InGaN/GaN Light Emitting Diodes With a p-down Structure

Transcription

1 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 8, AUGUST InGaN/GaN Light Emitting Diodes With a p-down Structure Y. K. Su, Senior Member, IEEE, S. J. Chang, Chih-Hsin Ko, J. F. Chen, Member, IEEE, Ta-Ming Kuan, Wen How Lan, Member, IEEE, Wen-Jen Lin, Ya-Tung Cherng, and Jim Webb Abstract Nitride-based p-down blue light emitting diodes (LEDs) were successfully fabricated. It was found that we could improve the crystal quality of these nitride-based p-down LEDs by inserting a codoped interlayer between the p-type cladding layer and MQW active layers. It was also found that the turn-on voltage could be reduced from 15 V to less than 5 V for the p-down LED with codoped layer and tunnel layer. The 20 ma output power was 1 mw for the p-down LED with an Mg+Si codoped interlayer and a rough p-tunnel layer. Index Terms Double crystal X-ray diffraction (DCXRD), electroluminescence (EL), InGaN/GaN, light emitting diodes (LEDs), multiquantum well (MQW), p-down, photoluminescence (PL). I. INTRODUCTION HIGH-BRIGHTNESS nitride-based light emitting diode (LED) was first demonstrated in 1993 [1] and the development of these III-nitride LEDs is very successful over the past ten years [2] [7]. Fig. 1(a) shows the schematic structure of a typical nitride-based LED. As shown in Fig. 1(a), the multiquantum well (MQW) active layers are grown on top of the n-type cladding layer, and the p-type cladding layer is grown on top of the MQW active layers in this structure. We call such a structure n-down structure. Such an n-down structure is adopted not only for nitride-based blue/green LEDs but also for the AlGaInP-based red/yellow LEDs. The first reason for using such an n-down structure is the crystal quality of MQW active layers. Generally speaking, the crystal quality of n-type cladding layer is normally better than that of p-type cladding layer. Thus, crystal quality of MQW active layers will also be better if they are grown on top of the n-type cladding layer. Also, the conductivity of n-type layer is normally much larger than that of the p-type layer. Thus, we can achieve a much better current spreading in the bottom n-gan layer of the nitride-based n-down LED. As a result, we can achieve a smaller turn-on voltage. The other important issue for LED is the transparent upper contact. For nitride-based n-down LEDs, Manuscript received February 25, This work was supported in part by the National Science Council, Taiwan, R.O.C., under Contract NSC E , and by the Chung-Shan Institute of Science and Technology. The review of this paper was arranged by Editor P. Bhattacharya. Y. K. Su, S. J. Chang, C.-H. Ko, J. F. Chen, and T.-M. Kuan are with the Institute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, R.O.C. W. H. Lan, W.-J. Lin, and Y.-T. Cherng are with the Materials Research and Development Center, Chung-Shan Institute of Science and Technology, Taoyuan, Taiwan, R.O.C. J. Webb is with the Institute for Microstructural Sciences, National Research Council Canada, Ottawa, ON, K1A 0R6 Canada. Publisher Item Identifier /TED semi-transparent Ni/Au is normally used as the upper p-contact. The other possible contact material is the transparent ITO. In fact, ITO has already been used in AlGaInP-based LEDs as the transparent upper contact. However, it has been reported that low ITO contact resistance is only achievable on n-gan layer, while ITO on p-gan normally forms Schottky contacts. Thus, if we want to use transparent ITO as the upper contact material, we need to use the p-down structure, as shown in Fig. 1(b). To realize such an ITO p-down nitride-based LED, we need to overcome problems such as low crystal quality in the MQW active layers and poor current spreading in the bottom p-gan layer. Previously, it has been reported that one can improve the crystal quality of GaN epilayers, InGaN/GaN MQW, and GaN/AlGaN MQW by introducing a small amount of Si [8], [9], and/or In [10] [12] doping during growth. Such improvement is probably due to the fact that Si and/or In doping can effectively suppress island-like spiral structure initialed by threading dislocations in GaN. Also, it was reported that Si and/or In doping can fill point defects in GaN and reduce the number of luminescence killers. As a result, the crystal quality and optical properties can both be improved. We thus use such concepts to improve the crystal quality of nitride-based p-down LED by inserting a Mg Si or Mg In codoped layer in between p-gan layer and MQW active layers, as shown in Fig. 1(c). Another obstacle for the practical application of p-down arrangement is the high resistive bottom p-gan layer. This problem can be solved by depositing a heavily Si doped low resistive n -GaN layer, followed by a tunnel layer and a p-gan layer, as shown in Fig. 1(d). By using such a structure, carriers can tunnel freely through the tunnel layer. At the same time, the poor current spreading can also be solved with the low resistive n -GaN bottom layer. In this paper, we report the study of optical and electrical properties of these nitride-based p-down LEDs. II. EXPERIMENTS Samples used in this study were all grown by metalorgainic chemical vapor deposition (MOCVD) on (0001) sapphire substrates. During the growth, trimethylgallium (TMGa), trimethylindium (TMIn) and ammonia (NH ) were used as gallium, indium and nitrogen sources, respectively. Biscyclopentadienyl magnesium (CP Mg) and disilane (Si H ) were used as the p-type and n-type doping sources, respectively. Prior to the growth of LED structures, a low temperature 20-nm-thick GaN nucleation layer and a 1- m-thick unintentionally doped GaN buffer layer were grown on top of the /02$ IEEE

2 1362 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 8, AUGUST 2002 (a) (b) Fig. 1. layer. (c) (d) Schematic diagrams of (a) n-down LED, (b) o-down LED, (c) p-down LED with codoped interlayer, (d) p-down LED with codoped interlayer and tunnel sapphire substrates. As shown in Fig. 1(a), we subsequently grow a 2- m-thick Si-doped GaN, MQW active layers, and a 0.4- m-thick Mg-doped GaN cap layer on top of the undoped GaN buffer layer when we prepare the conventional n-down LED. The MQW active layers consist of nine pairs of 3-nm-thick undoped In Ga N well layers and 7-nm-thick Si-doped GaN barrier layers for all samples. For the p-down LED, we deposited a 2- m-thick Mg-doped GaN, MQW active layers, and a 0.4- m-thick Si-doped GaN cap layer on top of the undoped GaN buffer layer, as shown in Fig. 1(b). P-down LED with codoped layer is to insert either a ramped Mg Si codoped layer, a ramped Mg In codoped layer or a constant Mg Si codoped layer in between the p-type GaN bottom layer and MQW layers of the p-down LED, as shown in Fig. 1(c). The thickness of the codoped layer is 0.1 m. Fig. 2(a) (c) show the molar flow rate of the ramped Mg Si codoped layer (concentration is cm ), ramped Mg In codoped layer (concentration is cm ) and constant Mg Si codoped layer (concentration is cm ), respectively, during the growth. Fig. 1(d) shows the schematic structure of the p-down LED with codoped layer and tunnel layer. As can be seen from Fig. 1(d), we grow a thick heavily Si-doped GaN layer on top of the undoped GaN buffer layer. A thin tunnel layer, a p-gan layer, MQW active layers and an n-gan cap layer were then deposited on top of the heavily Si-doped GaN layer Two different tunnel layers were used in this structure. The first method to achieve the thin tunnel layer is to prepare a rough three-dimensionally (3-D) grown p-gan by increasing the flow rate of CP Mg during growth. The other method is to prepare a 20-pair short period superlattice. Each short period superlattice consists of a 2-nm-thick In Ga N layer and a 2-nm-thick GaN layer. With this structure, we can achieve a low resistive bottom layer of our nitride-based p-down LEDs. It should be noted that MQW active layers were grown after the growth of p-cladding layer for p-down LEDs, and nitrogen was used as the carrier gas when we grew the InGaN/GaN MQW active layers [7] at 760 C. Thus, Mg can be automatically activated during the growth of MQW active layers. As a result, no postgrowth thermal annealing is needed to active Mg for the p-down LEDs. The crystal quality of these epitaxial layers was evaluated by room temperature (RT) photoluminescence (PL) and double crystal X-ray diffraction (DCXRD). A Bio-Rad rpm 2000 system with a low 7-mW HeCd laser operated at 325 nm was used for PL measurement and a Bede QC2A system was used for DCXRD measurement. After PL and DCXRD measure-

3 SU et al.: InGaN/GaN LIGHT EMITTING DIODES WITH A p-down STRUCTURE 1363 Fig. 3. I V characteristics of samples A, E, F, and G. Fig. 4. RT PL spectra of n-down LED (i.e., sample A), p-down LED without codoped interlayer (i.e., sample B) and p-down LED with codoped interlayer and tunnel layer (i.e., sample G). Fig. 2. Molar flow rates of (a) ramped Mg+Si codoped layer, (b) ramped Mg+In codoped layer, and (c) constant Mg+Si codoped layer. ments, LEDs were fabricated through standard lithography and etching. Metal and/or ITO contacts were subsequently deposited onto the surface of the samples to complete the fabrication of these LEDs. RT electroluminescence (EL) characteristics of these fabricated LEDs were evaluated by injecting a different amount of DC current into these LEDs. The current voltage ( ) measurements were also performed at RT by an HP4156 semiconductor parameter analyzer. III. RESULTS AND DISCUSSIONS Fig. 3 shows the characteristics of samples A, E, F, and G. It can be seen that although the turn on voltage of p-down LED (sample E) is the highest, we can significantly reduce the turn on voltage by inserting the codoping layer and/or tunneling layer. Fig. 4 shows the RT PL spectra of n-down LED (i.e., sample A), p-down LED without codoped interlayer (i.e., sample B) and p-down LED with codoped interlayer and tunnel layer (i.e., sample G). It could be seen that sample G has the strongest RT PL intensity among the three samples. Table I lists the RT PL intensity and full-with-half-maximum (FWHM) of DCXRD spectra for all samples used in this study. It could be seen that RT PL intensity of p-down LED without codoped interlayer was much weaker than that of the n-down LED due to its poor crystal quality. It was also found that the insertion of a codoped interlayer could increase the RT PL intensity of p-down LED by more than two orders of magnitude. Such observations agree well with the DCXRD FWHM results also shown in Table I that samples with a large PL intensity exhibits a smaller DCXRD FWHM. Such a result also agrees with previous reports that the incorporation of a small amount of Si and/or In can indeed improve the crystal quality of the nitride-based epitaxial layers. Figs. 5(a) and 6(b) show the high resolution transmission electron microscopy (TEM) pictures

4 1364 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 8, AUGUST 2002 Fig. 6. RT EL spectra of p-down LED without codoped layer (i.e., sample B) and p-down LED with Mg+Si codoped layer and rough p-tunnel layer (i.e., sample G) under a 20 ma forward current injection. TABLE I RESULTS OF THE PL AND DCXRD MEASUREMENT OF THE n-down AND p-down LAYERS WITH VARIOUS MODIFICATIONS Fig. 5. TEM images of the InGaN/GaN MQW structure under (a) Mg+In codoped layer (sample D) and (b) Mg+Si codoped layer (sample C). of p-down LED with Mg In codoped interlayer (i.e., sample D) and p-down LED with Mg Si codoped interlayer (i.e., sample C). It can be seen that sample with Mg Si codoped interlayer seems to have a smoother interface than that of sample with Mg In codoped interlayer. Similar results are observed from the DCXRD and PL measurements. As can be seen from Table I, sample C has a larger PL intensity and a smaller DCXRD FWHM as compared to sample D. These observations all suggest that Mg Si codoping seems to be more effective than Mg In codoping in improving the crystal quality of nitride-based epitaxial layers. Fig. 6 shows the RT EL spectra of p-down LED without codoped layer (i.e., sample B) and p-down LED with Mg Si codoped layer and rough p-tunnel layer (i.e., sample G) under a 20 ma forward current injection. It can be seen that the EL intensity of sample G is more than 20 times larger than that of sample B. Table II lists the turn on voltage and 20 ma output power of samples A, B, E, F and G and Fig. 3 shows the curve. It was found that p-down LED without codoped inter- TABLE II DEVICE CHARACTERISTICS OF SEVERAL LED STRUCTURES layer (i.e., sample B) has the largest turn on voltage and the smallest 20 ma output power. It was also found that the insertion of an Mg Si codoped interlayer (i.e., sample E) could slightly low down the turn on voltage so as to achieve a small 0.3 mw 20 ma output power. Such an improvement is due to the improved crystal quality in sample E. On the other hand, p-down

5 SU et al.: InGaN/GaN LIGHT EMITTING DIODES WITH A p-down STRUCTURE 1365 LEDs with Mg Si codoped layer and tunnel layer (i.e., sample F and sample G) both has a much lower turn on voltage. Such a drastic reduction in turn on voltage from 15 V to less than 5 V is due to the much smaller resistivity in the heavily Si-doped n-gan bottom layer. It was also found that sample G with a rough p-tunnel layer seems to have a smaller turn on voltage and a larger 20 ma output power as compared to sample F with a superlattice tunnel layer. The reason of such an observation is not know yet and a more detailed study is under way. Also, it was found that the conventional n-down LED (i.e., sample A) still has the lowest turn on voltage and the largest 20 ma output power among these samples. Such a result suggests that a further improvement through structure optimization is needed to realize a feasible nitride-based p-down LED. IV. SUMMARY In summary, nitride-based p-down blue LEDs were fabricated. It was found that we could improve the crystal quality of these nitride-based p-down LEDs by inserting a codoped interlayer between the p-type cladding layer and MQW active layers. It was also found that the turn on voltage could be significantly reduced to less than 5 V for the p-down LED with codoped layer and tunnel layer. The 20 ma output power was 1 mw for the p-down LED with an Mg Si codoped interlayer and a rough p-tunnel layer. [12] H. M. Chung, W. C. Chuang, Y. C. Pan, C. C. Tsai, M. C. Lee, W. H. Chen, W. K. Chen, C. I. Chiang, C. H. Lin, and H. Chang, Electrical characterization of isoelectronic In-doping effects in GaN films grown by metalorganic vapor phase epitaxy, Appl. Phys. Lett., vol. 76, no. 7, pp , Y. K. Su (SM 90) was born in Kaohsiung, Taiwan, R.O.C., on August 23, He received the B.S. and Ph.D. degrees in electrical engineering from National Cheng Kung University (NCKU), Tainan, 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, Murray Hil, NJ, as a Visiting Scholar. He was also a Visiting Professor at Stuttgart University, Stuttgart, Germany, in In 1991, he became an Adjunct Professor at the State University of New York, Binghamton. He is now a Professor in the Department of Electrical Engineering, NCKU, and Director General of the Department of Engineering and Applied Science, National Science Council of Taiwan. 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), Taiwan, and He also received the Best Teaching Professor Fellowship from the Ministry of Education, Taiwan, in 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). 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, no. 13, pp , [2] S. Nakamura, Zn-doped InGaN growth and InGaN/AlGaN double-heterostructore blue-light-emitting diodes, J. Cryst. Growth, vol. 145, pp , [3] S. Nakamura, III V nitride based light-emitting devices, Solid-State Commun., vol. 102, no. 2-3, pp , [4], InGaN-based blue light-emitting diodes and laser diodes, J. Cryst. Growth, vol , pp , May [5] T. Mukai, D. Morita, and S. Nakamura, High-power UV InGaN/AlGaN double-heterostructure LEDs, J. Cryst. Growth, vol , pp , [6] J. K. Sheu, J. M. Tsai, S. C. Shei, W. C. Lai, T. C. Wen, C. H. Kou, Y. K. Su, S. J. Chang, and G. C. Chi, Low-operation voltage of InGaN/GaN light-emitting diodes with Si-doped In Ga N/GaN short-period superlattice tunneling contact layer, IEEE Electron Device Lett., vol. 22, pp , Oct [7] W. C. Lai, S. J. Chang, M. Yokoyama, J. K. Sheu, and J. F. Chen, InGaN/AlInGaN light emitting diodes, IEEE Photon. Technol. Lett., vol. 13, pp , June [8] T. Yamamoto and Y. H. Katayama, Electronic structures of p-type GaN codoped with Be or Mg as the acceptors and Si or O as the donor codopants, J. Cryst. Growth, vol , pp , [9] K. S. Kim, G. M. Yang, and H. J. Lee, The study on the growth and properties of Mg doped and Mg Si codoped p-type GaN, Solid-State Electron., vol. 43, no. 9, pp , [10] C. K. Shu, J. Ou, H. C. Lin, W. K. Chen, and M. C. Lee, Isoelectronic In-doping effect in GaN films grown by metalorganic chemical vapor deposition, Appl. Phys. Lett., vol. 73, pp , [11] S. Yamaguchi, M. Kariya, S. Nitta, H. Amano, and I. Akasaki, Strain relief and its effect on the propereies of GaN using isoelectronic In doping grown by metalorganic vapor phase epitaxy, Appl. Phys. Lett., vol. 75, no. 26, pp , S. J. Chang was born in Taipei, Taiwan, R.O.C., on January 17, He received the B.S.E.E. degree from the National Cheng Kung University (NCKU), Tainan, Taiwan, in 1983, the M.S.E.E. degree from the State University of New York, Stony Brook, in 1985, and the Ph.D.E.E. degree from the University of California, Los Angeles, in He was a Research Scientist with NTT Basic Research Laboratories, Japan, from 1989 to In 1992, he became an Associate Professor with the Electrical Engineering Department, NCKU, and was promoted to full Professor in Currently, he also serves as the Director of the Semiconductor Research Center, NCKU. He was a Visiting Scholar in the Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, Japan, from July 1999 to February 2000, and a Royal Society Visiting Scholar with the University of Wales, Swansea, U.K., from January 1999 to March His current research interests include semiconductor physics and optoelectronic devices. Chih-Hsin Ko is currently pursuing the Ph.D. degree at National Cheng Kung University (NCKU), Tainan, Taiwan, R.O.C. From 1997 to 2001, he joined the Materials and Electro-Optics Research Division, Chung-Shan Institute of Science and Technology, Taiwan. In 2000, he began study at NCKU, Tainan. Since 2001, he has been with the Institute for Microstructural Sciences, National Research Council, Ottawa, ON, Canada. His current research interests are in MOCVD and Gas-Source MBE of III V compound semicounductor materials and devices. Specific research interests include growth of wide-bandgap semiconductors (GaN-based), and their application to blue LEDs, lasers, and high-power electronic devices.

6 1366 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 8, AUGUST 2002 J. F. Chen (M 93) received the B.S. degree from the National Cheng Kung University (NCKU), Tainan, Taiwan, R.O.C., in 1990, and the M.S. and Ph.D. degrees from the University of California, Berkeley, in 1995 and 1998, all in electrical engineering. Currently, he is with the Institute of Microelectronics and the Department of Electrical Engineering, NCKU, as an Assistant Professor. His main field of research includes optoelectronic devices and reliability of integrated circuits. Wen-Jen Lin received the B.S. and M.S. in physics from the Fu-Jen University, Taipei, Taiwan, R.O.C., in 1978 and 1982, respectively, and the Ph.D. degree in electrical engineering from National Chiao-Tung University, Hsinchu, Taiwan, in He is currently the Director of the MOCVD Facility of the Materials and Electro-Optics Research Division, Chung-Shan Institute of Science and Technology. His current research focuses on GaN materials for optical and electrical applications. Ta-Ming Kuan received the B.E. degree in electrical engineering from National Cheng Kung University (NCKU), Tainan, Taiwan, R.O.C., in 2000, where he is currently a Ph.D. student in the Optical and Microwave Device Laboratory, Institute of Microelectronics. Since 2000, he has been with the Materials and Electro-Optics Research Division, Chung-Shan Institute of Science and Technology, Taiwan. His main research aspects are in crystal growth and characterization of group-iii nitrides compound semiconductor materials and devices by MOCVD. Specific research interests include growth of wide-bandgap semiconductors (GaN-based) and their optoelectronic applications. Ya-Tung Cherng received the M.S. degree in solid-state science from Syracuse University, Syracuse, NY, in 1978, and the Ph.D. degree in materials science and technology from the University of Utah, Salt Lake City, in He is the Director of the Opto-Electronic Device and Materials Section, Materials Research and Development Center, Chung Shun Institute of Science and Technology, Lung-Tan, Taiwan, R.O.C. His research interests include III V and II VI infrared detectors and materials, OMVPE of compound semiconductor materials, and electro-optic and infrared image systems. Wen How Lan (M 91) was born in Chia Yi, Taiwan, R.O.C., on December 19, He received the Ph.D. degree from the National Chiao-Tung University, Hsinchu, Taiwan, R.O.C., in While a Ph.D. candidate, he worked on several industrial projects, including photochemical vapor deposition, metal oxide semiconductor analysis, and vertical cavity semiconductor fabrication. In 1991, he joined the Research Group at Chung Shan Institute of Science and Technology to develop the technologies about the epitaxy/process/characterization of As and P-based III V compound semiconductor. In 1995, he led a group to establish a II VI molecular beam epitaxy system to affiliate the development in blue laser and succeed in the ZnSe based blue laser (RT CW) in In 1997, he also worked on the technology development in N-based III V compounds with metal organic chemical vapor deposition systems. His research interests and activities cover optoelectronic compound semiconductor devices, high band-gap materials, solid-state epitaxial growth, and electronic device characterization. He is the author of more than 40 technical papers and presentations and holds one patent in the fabrication of semiconductor devices. Jim Webb received the B.S., M.S., and Ph.D. degrees from the Physics Department, University of Waterloo, Waterloo, ON, Canada, in 1970, 1972, and 1975, respectively. He is currently with the Epitaxy Group, Institute for Microstructural Sciences, National Research Council, Ottawa, ON. His current interest is using MEB systems to grow high quality nitride-based materials and the fabrication of high speed nitride-based HFETs.