HIGH-PERFORMANCE blue and green light emitting

IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 5, NO. 2, JUNE 2005 277 ESD Engineering of Nitride-Based LEDs Y. K. Su, S. J. Chang, S. C. Wei, Shi-Ming Chen, and Wen-Liang Li Abstract GaN-based light emitting diodes (LEDs) with p-cap layers grown at various temperatures were fabricated. It was found that the LED with 900 C-grown p-cap layer could only endure negative 1100 V electrostatic discharge (ESD) pulses while the LED with 1040 C-grown p-cap layer could endure ESD pulses as high as negative 3500 V. It was also found that the ESD performances of the LEDs with 900 and 1040 C-grown p-cap layers were limited by the V-shape defects and the bonding pad design, respectively. Index Terms Electrostatic discharge (ESD), GaN, growth temperature, LED, V-defect. I. INTRODUCTION HIGH-PERFORMANCE blue and green light emitting diodes (LEDs) have been developed using GaN-based materials grown on sapphire substrates in recent years [1] [3]. This has resulted in a variety of applications such as traffic light and full color display [4]. Although these LEDs are already commercially available, it has been shown that GaN-based LEDs often suffer from electrostatic discharge (ESD) due to the insulating nature of sapphire substrates. Thus, it is extremely important to improve ESD reliability of nitride-based devices. However, only very few reports regarding ESD effects could be found in the literature, particularly for those related to GaN-based optoelectronic devices [5] [8]. Previously, it has been shown that one can either combine GaN-based LEDs with Si-based Zener diodes through flip-chip process [6] or build an internal GaN Schottky diode inside the LED chips [7] to improve the ESD characteristics of nitride-based LEDs. Although these two methods can both effectively improve the ESD characteristic of nitride-based LEDs, it is also necessary to take extra complex processing steps in both cases. These steps might result in lower production yields and higher production costs. For nitride-based LEDs, growth temperature of the top p-gan layers is also important [9], [10]. Since p-gan layers were grown on top of the InGaN/GaN multiquantum well (MQW) active regions, high-temperature grown p-gan layers might result in degraded optical and structural properties of the LEDs. In other words, quantum well intermixing and dopant redistribution might occur when a high-temperature grown p-gan cap layer is deposited onto the low-temperature grown InGaN/GaN MQW. However, crystal quality of the top Manuscript received August 4, 2004; revised December 1, 2004. This work was supported by the National Science Council under Contract NSC-89-2215-E-006-095. Y. K. Su, S. J. Chang, and S. C. Wei are with the Institute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, R.O.C. (e-mail: changsj@mail.ncku.edu.tw). S.-M. Chen and W.-L. Li are with Epitech Technology Corporation, Tainan 74145, Taiwan, R.O.C. Digital Object Identifier 10.1109/TDMR.2005.847197 Fig. 1. Schematic diagram of a processed LED chip. p-gan layers grown at high temperatures should be better, which might result in improved ESD characteristics of the nitride-based LEDs. In this study, we fabricated nitride-based LEDs with top p-gan layers grown at various temperatures. The physical and ESD properties of these fabricated LEDs will be discussed. II. EXPERIMENT Samples used in this study were all grown on (0001) sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) [11] [13]. Trimethylgallium (TMGa), trimethylindium (TMIn) and ammonia (NH ) were used as the gallium, indium and nitrogen sources, respectively. Bi-scyclopentadienyl magnesium (CP Mg) and disilane (Si H ) were used as the p-type and n-type doping sources, respectively. The InGaN/GaN MQW LED structure consists of a 30-nm-thick GaN nucleation layer grown at 520 C, a 1- m-thick undoped GaN layer grown at 1050 C, a 2- m-thick Si-doped GaN n-cladding layer also grown at 1050 C, an InGaN/GaN MQW active region grown at 700 C and a 0.2- m-thick Mg-doped GaN cap layer grown at various temperatures. The MQW active region consists of five periods of 3-nm-thick InGaN well layers and 15-nm-thick GaN barrier layers. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were then used to physically characterize surface morphologies and cross sections of the as-grown samples. 1530-4388/$20.00 2005 IEEE

278 IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 5, NO. 2, JUNE 2005 Fig. 2. (a) TEM micrograph of the GaN-based LED with a p-gan cap layer grown at 900 C. (b) Shows an enlarged image of the same sample. Fig. 3. SEM micrographs of the LEDs with p-cap layers grown at (a) 900 C and (b) 1040 C. Surfaces of the as-grown LED epitaxial layers were then partially etched until the n-type GaN layers were exposed. Ni/Au transparent contact layers, with various Au layer thicknesses, were subsequently evaporated onto the p-gan cap layers to serve as the transparent contact layers (TCLs). A thick Au layer was then deposited onto part of the TCL to serve as the p-electrodes (i.e., anodes). On the other hand, Ti/Al/Ti/Au contacts were deposited onto the exposed n-type GaN layers to serve as the n-type electrodes (i.e., cathodes), to complete the fabrication of the LEDs. The size of each LED chip was 14 mil 14 mil. Fig. 1 shows the schematic diagram of a processed LED chip. The ESD characteristics of these samples were then measured by an Electro-tech system ESD simulator Model 910, which could produce electrical pulses similar to those originated from human body. In this study, we applied both positive and negative ESD biases onto the LEDs. We started at 1000-V human body mode (HBM) and successively increased the ESD pulse amplitude with a step of 100 V. After each test, we applied a 5-V reverse bias onto the LEDs and measured the leakage current. If the leakage current was larger than 2 A, we then concluded that the chip was failed. Finally, an optical microscope was used to evaluate the ESD-induced surface damages. III. RESULTS AND DISCUSSION Fig. 2(a) shows TEM micrograph of an as-grow sample (i.e., prior to LED processing) with a p-gan cap layer grown at 900 C. It can be seen clearly that there exist a large number of defects in this particular sample. Fig. 2(b) shows an enlarged image of the same sample. It can be seen that the defects are V-shaped with a threading dislocation connected at the bottom. The formation of these V-shape defects could be attributed to the fact that Ga atoms might not have enough energy to migrate to proper sites at such a low temperature (i.e., 900 C) [9], [10]. Thus, lateral growth rate of GaN will become smaller. Fig. 3(a) and (b) shows SEM micrographs of the as-grown samples with p-cap layers grown at 900 C and 1040 C, respectively. As shown in Fig. 3(a), it was found that surface morphology of the sample with 900 C-grown p-cap layer was rough [9], [10]. This can again be attributed to the low migration speed

SU et al.: ESD ENGINEERING OF NITRIDE-BASED LEDs 279 Fig. 4. Measured ESD results for the LEDs with p-cap layers grown at various temperatures. Fig. 5. Output power of the LEDs with p-cap layers grown at various temperatures, prior to ESD stressing. of Ga atoms at 900 C. In contrast, surface morphology of the sample with 1040 C-grown p-cap layer was much smoother with almost no visible V-shape defects, as shown in Fig. 3(b). Fig. 4 shows measured ESD results for the processed LEDs with p-cap layers grown at various temperatures. During positive ESD measurements, we applied a positive ESD stress onto the anode (i.e., p-electrode) while the cathode (i.e., n-electrode) was grounded. For negative ESD measurements, a negative ESD stress was applied onto the anode while the cathode was grounded. It can be seen clearly that all LEDs used in this study could endure positive ESD voltage higher than positive 7000 V since current could flow across the p-n junction easily. On the other hand, negative ESD characteristics of the LEDs are much poorer. It was found that the LED with 900 C-grown p-cap layer could only endure negative 1100 V ESD pulses. However, it was also found that LEDs with 1040 C and 1100 C-grown p-cap layers could both endure ESD pulses as high as negative 3500 V. In other words, we can significantly enhance the ESD characteristics of GaN-based LEDs by simply increasing the growth temperature of p-gan cap layers. Output power of the LEDs prior to ESD stressing was also investigated. As shown in Fig. 5, it was found that LEDs with 900 C, 1000 C, and 1040 C-grown p-cap layers were all around 5.5 mw. In contrast, output power of the LED with 1100 C-grown p-cap layer was only around 2.8 mw. When we increased growth Fig. 6. Photographs of the ESD damaged LEDs with (a) and (b) 900 C and (c) 1040 C-grown p-cap layers. temperature of the p-cap layer to 1100 C, we also found that surface of the as-grown sample became darker. These results should all relate to indium out-diffusion at the extremely high 1100 C. Fig. 6(a) and (b) shows photographs of the ESD damaged LEDs with 900 C-grown p-cap layers. For these two photographs, we applied negative 1100-V HBM pulses onto the LEDs with 900 C-grown p-cap layers. It can be seen clearly that the dead spots are randomly distributed across the LED surface. We believe these dead spots are related to the V-shape defects, which are also the weakest points in the LEDs [14]. In contrast, a whole dead area instead of spots was observed from

280 IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 5, NO. 2, JUNE 2005 the LED with 1040 C-grown p-cap layer, as shown in Fig. 6(c). For this photograph, we applied negative 3500 V HBM pulse onto the LED with 1040 C-grown p-cap layer. Since the LED with 1040 C-grown p-cap layer can endure a much larger ESD pulse voltage, damages should occur at the place with the largest electric field. As also shown in Fig. 6(c), the location of the dead area agrees well with our bounding pad design. IV. SUMMARY GaN-based LEDs with p-cap layers grown at various temperatures were fabricated. It was found that there exist a large number of V-shape defects in LEDs with 900 C-grown p-cap layers. These V-shape defects will result in a degraded ESD performance. It was also found that we could significantly improve the ESD characteristics of GaN-based LEDs by raising the p-cap growth temperature to 1040 C. REFERENCES [1] S. Nakamura, T. Mukai, and M. Senoh, High-brightness InGaN/AlGaN double-heterostructure blue-green-light-emitting diodes, J. Appl. Phys., vol. 76, no. 12, pp. 8189 8191, 1994. [2] S. Nakamura, M. Senoh, and T. Mukai, P-GaN/n-InGaN/n-GaN double-heterostructure blue-light-emitting diodes, Jpn. J. Appl. Phys. Lett., vol. 32, no. 1A-B, pp. L8 L11, 1993. [3] S. J. Chang, C. H. Chen, P. C. Chang, Y. K. Su, P. C. Chen, Y. D. Jhou, H. Hung, C. M. Wang, and B. R. Huang, Nitride-based LEDs with p-ingan capping layer, IEEE Trans. Electron Dev., vol. 50, no. 12, pp. 2567 2570, Dec. 2003. [4] C. H. Chen, S. J. Chang, Y. K. Su, J. K. Sheu, J. F. Chen, C. H. Kuo, and Y. C. Lin, Nitride-based cascade near white light emitting diodes, IEEE Photon. Technol. Lett., vol. 14, no. 7, pp. 908 910, 2002. [5] S. P. Sim, M. J. Robertson, and R. G. Plumb, Catastrophic and latent damage in GaAlAs laser caused by electrical transient, J. Appl. Phys., vol. 55, pp. 3960 3955, 1984. [6] T. Inoue, Light-Emitting Devices, Japanese Patent H11-040 848, 1999. in Japanese. [7] S. J. Chang, C. H. Chen, Y. K. Su, J. K. Sheu, W. C. Lai, J. M. Tsai, C. H. Liu, and S. C. Chen, Improved ESD protection by combining InGaN-GaN MQW LEDs with GaN Schottky diodes, IEEE Electron Dev. Lett., vol. 24, no. 3, pp. 129 131, Mar. 2003. [8] G. Meneghesso, S. Podda, and M. Vanzi, Investigation on ESD-stressed GaN/InGaN-on-sapphire blue LEDs, Microelectron. Rel., vol. 41, no. 9 10, pp. 1609 1614, Oct. 2001. [9] S. J. Chang, L. W. Wu, Y. K. Su, Y. P. Hsu, W. C. Lai, J. M. Tsai, J. K. Sheu, and C. T. Lee, Nitride-based LEDs with 800 C-grown p-alingan/gan double cap layers, IEEE Photon. Technol. Lett., vol. 16, no. 6, pp. 1447 1449, Jun. 2004. [10] L. W. Wu, S. J. Chang, Y. K. Su, R. W. Chuang, Y. P. Hsu, C. H. Kuo, W. C. Lai, T. C. Wen, J. M. Tsai, and J. K. Sheu, In Ga N/GaN MQW LEDs with a low temperature GaN cap layer, Solid State Electron., vol. 47, no. 11, pp. 2027 2030, Nov. 2003. [11] 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. Topics Quantum Electron., vol. 8, no. 2, pp. 278 283, Mar./Apr. 2002. [12] 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. Topics Quantum Electron., vol. 8, no. 4, pp. 744 748, Jul./Aug. 2002. [13] 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. [14] X. A. Cao, J. M. Teetsov, M. P. D Evylen, D. W. Marfeld, and C. H. Yan, Electrical characteristics of InGaN/GaN light-emitting diodes grown on GaN and sapphire substrates, Appl. Phys. Lett., vol. 85, no. 1, pp. 7 9, Jul. 2004. Y. K. Su was born in Kaohsiung, Taiwan, R.O.C., on August 23, 1948. He received the B.S. and Ph.D. degrees in electrical engineering from National Cheng Kung University (NCKU), Taiwan. From 1979 to 1983, he was with the Department of Electrical Engineering, NCKU, as an Associate Professor and was engaged in research on compound semiconductors and optoelectronic materials. In 1983, he was promoted to Full Professor in the Department of Electrical Engineering. From 1979 to 1980 and 1986 to 1987, he was on leave, working at the University of Southern California, Los Angeles, and AT&T Bell Laboratories as a Visiting Scholar. He was also a Visiting Professor at Stuttgart University, Germany, in 1993. In 1991, he became an Adjunct Professor at the State University of New York, Binghamton. Now he is a Professor in the Department of Electrical Engineering and Dean of Academic Affair at NCKU. His research activities have been in compound semiconductors, integrated optics, and microwave devices. He has published over 200 papers in the area of thin-film materials and devices and optoelectronic devices. Dr. Su is a member of SPIE, the Materials Research Society, and Phi Tau Phi. He received the Outstanding Research Professor Fellowship from the National Science Council (NSC), R.O.C., during 1986 1992 and 1994 1995. He also received the Best Teaching Professor Fellowship from the Ministry of Education, R.O.C., in 1992. In 1995, he received the Excellent Engineering Professor Fellowship from the Chinese Engineering Association. In 1996 and 1998, he received the Award from the Chinese Electrical Engineering Association. In 1998, he also received the Academy Member of Asia-Pacific Academy of Materials (APAM). S. J. Chang was born in Taipei, Taiwan, R.O.C., on January 17, 1961. He received the B.S.E.E. degree from National Cheng Kung University (NCKU), Tainan, Taiwan, in 1983, the M.S.E.E. degree from State University of New York, Stony Brook, in 1985 and the Ph.D.E.E. from the University of California, Los Angeles in 1989. He was a Research Scientist with NTT Basic Research Laboratories, Musashino, Japan, from 1989 to 1992. In 1992, he became an Associate Professor in Department of Electrical Engineering, NCKU, and was promoted to full Professor in 1998. Currently, he also serves as the Director of Semiconductor Research Center, NCKU. He was a Royal Society Visiting Scholar in University of Wales, Swansea, U.K., from January 1999 to March 1999, a Visiting Scholar in Research Center for Advanced Science and Technology, University of Tokyo, Japan, from July 1999 to February 2000, a Visiting Scholar in Institute of Microstructural Science, National Research Council, Canada, from August 2001 to September 2001, a Visiting Scholar in Institute of Physics, Stuttgart University, Germany, from August 2002 to September 2002, and a Visiting Scholar to Faculty of Engineering, Waseda University, Japan, from July 2004 to September 2004. His current research interests include semiconductor physics and optoelectronic devices. Dr. Chang received the Outstanding Research Award from the National Science Council, Taiwan, in 2004. He is also an honorary professor of Changchun University of Science and Technology, China. S. C. Wei was born in Miao-Li, Taiwan, R.O.C., in 1972. He received the M.S. degree from the Department of Electrical Engineering, National Sun Yat-San University, Taiwan, in 1996. He is currently working toward the Ph.D. degree in the Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, where his research is focused on GaN-based optical and electrical devices.

SU et al.: ESD ENGINEERING OF NITRIDE-BASED LEDs 281 Shi-Ming Chen was born in Taiwan, R.O.C., on February 20, 1966. He received the B.S. degree in electrical engineering from the National Cheng Kung University in 1989, the M.S. degree from the National Sun-Yat-San University in 1991, and the Ph.D. degree in electrical engineering from the National Cheng Kung University in 1995. He is a President at Epitech Corporation, Ltd., Taiwan. His research interests are focused on quantum photo-devices. Wen-Liang Li was born in Kaohsiung, Taiwan, R.O.C., on November 18, 1965. He received the B.S. degree in electrical engineering from National Cheng Kung University in 1989, the M.S. degree from the National Sun-Yat-San University in 1991, and the Ph.D. degree in electrical engineering from the National Cheng Kung University in 1997. He is a Vice President at Epitech Corporation, Ltd., Taiwan. His research interests are focused on quantum photo-devices.