Nitride-Based LEDs With a Hybrid Al Mirror +TiO2=SiO2 ... - IEEE Xplore

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 17, SEPTEMBER 1, 2008

3131

Nitride-Based LEDs With a Hybrid Al Mirror +TiO2=SiO2 DBR Backside Reflector S. J. Chang, C. F. Shen, M. H. Hsieh, C. T. Kuo, T. K. Ko, W. S. Chen, and S. C. Shei

Abstract—Nitride-based light-emitting diodes (LEDs) with a hybrid backside reflector combining a TiO2 SiO2 distributed Bragg reflector (DBR) and an Al mirror were proposed and realized. It was found that we can significantly enhance the 35% reflectivity of the 2-pair TiO2 SiO2 DBR to 94% by combining with Al mirror (hybrid reflector). Furthermore, reflectivity of the proposed reflector depends only slightly on incident light wavelength and the incident angle. With 350-mA current injection, it was found that the output powers were 145.7, 178.2, and 201.9 mW for the LEDs with 2-pair DBR, with an Al mirror and with a hybrid reflector, respectively, when packaged in TO-cups. It was also found that reliability for the LED with the hybrid reflector is good. Index Terms—Al, distributed Bragg reflector (DBR), GaN, lightemitting diode (LED), reflector.

I. INTRODUCTION AN-BASED materials have attracted much attention due to their potential applications for optoelectronic devices. With the wide direct band gap, these materials are particularly useful for light-emitting diodes (LEDs) covering ultraviolet, blue, and green wavelength regions [1]–[3]. In recent years, these nitride-based LEDs have been commercially available for traffic signals, outdoor full-color displays, back light for liquid-crystal display (LCD) panels, and solid-state lighting. For the case of lighting, light bulbs and fluorescence tubes are currently the most commonly used light sources in our daily life. However, light bulbs consume large power. The lifetime of light bulbs is also short. On the other hand, fluorescence tubes contain materials that are hazardous to the environment. In contrast, LEDs are environment friendly, reliable and consume less power. Although it has been shown that we could combine nitride-based blue LED chips with yellow phosphors to generate white light [4]–[6], the output power of the nitride-based

G

Manuscript received December 12, 2007; revised March 16, 2008. December 19, 2008. This work was supported in part by the Center for Frontier Materials and Micro/Nano Science and Technology, National Cheng Kung University, Taiwan (D97-2700), and in part by the Advanced Optoelectronic Technology Center, National Cheng Kung University, under projects from the Ministry of Education. S. J. Chang, C. F. Shen, and W. S. Chen are with the Institute of Microelectronics and Department of Electrical Engineering, Center for Micro/Nano Science and Technology, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan, R.O.C. (e-mail: changsj@mail. ncku.edu.tw). M. H. Hsieh, C. T. Kuo, and T. K. Ko are with the Epistar Corporation, Hsin-Shi 744, Taiwan, R.O.C. S. C. Shei is with the Department of Electronic Engineering, National University of Tainan, Tainan 700, Taiwan, R.O.C. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2008.923927

white LED lamps is still low. In other words, we need to further improve the output intensity of nitride-based blue LED chips before we can realize feasible nitride-based white LED lamps. Unlike laser diodes, photons generated from LED chips could be emitted in any direction. As a result, a large portion of photons emitted from a LED chip could be lost, particularly for those photons being emitted downward to the substrate. Thus, if we could effectively reflect those photons emitted downward, we should be able to enhance the LED output intensity significantly. Since nitride-based LED structures are normally grown on transparent sapphire substrates, we should be able to reflect downward emitting photons by depositing a reflector at the backside of sapphire substrates. Indeed, it has been demonstrated that one can use either metallic mirror [7], distributed Bragg reflector (DBR) [8], or omni-directional reflector (ODR) [9] to serve as the LED backside reflector. Among the various metals, Ag exhibits the highest reflectivity in the visible wavelength region. However, the adhesion between Ag and sapphire substrate is poor. In contrast, the adhesion between Al and sapphire substrate is much better although the reflectivity of Al is slightly lower. Al is also inexpensive and can be evaporated easily. It has been shown that one can achieve a 12% increase in LED output power by simply depositing an Al mirror at the backside of sapphire substrates [7]. On the other hand, highly reflective DBR and ODR are formed from a repeated periodic stack of alternating high and low index layers. However, we need to precisely control the thickness of each layer and to choose two different materials with a large reflective index difference. To increase the reflectivity, we also need to increase the number of pairs of the DBR and/or ODR which is materials and time consuming. If we can combine the advantage of DBR(ODR) with that of metallic mirror, we should be able to achieve a high backside reflectivity while reducing the number of DBR(ODR) pairs and thus shortening the process time. In this study, we report the fabrication of GaN-based LEDs with a hybrid backside reflector combining a TiO SiO DBR and an Al mirror. Detailed fabrication process and the performance of the fabricated LEDs will also be reported. II. EXPERIMENTAL The InGaN/GaN MQW LED epitaxial layers used in this study were all grown by metalorganic chemical vapor deposition (MOCVD) on c-face 2-in sapphire Al O (0001) substrates. Details of the growth procedures can be found elsewhere [10]–[12]. The LED structure consists of a 30-nm-thick GaN nucleation layer grown at 550 C, a 3- m-thick Si-doped n-GaN buffer layer grown at 1050 C, an unintentionally doped InGaN/GaN multiquantum well (MQW) active region grown at

0733-8724/$25.00 © 2008 IEEE

3132

770 C, a 50-nm-thick Mg-doped Al Ga N electron blocking layer grown at 1050 C, a 0.6- m-thick Mg-doped p-GaN layer grown at 1050 C. To enhance output power, we textured the sample surface by subsequently stopping the TMGa flow while maintaining CP Mg flow for 5 min. A second p-GaN contact layer was then grown again after this Mg-treatment process [13]. Finally, a Si-doped -short period superlattice (SPS) tunnel contact structure was grown on the p-GaN contact layer to improve the p-ohmic contact [14]. The InGaN/GaN MQW active region consists of five pairs of 3-nm-thick In Ga N well layers and 7-nm-thick GaN barrier layers. The -SPS tunnel contact structure consists of In Ga N/GaN (5 /5 ). The as-grown four pairs of samples were subsequently annealed at 750 C in N ambient to active Mg in the p-type layers [15], [16]. It should be noted that we can achieve a rough LED sample surface with the Mg-treatment process. As a result, we can reduce total internal reflection inside the LEDs and thus enhance the LED light extraction efficiency, as compared to the previously reported LEDs with a flat sample surface [7]. Surfaces of the samples were partially etched to expose the n-type GaN layer. We subsequently deposited a 230-nm-thick indium-tin oxide (ITO) layer onto the sample surface (i.e., -SPS) to serve as the transparent contact layer Si-doped (TCL). On the other hand, Ti/Al/Ti/Au contact was evaporated onto the exposed n-type GaN layer to serve as the n-type electrode. The epitaxial wafers were then lapped down to 100 m. The wafers were then soft polished for 60 min to remove the stress between the lapped surface and the epitaxial surface. The lapped and polished LED wafers were then cleaned in HCl and HF to remove residual contaminants on both sides of the wafers. An Al mirror layer or a SiO TiO DBR structure was subsequently deposited onto the backside surface of the lapped and polished LED wafers, by e-gun evaporation, to serve as the reflector. During deposition, a thickness controller with a quartz oscillator was used to monitor the thickness of each layer. To maximize the reflectivity, we used different metallic mirror, DBR structures with different numbers of pairs, and DBR or M-DBR). hybrid backside reflectors (i.e., metal During deposition, we kept the thickness of the metallic mirrors at 500 nm. For the DBR deposition, we kept the thickness of layer and each TiO layer at each SiO 787 and 456 , respectively. After deposition, we used a Hitachi U3010 spectrophotometer to measure the reflectance of the deposited reflectors. We then used scribe and break to fabricate 1 1 mm LED chips. Fig. 1 shows schematic diagram of the LED chip with hybrid backside reflector proposed in this study. It can be seen clearly that surface of the fabricated LED chips were textured. To package these LEDs, we can use either SMD-cups or TO-cups. Compared with TO-cups, it is known that SMD-cups are more reflective in the blue/green region. To minimize the effects of packaging, we thus placed each of the LED chips in a TO-cup and then packaged them into LED lamps. Current-voltage (I-V) measurements of the fabricated LED lamps were then performed by an HP4155 semiconductor parameter analyzer. Intensity-current (L-I) characteristics were measured from top of devices using molded LEDs with an integrated

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 17, SEPTEMBER 1, 2008

Fig. 1. Schematic diagram of the LED with hybrid backside reflector proposed in this study.

Fig. 2. Structure used for the calculation of reflection spectra.

Fig. 3. Calculated reflection spectra for DBR structures with an incident angle of 5 .

sphere detector by injecting different amounts of DC current into these samples. Also, the reliability of these LED lamps was also evaluated by injection 450-mA DC current at room temperature for more than 1000 h. III. RESULTS AND DISCUSSION We first performed some theoretical studies on the DBR backside reflectors used in this study. In our simulation, we used TFCalc. to calculate reflection spectra of the structure shown , , in Fig. 2. In the calculation, we used and as the reflective indices of sapphire substrate, and SiO , respectively. . We also TiO used 787 and 456 as the thickness of each SiO layer and

CHANG et al.: NITRIDE-BASED LEDS WITH A HYBRID AL MIRROR

TiO

SiO DBR BACKSIDE REFLECTOR

3133

Fig. 4. Reflection spectra for the DBR structures measured with an incident angle of (a) 5 , (b) 30 , and (c) 60 .

Fig. 5. Reflection spectra of Ag, Al and M-DRB measured with an incident angle of (a) 5 , (b) 30 , and (c) 60 .

each TiO layer, respectively. Fig. 3 shows calculated reflection spectra for the DBR structures measured with an incident angle

of 5 . On the other hand, Fig. 4(a) shows measured reflection spectra for the DBR structures measured with an incident angle

3134

of 5 . It can be seen that reflectivity for the 2-pair DBR structure (i.e., 2DBR) is only 35% at 460 nm. As we increased number of DBR pairs, however, it was found that reflectivity can reach 92%, 97%, and 98% at 460 nm as we increased the number of DBR pair to 8, 10, and 12, respectively. This should be attributed to the fact that only portion of the light wave will be reflected at each DBR interface. Thus, we could increase the reflectivity by increasing the number of the DBR pairs. It should be noted that the experimentally measured reflection spectra shown in Fig. 4(a) seem to be lower and narrower as compared with those calculated spectra shown in Fig. 3. This is probably due to the thickness deviation between the designated SiO TiO layers and the actually deposited SiO TiO layers. Fig. 4(b) and (c) shows reflection spectra for the DBR structures measured with an incident angle of 30 and 60 , respectively. It was found again that reflectivity increased as we increased the number of DBR pairs. It was also found from these three figures that the reflection spectra of DBR depend strongly on the light incident angle. Furthermore, the highly reflective bands became narrower as we increased the incident angle. Compared with DBR structures, reflectivity of metallic mirrors is much less wavelength dependent and much less incident angle dependent. Fig. 5(a) shows reflection spectra of Ag and Al measured with an incident angle of 5 . It can be seen that in the wavelength rereflectivity of Ag is around gion between 450 to 530 nm. On the other hand, reflectivity in the same wavelength region. of Al is around Although Ag exhibits higher reflectivity, we chose Al mirror for the hybrid reflector used in this study due to the aforementioned reason. Reflection spectrum of the hybrid reflector combining Al mirror with 2-pair TiO SiO DBR (M-2DBR) is also plotted in Fig. 5(a). With the Al mirror, it was found that we can significantly enhance the 35% at 460-nm reflectivity shown in Fig. 4(a) to 94%. Such a reflectivity is also much higher than the 82% reflectivity measured from the single Al mirror. Reflection spectra of M-4DBR, M-6DBR, and M-8DBR are also plotted in the same figure. Although the reflectivities of these three reflectors are all larger than 97% at 460 nm, the 94% reflectivity measured from M-2DBR should be good enough for practical device applications. Fig. 5(b) and (c) shows reflection spectra for Al mirror, Ag mirror and M-2DBR reflector measured with an incident angle of 30 and 60 , respectively. It can be seen that reflectivity of the M-2DBR reflector is also high and depends only slightly on incident light wavelength at these two incident angles. Fig. 6 shows normalized electroluminescence (EL) spectra of the fabricated LEDs. With 350-mA current injection, it was found that peak wavelengths were 457.1, 458.7, and 457.9 nm, respectively, while full-width-half-maxima (FWHM) of the EL peaks were 23.4, 21.8, and 22.3 nm, respectively, for the LEDs with 2-DBR, with an Al mirror and with a hybrid (M-2DBR) reflector. The almost identical peak wavelength and FWHM should be attributed to exactly the same MQW structure used in these three devices. Fig. 7 shows L-I-V characteristics of LED with an Al mirror, LED with 2-DBR and LED with a hybrid (M-2DBR) reflector. It was found that I-V curves of these three LEDs are almost identical again due to the same epitaxial structure. With the same 350-mA current injection, it was found that

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 17, SEPTEMBER 1, 2008

Fig. 6. Room temperature EL spectra of the LEDs with Al mirror, 2DBR mirror and M-2DBR mirror.

Fig. 7. L-I-V characteristic of the fabricated LEDs.

forward voltages were all 3.59 V for these devices. On the other hand, L-I characteristics of these three LEDs are quiet different. Among the three devices, it was found that output intensity of the LED with a hybrid (M-2DBR) reflector was the highest, followed by the LED with an Al mirror. On the other hand, output intensity of the LED with 2-DBR was the weakest. With 350-mA current injection, it was found that output powers were 145.7, 178.2, and 201.9 mW for the LEDs with 2-DBR, with an Al mirror and with a hybrid (M-2DBR) reflector, respectively. In other words, we achieved the largest output power from the LED with hybrid reflector again due to the high reflectivity of the hybrid reflector. The largest output power observed from the LED with a hybrid reflector should be attributed to the high reflectivity of the hybrid reflector. Fig. 8 shows life tests of relative luminous intensity measured from the three fabricated LEDs, normalized to their respective initial readings. During life test, the LEDs were driven by 450-mA injection current at room temperature. It can be seen that the EL intensity decreased by only 6.8% after 1000 h for the LEDs with 2-DBR. Such a result indicates that heat conduction of the 2-pair dielectric DBR is still reasonably good. On the other hand, EL intensity decreased by 8.3% after 1000 h for the

CHANG et al.: NITRIDE-BASED LEDS WITH A HYBRID AL MIRROR

TiO

SiO DBR BACKSIDE REFLECTOR

3135

[4] M. Yamada, Y. Narukawa, and T. Mukai, “Phosphor free high-luminous-efficiency white light-emitting diodes composed of InGaN multiquantum well,” Jpn. J. Appl. Phys., vol. 41, pp. L246–L248, 2002. [5] S. Nakamura and G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers. Heidelberg, Germany: Springer, 1997. Ce [6] J. H. Yum, S. Y. Seo, S. H. Lee, and Y. E. Sung, “Y Al O phosphor coatings on gallium nitride for white light emitting diodes,” J. Electrochem. Soc., vol. 150, pp. H47–H52, 2003. [7] C. S. Chang, S. J. Chang, Y. K. Su, W. S. Chen, C. F. Shen, S. C. Shei, and H. M. Lo, “Nitride based power chip with indium-tin-oxide p-contact and Al back-side reflector,” Jpn. J. Appl. Phys., vol. 44, pp. 2462–2464, 2005. [8] Y. S. Zhao, D. L. Hibbard, H. P. Lee, K. Ma, W. So, and H. Liu, “Efficiency enhancement of InGaN/GaN light-emitting diodes with a back-surface distributed bragg reflector,” J. Electron. Mater., vol. 32, pp. 1523–1526, 2003. [9] C. H. Lin, J. Y. Tsai, C. C. Kao, H. C. Kuo, C. C. Yu, J. R. Lo, and K. M. Leung, “Enhanced light output in InGaN-based light-emitting diodes with omnidirectional one-dimensional photonic crystals,” Jpn. J. Appl. Phys., vol. 45, pp. 1591–1593, 2006. [10] S. J. Chang, S. C. Wei, Y. K. Su, R. W. Chuang, S. M. Chen, and W. L. Li, “Nitride-based LEDs with MQW active regions grown by different temperature profiles,” IEEE Photon. Technol. Lett., vol. 17, no. 9, pp. 1806–1808, Sep. 2005. [11] S. J. Chang, C. S. Chang, Y. K. Su, C. T. Lee, W. S. Chen, C. F. Shen, Y. P. Hsu, S. C. Shei, and H. M. Lo, “Nitride-based flip-chip ITO LEDs,” IEEE Tran. Adv. Packag., vol. 28, no. 2, pp. 273–277, May 2005. [12] 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. [13] C. M. Tsai, J. K. Sheu, W. C. Lai, Y. P. Hsu, P. T. Wang, C. T. Kuo, C. W. Kuo, S. J. Chang, and Y. K. Su, “Enhanced output power in GaN-based LEDs with naturally textured surface grown by MOCVD,” IEEE Electron. Device Lett., vol. 26, no. 7, pp. 464–466, Jul. 2005. [14] 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 SP S IT O upper contacts,” IEEE J. Quant. Electron., vol. 39, no. 11, pp. 1439–1443, Nov. 2003. [15] S. Nakamura, T. Mukai, M. Senoh, and N. Iwasa, “Thermal annealing effects on p-type Mg-doped GaN films,” Jpn. J. Appl. Phys., vol. 31, pp. L139–L142, 1992. [16] S. Nakamura, N. Iwasa, M. Senoh, and T. Mukai, “Hole compensation mechanism of p-type GaN films,” Jpn. J. Appl. Phys., vol. 31, pp. 1258–1266, 1992.

:

Fig. 8. Life tests of relative luminous intensity measured from the fabricated LEDs, normalized to their respective initial readings.

LED with an Al mirror. The slightly larger EL intensity decay is probably due to the oxidation of the Al layer, which may result in a reduced reflectivity. It should be noted that the 8.3% decay is acceptable for high power LEDs. On the other hands, EL intensity decreased by 7.3% for the LED with hybrid (M-2DBR) reflector after 1000 h. It should be noted that the hybrid reflector suffers from both Al oxidation and relatively poor thermal conduction of the dielectric DBR. Compared to the LED with an Al mirror, the smaller degradation observed from the LED with hybrid reflector suggests the high reflectivity of DBR could result in less heat generation. It is possible that the effect of less heat generation is more important than that of the relatively poor thermal conduction induced by the dielectric DBR. Further experiments on junction temperature measurement are needed to clarify this point.

+

IV. CONCLUSION In summary, nitride-based LEDs with a hybrid Al mirror TiO SiO DBR backside reflector were proposed and realized. With the Al mirror, it was found that we can significantly enhance the 35% reflectivity of the 2-pair TiO SiO DBR to 94%. It was also found that reflectivity of the M-2DBR reflector depends only slightly on incident light wavelength and the incident angle. With 350-mA current injection, it was found that output powers were 145.7, 178.2, and 201.9 mW for the LEDs with 2-DBR, with an Al mirror and with a hybrid (M-2DBR) reflector, respectively, when packaged in TO-cups. Furthermore, it was found that we can thus achieve better device reliability from the LED with the hybrid reflector, as compared to the LED with a simple Al mirror. REFERENCES [1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamade, H. Kiyoko, and Y. Sugimoto, “InGaN-based multi-quantum-well-structure laser diodes,” Jpn. J. Appl. Phys., vol. 35, pp. L74–L76, 1996. [2] 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 Quant. Electron., vol. 8, no. 2, pp. 278–283, Mar. –Apr. 2002. [3] C. F. Shih, N. C. Chen, C. A. Chang, and K. S. Liu, “Blue, green and white InGaN light-emitting diodes grown on Si,” Jpn. J. Appl. Phys., vol. 44, pp. L140–L143, 2005.

S. J. Chang was born in Taipei, Taiwan, R.O.C., 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 at Nippon Telegraph and Telephone (NTT) Basic Research Laboratories, Musashino, 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 is currently the Deputy Director of the Center for Micro/Nano Science and Technology, and the Director of Semiconductor Research Center, NCKU. He was a Royal Society Visiting Scholar at the University of Wales, Swansea, U.K. from January 1999 to March 1999; a Visiting Scholar at the Research Center for Advanced Science and Technology, University of Tokyo, Japan, from July 1999 to February 2000; a Visiting Scholar at the Institute of Microstructural Science, National Research Council, Canada, from August 2001 to September 2001; a Visiting Scholar at the Institute of Physics, Stuttgart University, Germany, from August 2002 to September 2002; and a Visiting Scholar at the Faculty of Engineering, Waseda University, Japan, from July 2005 to September 2005. He is also an honorary Professor at the Changchun University of Science and Technology, China. His current research interests include semiconductor physics, optoelectronic devices, and nanotechnology. Prof. Chang is the recipient of the outstanding research award from the National Science Council, Taiwan, in 2004.

3136

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 17, SEPTEMBER 1, 2008

C. F. Shen was born in Yun-Lin, Taiwan, R.O.C., in 1981. He received his B.S. degree from the Department of Electrical Engineering and the M.S. degree from the Institute of Microelectronics, National Cheng Kung University (NCKU), Tainan, Taiwan, in 2003 and 2005, respectively. Currently, he is pursuing the Ph.D. degree in nitride-based optoelectronic devices at the Institute of Microelectronics, NCKU, Tainan.

W. S. Chen was born in Chia-Yi, Taiwan, R.O.C., in 1977. He received the B.S. degree from the Department of Electrical Engineering and the M.S. degree from the Institute of Microelectronics, National Cheng Kung University (NCKU), Tainan, Taiwan, in 2001 and 2003, respectively. Currently, he is pursuing the Ph.D. degree in nitride-based optoelectronic devices at the Institute of Microelectronics, NCKU, Tainan.

M. H. Hsieh, photograph and biography not available at the time of publication.

S. C. Shei received the B.S., M.S., and Ph.D. degrees from the Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, R.O.C., in 1988, 1990, and 1995. His major field is focused on III-V optoelectronic semiconductors. Currently, he is an Assistant Professor with the Department of Electronic Engineering, National University of Tainan, Tainan.

C. T. Kuo, photograph and biography not available at the time of publication.

T. K. Ko was born in Chang-Hua, Taiwan, R.O.C., in 1980. He received the B.S. degree from the Department of Electrical Engineering and the M.S. and Ph.D. degrees from the Institute of Microelectronics, National Cheng Kung University, Tainan, Taiwan, in 2002, 2004, and 2007, respectively. Currently, he is a member of technical staff with the Epistar Corporation, Hsin-Shi, Taiwan.