-V characteristics measured from the fabricated LEDs. With 350 mA current injection, it is found that forward voltages for the reference LED, 3-layer SEI LED, 6-layer SEI LED and 9-layer SEI LED were 3.30, 3.24, 3.25 and 3.25 V, respectively. It can be seen clearly that forward voltage measured from the reference LED was much larger than those observed from the SEI LEDs. Previously, Sheu et al. reported the insertion of an InGaN layer prior to the growth of MQW active region [24]. With the inserted InGaN layer, it was found that forward voltage of the LED became significantly smaller due to the enhanced current spreading along the in-plane directions. Such a phenomenon has also been observed from the LEDs with charge asymmetric resonance tunnelling (CART) structure [25], [26]. It has been shown previously in Fig. 2(a) that output powers measured from the 3-layer SEI LED and the reference LED were about the same. The smaller forward voltage observed from the 3-layer SEI LED indicates its wall-plug efficiency should be larger than that of the reference LED. Fig. 3 shows normalized external quantum efficiency (EQE) as a function of injection current for the fabricated LEDs. In this figure, EQE was normalized to their respective maximum values. It can be seen that we can indeed effectively mitigate the drooping effect by introducing the SEI structure. As the injection current was increased to 1 A, it is found that normalized EQEs were reduced down to 70.1% and 70.3% of their respective maximum values for the 9-layer SEI LED and 6-layer SEI LED, respectively. In contrast, normalized EQEs for the 3-layer
Fig. 4. (a) Pulsed L-I and (b) I-V characteristics measured from the 9-layer SEI LEDs with and without the EBL.
SEI LED and reference LED were both reduced down to around 67.5% of their respective maximum value, as we increased the injection current to 1 A. In other words, we could improve the efficiency droop from 32.5% to 29.9% by introducing 9-layer SEI structure and to 29.7% by introducing 6-layer SEI structure. These improvements could again be attributed to the effective electron cooling by the SEI structure. On the other hand, the improvement for the 3-layer LED was almost negligible. This should again be attributed to the thicker layers in the SEI structure could result in partial relaxation and defect generation in the sample. To further investigate the effect of SEI structure, we prepared a LED sample with 9-layer SEI structure but without the EBL. Fig. 4(a) shows its pulsed L-I characteristics. For comparison, pulsed L-I characteristics measured from the LED with both 9-layer SEI structure and EBL were also plotted. With the same injection current, it can be seen that output power measured from the LED with both 9-layer SEI structure and EBL was slightly larger than that measured from the LED with 9-layer SEI structure. Although SEI structure could cool the electrons, it is likely that some of the injected electrons could still leak into the p-GaN layer for the LED without EBL. As a result, we observed a slightly smaller LED output power.
CHANG AND LIN: GaN-BASED LEDs WITH STAIRCASE ELECTRON INJECTOR STRUCTURE
Fig. 5. Normalized EQE as a function of injection current for the 9-layer SEI LEDs with and without EBL.
Fig. 4(b) shows I-V characteristics measured from these two LEDs. With 350 mA current injection, it was found that forward voltage for the LED with 9-layer SEI structure only was as high as 3.60 V. This value was significantly larger than the 3.25 V forward voltage measured from the LED with both 9-layer SEI structure and EBL. Fig. 5 shows normalized EQE as a function of injection current for these two LEDs. It can be seen that efficiency droop observed from the LED with 9-layer SEI structure but without EBL was only slightly worse than that observed from the LED with both 9-layer SEI structure and EBL. The almost the same droop behaviors observed from these two devices indicates that the improvement in efficiency droop is almost negligible by adding EBL into the LED with 9-layer SEI structure. On the other hand, the efficiency droop behavior observed from the LED with 9-layer SEI structure was much better than that observed from the reference LED with EBL only (i.e., without the SEI structure), as shown in Fig. 3. This could again be attributed to the effective cooling of the injected electrons by the SEI structure. IV. CONCLUSION In summary, we experimentally studied GaN-based LEDs with both an SEI structure and a conventional EBL. With the EBL, it was found that we could enhance LED output power, reduce forward voltage and mitigate efficiency droop by inserting the SEI structure. These improvements could all be attributed to the effective cooling of the injected hot electrons. However, it was also found that a significantly amount of injected electrons could still leak into the p-GaN layer in the LED with SEI structure but without the EBL. REFERENCES [1] S. Nakamura, T. Mukai, and M. Senoh, “Candela-class high brightness InGaN/AlGaN double-heterostructure blue light-emitting diodes,” Appl. Phys. Lett., vol. 64, pp. 1687–1689, 1994. [2] I. Akasaki and H. Amano, “Crystal growth and conductivity control of group III-nitride semiconductors and their applications to short wavelength light emitters,” Jpn. J. Appl. Phys., vol. 36, pp. 5393–5408, 1997.
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[3] S. J. Chang, W. C. Lai, Y. K. Su, J. F. Chen, C. H. Liu, and U. H. Liaw, “InGaN/GaN multiquantum well blue and green light emitting diodes,” IEEE J. Sel. Top. Quantum Electron., vol. 8, no. 2, pp. 278–283, Mar. –Apr. 2002. [4] S. J. Chang, C. H. Kuo, Y. K. Su, L. W. Wu, J. K. Sheu, T. C. Wen, W. C. Lai, J. F. Chen, and J. M. Tsai, “400 nm InGaN/GaN and InGaN/ AlGaN multiquantum well light-emitting diodes,” IEEE J. Sel. Top. Quantum Electron., vol. 8, no. 4, pp. 744–748, Jul.–Aug. 2002. [5] J. T. Chu, H. W. Huang, C. C. Kao, W. D. Liang, F. I. Lai, C. F. Chu, H. C. Kuo, and S. C. Wang, “Fabrication of large-area GaN-based light-emitting diodes on Cu substrate,” Jpn. J. Appl. Phys., vol. 44, pp. 2509–2511, 2005. [6] W. H. Lan, “Wavelength shift of gallium nitride light emitting diode with p-down structure,” IEEE Trans. Electron. Devices, vol. 52, no. 6, pp. 1217–1219, Jun. 2005. [7] M. H. Kim, M. F. Schubert, Q. Dai, J. K. Kim, E. F. Schubert, J. Piprek, and Y. J. Park, “Origin of efficiency droop in GaN-based LEDs,” Appl. Phys. Lett., vol. 91, 2007, Art. ID 183507. [8] D. F. Feezell, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Semipolar (20-2-1) InGaN/GaN light-emitting diodes for high-efficiency solid-state lighting,” J. Display Technol., vol. 9, pp. 190–198, 2013. [9] R. M. Farrell, E. C. Young, F. Wu, S. P. DenBaars, and J. S. Speck, “Materials and growth issues for high-performance nonpolar and semipolar light-emitting devices,” Semicond. Sci. Technol, vol. 27, 2012, Art. no. 024001. [10] H. Zhao, R. A. Arif, and N. Tansu, “Design analysis of staggered InGaN quantum wells light-emitting diodes at 500–540 nm,” IEEE J. Sel. Top. Quantum Electron., vol. 15, no. 6, pp. 1104–1114, Nov.–Dec. 2009. [11] H. Zhao, R. A. Arif, and N. Tansu, “Self consistent analysis of type-II ‘W’ InGaN-GaNAs quantum well lasers,” J. Appl. Phys., vol. 104, 2008, Art. ID. 043104. [12] J. Zhang and N. Tansu, “Optical gain and laser characteristics of InGaN quantum wells on ternary InGaN substrates,” IEEE Photon. J., vol. 5, no. 2, Apr. 2013, Art. ID 2600111. [13] J. Iveland, L. Martinelli, J. Peretti, J. S. Speck, and C. Weisbuch, “Direct measurement of Auger electrons emitted from a semiconductor light-emitting diode under electrical injection: Identification of the dominant mechanism for efficiency droop,” Phys. Rev. Lett., vol. 110, 2013, Art. ID 177406. [14] C. K. Tan, J. Zhang, X. H. Li, G. Y. Liu, B. O. Tayo, and N. Tansu, “First-principle electronic properties of dilute-As GaNAs alloy for visible light emitters,” J. Display Technol., vol. 9, no. 4, pp. 272–279, Apr. 2013. [15] S. Grzanka, G. Franssen, G. Targowski, K. Krowicki, T. Suski, R. Czernecki, P. Perlin, and M. Leszczynski, “Role of the electron blocking layer in the low-temperature collapse of electroluminescence in nitride light-emitting diodes,” Appl. Phys. Lett., vol. 90, 2007, Art. ID 103507. [16] S. Choi, M. H. Ji, J. Kim, H. J. Kim, M. M. Satter, P. D. Yoder, J. H. Ryou, R. D. Dupuis, A. M. Fischer, and F. A. Ponce, “Efficiency droop due to electron spill-over and limited hole injection in III-nitride visible light-emitting diodes employing lattice-matched InAlN electron blocking layers,” Appl. Phys. Lett., vol. 101, 2012, Art. ID 161110. [17] G. Y. Liu, J. Zhang, C. K. Tan, and N. Tansu, “Efficiency-droop suppression by using large-bandgap AlGaInN thin barrier layers in InGaN quantum wells light-emitting diodes,” IEEE Photon. J., vol. 5, 2013, Art. ID 2201011. [18] X. Ni, X. Li, J. Lee, S. Liu, V. Avrutin, U. Ozgur, H. Morkoc, A. Matulionis, T. Paskova, G. Mulholland, and K. R. Evans, “InGaN staircase electron injector for reduction of electron overflow in InGaN light emitting diodes,” Appl. Phys. Lett., vol. 97, 2010, Art. ID 031110. [19] F. Zhang, X. Li, S. Hafiz, S. Okur, V. Avrutin, U. Ozgur, H. Morkoc, and A. Matulionis, “The effect of stair case electron injector design on electron overflow in InGaN light emitting diodes,” Appl. Phys. Lett., vol. 103, 2013, Art. ID 051122. [20] L. W. Wu, S. J. Chang, T. C. Wen, Y. K. Su, W. C. Lai, C. H. Kuo, C. H. Chen, and J. K. Sheu, “Influence of Si-doping on the characteristics of InGaN/GaN multiple quantum well blue light emitting diodes,” IEEE J. Quantum Electron., vol. 38, no. 4, pp. 446–450, Apr. 2002. [21] S. J. Chang, W. S. Chen, S. C. Shei, C. F. Shen, T. K. Ko, J. M. Tsai, W. C. Lai, J. K. Sheu, A. J. Lin, and S. C. Hung, “GaN-based power flipchip LEDs with Cu submount,” IEEE J. Sel. Top. Quantum Electron., vol. 15, no. 4, pp. 1287–1291, Jul.–Aug. 2009.
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[22] C. Y. Chang, S. J. Chang, C. H. Liu, S. G. Li, and T. K. Lin, “GaNbased LEDs with double strain releasing MQWs and Si delta-doping layers,” IEEE Photon. Technol. Lett., vol. 24, no. 20, pp. 1809–1811, Oct. 2012. [23] S. J. Chang, C. S. Chang, Y. K. Su, R. W. Chuang, Y. C. Lin, S. C. Shei, H. M. Lo, H. Y. Lin, and J. C. Ke, “Highly reliable nitride based LEDs with SPS+ITO upper contacts,” IEEE J. Quantum Electron., vol. 39, no. 11, pp. 1439–1443, Nov. 2003. [24] J. K. Sheu, G. C. Chi, and M. J. Jou, “Enhanced output power in an InGaN-GaN multiquantum-well light-emitting diode with an InGaN current-spreading layer,” IEEE Photon. Technol. Lett., vol. 13, no. 11, pp. 1164–1166, Nov. 2001. [25] C. H. Chen, Y. K. Su, S. J. Chang, G. C. Chi, J. K. Sheu, J. F. Chen, C. H. Liu, and U. H. Liaw, “High brightness green light emitting diode with charge asymmetric resonance tunneling structure,” IEEE Electron. Device Lett., vol. 23, no. 1, pp. 130–132, Jan. 2002. [26] C. H. Chen, S. J. Chang, Y. K. Su, G. C. Chi, J. K. Sheu, and J. F. Chen, “High efficient InGaN/GaN MQW green light emitting diodes with CART and DBR structures,” IEEE J. Sel. Top. Quantum Electron., vol. 8, no. 2, pp. 284–288, Mar.–Apr. 2002. Shoou-Jinn Chang (M’06–SM’10–F’14) was born in Taipei, Taiwan on January 17, 1961. He received the B.S. degree from the National Cheng Kung University (NCKU), Tainan, Taiwan in 1983, the M.S. degree from the State University of New York, Stony Brook in 1985 and the Ph.D. degree from the University of California, Los Angeles in 1989, all in electrical engineering. From 1989 to 1992, he was a Research Scientist with Nippon Telegraph and Telephone (NTT) Basic Research Laboratories, Musashino, Tokyo, Japan. He joined the Department
of Electrical Engineering, NCKU in 1992 as an Associate Professor, where he was promoted to Full Professor in 1998. He served as the Director of the Institute of Microelectronics, NCKU from August 2008 to July 2011. He is currently the Deputy Director of the Advanced Optoelectronic Technology Center, NCKU. He is also an honorary professor of the Changchun University of Science and Technology, China. His current research interests include semiconductor physics, optoelectronic devices, and nanotechnology. Professor Chang is the recipient of the outstanding research award from the National Science Council, Taiwan in 2004. He is a fellow of SPIE and a fellow of OSA.
Yu-Yao Lin was born in Kaohsiung, Taiwan on January 28, 1983. He received the BS degree in electrical engineering in 2005 from Chung Yung Christian University, Jhungli, Taiwan. In 2008 and 2013, he received the MS degree and PhD degree, respectively, from the Department of Photonics, National Cheng Kung University, Tainan, Taiwan. Currently, he is a research staff with the Epistar Corporation in Hsinshi, Tainan, Taiwan. His research interest is focused on the growth, fabrication, characterization and simulation of GaN-based light-emitting devices.
