LIGHT-EMITTING DIODES (LEDs) based on InGaN

Transcription

1 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 11, NOVEMBER Efficiency Improvement of Near-Ultraviolet InGaN LEDs Using Patterned Sapphire Substrates Woei-Kai Wang, Dong-Sing Wuu, Shu-Hei Lin, Pin Han, Ray-Hua Horng, Ta-Cheng Hsu, Donald Tai-Chan Huo, Ming-Jiunn Jou, Yuan-Hsin Yu, and Aikey Lin Abstract The use of conventional and patterned sapphire substrates (PSSs) to fabricate InGaN-based near-ultraviolet (410 nm) light-emitting diodes (LEDs) was demonstrated. The PSS was prepared using a periodic hole pattern (diameter: 3 m; spacing: 3 m) on the (0001) sapphire with different etching depths. From transmission-electron-microscopy and etch-pit-density studies, the PSS with an optimum pattern depth ( =15 m) was confirmed to be an efficient way to reduce the thread dislocations in the GaN microstructure. It was found that the output power increased from 8.6 to 10.4 mw, corresponding to about 29% increases in the external quantum efficiency. However, the internal quantum efficiency (@ 20 ma) was about 36% and 38% for the conventional and PSS LEDs, respectively. The achieved improvement of the output power is not only due to the improvement of the internal quantum efficiency upon decreasing the dislocation density, but also due to the enhancement of the extraction efficiency using the PSS. Finally, better long-time reliability of the PSS LED performance was observed. Index Terms GaN, InGaN, light-emitting diode (LED), near ultraviolet (UV), patterned sapphire substrate (PSS). I. INTRODUCTION LIGHT-EMITTING DIODES (LEDs) based on InGaN semiconductor materials have become commercialized products in recent years [1], [2]. These devices have already been extensively used in outdoor displays, traffic lights and general lighting and show a greater potential to replace incandescent bulbs and fluorescent lamps. Nevertheless, further progress is still strongly desired in order for these devices to reach efficiency levels achievable with other III V materials systems. Currently, the most commonly used method to achieve white-light LEDs is to combine an yttrium-aluminum-garnet phosphor wavelength converted with a GaN blue LED chip. Ultraviolet (UV) LEDs can be used as a pumping source Manuscript received May 4, 2005; revised August 5, This work was supported by National Science Council under Contract NSC E W.-K. Wang and D.-S. Wuu are with the Department of Materials Engineering, National Chung Hsing University, Taichung 402, Taiwan, R.O.C. ( dsw@dragon.nchu.edu.tw). S.-H. Lin is with the Institute of Electro Optics and Materials Science, National Fomosa University, Taiwan, R.O.C. P. Han and R.-H. Horng are with the Institute of Precision Engineering, National Chung Hsing University, Taichung 402, Taiwan, R.O.C. ( huahorng@dragon.nchu.edu.tw). T.-C. Hsu, D. T.-C. Huo, and M.-J. Jou are with Epistar Corporation, Hsinchu 300, Taiwan, R.O.C. Y.-H. Yu and A. Lin are with Wafer Works Corporation, Taoyuan 326, Taiwan, R.O.C. Digital Object Identifier /JQE for developing white-light LEDs to solve the low color-rendering-index problem [3], [4]. However, UV LEDs are more sensitive to dislocation than blue GaN-based LEDs, as indicated from previous studies [5]. It is well known that a dislocation density in the order of cm is inherent in the epitaxial GaN films on sapphire substrates due to the large lattice mismatch. High dislocation density will influence the device characteristics, such as device lifetime, electron mobility, and the quantum efficiency of radiative recombination. Therefore, how to further reduce the dislocation density is an important issue for fabricating high-performance UV LEDs. Many different growth approaches have been proposed for threading dislocation density reduction [6], [7]. Lateral epitaxial overgrowth is a commonly used technique utilizing metal organic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy to reduce the threading dislocation density to the 10 cm range. In this method, a GaN epilayer with several m in thickness is first grown onto a sapphire substrate [8], [9]. Subsequently, a SiN or SiO strip type mask is produced, followed by epitaxial growth. Recently, several groups have recently demonstrated direct lateral epitaxy growth onto a stripe-type patterned sapphire substrate (PSS). Tadatomo. et al. have adopted models of parallel grooves along the (11 2 0) sapphire to fabricate nitride LEDs [10], [11]. Bell. et al. have used the trenches of sapphire (11 2 0) direction to grow the Mg-doped AlGaN epilayers [12]. A hexagon pattern parallel with the a axis of the sapphire was reported by Yamada. et al. [13]. Chang et al. [14] have described the use of parallel stripes along the sapphire (1 00) direction for their blue LED growth. In our previously study [15], a considerably improved output power of InGaN-based UV LEDs on PSSs was obtained. The PSS was prepared using a periodic hole pattern (diameter: 3 m; spacing: 3 m) on the (0001) sapphire with different etching depths. The proposed method can reduce the TDs via a single growth process without any interruption or deposition onto the SiO mask. It also eliminates the need for a precise photolithography process to transfer a special pattern axis and prevents the induced contamination. We also found the enhancement of optical reflection from the GaN/sapphire interface. To understand and apply the PSS to GaN device fabrication, it is important to know the influence of the optical scattering and dislocations in the material. In this paper, we report on the microstructure, electrical and optical properties of near-uv InGaN-based LEDs grown on both PSS and conventional sapphire substrates. The light emissions and physical characteristics of the fabricated LEDs were investigated. The reliability of these fabricated LEDs will also be described /$ IEEE

2 1404 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 11, NOVEMBER 2005 Fig. 1. (a) Schematic diagram of near-uv InGaN PSS LED. (b) SEM micrograph of bare PSS (D =1:5 m) before MOCVD growth. II. EXPERIMENT The samples used in this study were all grown over 2-in (0001) sapphire substrates in a MOCVD system. A schematic diagram of the InGaN PSS-LED structure is shown in Fig. 1(a). The PSS was prepared using a periodic hole pattern, where the hole etching depths ranged from 0.5 to 1.5 m. The hole array (diameter: 3 m; spacing: 3 m) was generated using a standard photolithography process. The sapphire substrate was etched using BCl Cl in an inductively coupled-plasma etcher at a typical dc bias of 100 V. After etching, the corresponding micrograph of the PSS was examined by scanning electron microscopy (SEM, (JEOL-JSM 6360) and shown in Fig. 1(b), where a 1.5- m etching depth with a sidewall angle of 75 was achieved. During the MOCVD growth, trimethylgallium, trimethylindium and ammonia were used as the gallium, indium and nitrogen precursors. Biscyclopentadienyl magnesium and disilane were employed as the p- and n-type dopant sources, respectively. The carrier gas was hydrogen through the growth except that nitrogen was used for the InGaN growth. The reactor pressure was maintained at 100 torr throughout the growth process. Prior to the growth, sapphire substrates were thermally baked at 1100 C in hydrogen gas to remove surface contamination. The LED structure consisted of a 30-nm-thick GaN low-temperature buffer layer, a 1.5- m-thick layer of undoped GaN, a 30-nm-thick GaN low-temperature buffer layer, a 1.5- m-thick layer of undoped GaN, a 4- m-thick layer of n-type GaN: Si, a n-type Al Ga N GaN layer, an multiple quantum-wells (MQWs) active layer, a p-type Al Ga N GaN layer and a 0.3- m-thick p-type GaN: Mg layer. The LED sample used in this research had a chip size of 365 m 365 m, fabricated using standard photolithography and dry etch techniques. The Ni Au and Ti Al Ti Au metal contacts were deposited as p-type and n-type GaN, respectively. Note that the active layers in the conventional and PSS LEDs were grown under the same growth run. The GaN sample was polished with a chemical mechanical polishing (Logitech MP-5) process to produce tilt angle GaN surface and the microstructure of the sample was characterized cross-sectional transmission electron microscopy (TEM, JEOL-JEM 2010). TEM samples were prepared using standard mechanical polishing (down to a thickness below 20 m) and Ar ion-milling (Gatan 600-DIF, 1 ma@ 4 kev) techniques to Fig. 2. Cross section TEM image of GaN epilayer grown on (a) conventional sapphire substrate, (b) PSS with D =0:5 m, and (c) PSS with D = 1:5 m. achieve electron transparency. Photoluminescence (PL) mapping (ACC-PLM 100) is used to investigate optical properties of the samples at room temperature using a He Cd laser (325 nm) as an excitation source. The current voltage ( ) characteristics of the LEDs were measured using an Agilent 4155B semiconductor parameter analyzer. The output power of the LED lamp was measured using an integrated sphere detector and the measured deviation was around 5%. III. RESULTS AND DISCUSSION Cross-sectional TEM measurements were performed to investigate the dislocation distribution of the GaN-on-PSS samples with various values. For comparison, the TEM micrograph of the GaN grown on a conventional sapphire substrate is also illustrated in Fig. 2(a). It can be seen clearly that a large number of extended TDs propagate throughout the GaN film, originating from the GaN sapphire interface. The generation of these dislocations is caused by the large lattice mismatch between GaN and sapphire. For the sample grown on the PSS with m, the GaN epilayer buried the cavity incompletely and some small voids near the cavity edge were observed

3 WANG et al.: EFFICIENCY IMPROVEMENT OF NEAR-ULTRAVIOLET InGaN LEDs USING PATTERNED SAPPHIRE SUBSTRATES 1405 Fig. 3. X-ray rocking curves of (0002) reflections for GaN grown on PSS (D = 1:5 m) and conventional sapphire substrate. as shown in Fig. 2(b). These dislocations were generated randomly. For the sample grown on the PSS with m, the GaN epilayer grew laterally from the top of the sapphire substrate and some voids (0.5 m in size) that formed when two growth front boundaries coalesced were observed on the pattern sidewall. The undoped GaN also grew into the sidewall of the trench. Fig. 2(c) shows the representative distributions of the 90 bending dislocations in the lateral growth region in the GaN-on-PSS m heterostructure. The observed angle might be determined using the lateral/vertical growth rate ratio. These dislocations did not subsequently propagate to the surface of the overgrown GaN layer. Above these voids, the dislocations do seldom observed. These voids are associated with the relaxed morphologies of the GaN film side faces and usually lead to threading dislocation bending in the direction of these voids. Hence, free standing laterally grown GaN films were achieved. Evidence of dislocation reduction is also obtained from the etch pit density (EPD) measurements, where the GaN samples were chemically etched in a H SO H PO (1:3) mixture at 250 C for 10 min. It was found that the EPD was around cm for the sample with m (i.e., conventional sapphire substrate) and decreased to cm for the sample with m. These results indicated that a significant reduction in the dislocation density was achieved via the lateral epitaxial overgrowth on the PSS without a SiO mask. Fig. 3 shows the crystallinity of the GaN samples examined by double-crystal X-ray diffractometry. The average full-width at half maximum of the (0002) X-ray rocking curves of GaN films grown on PSS and conventional sapphire substrate were found to be 320 and 360 arcsec, respectively. The crystallinity improvement examined by X-ray measurements also showed in good agreement with the result revealed by TEM observations. The growth evolution of the GaN-on-PSS can be examined by plane-view SEM as shown in Fig. 4(a), where the sample was polished with an inclined angle. A schematic diagram of the corresponding epi-structure is shown in Fig. 4(b). The GaN was vertically grown from the bottom of the sapphire substrate and extended forming lateral epitaxial growth over the trenches with (11 2 0) facets. The GaN was kept (0001) and (11 20)or (11 2 2) facets until coalescence. It can be observed that the void in the epilayer was gradually changed from circle to triangle and Fig. 4. (a) SEM micrograph of GaN-on-PSS sample after tilt-angle polishing. (b) Schematic diagram of cross section heterostructure in (a). Fig. 5. Schematic diagram of evolution of GaN grown on patterned sapphire substrate. finally disappeared. Based on the above TEM and SEM observations, the growth evolution of the GaN-on-PSS can be schematically shown in Fig. 5. A thin (0001) GaN epilayer was first grown onto a (0001) PSS [Fig. 5(a)]. The GaN also grew into the bottom and sidewall of the trench. Then the GaN epilayer grew laterally with a new shape (11 2 2) facets [Fig. 5(b)]. When the growth continued, the coalescence of the GaN occurred and the epitaxial lateral overgrowth was formed [Fig. 5(c)]. Finally, a flat GaN epilayer was obtained on the PSS [Fig. 5(d)]. Fig. 6 shows the room-temperature PL intensity mapping obtained from the near-uv InGaN LED sample grown on a 2-in sapphire substrate where there exist four zones with various

4 1406 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 11, NOVEMBER 2005 Fig. 6. Room temperature photoluminescence intensity mapping of PSS LEDs with different etching depths (D =0, 0.5, 1.0 and 1.5 m). Fig. 8. Internal quantum efficiency of conventional and PSS (D = 1:5 m) InGaN LEDs measured at various temperatures (@20 ma). Fig. 7. Trace-Pro simulated EL intensity of PSS LED with various D values. etching depths (i.e., and m). It was found that the PL peak around 3.07 ev (403 nm) increased remarkably as the increased. At the m zone, the near bandedge PL intensity is obviously stronger than that of the LED with m (conventional sapphire substrate). Such a significant enhancement in PL intensity due to the decreases of trap densities and the dislocation-induced nonradiative recombination centers indicates that the PSS can significantly improve the quality of GaN epilayers. It has been reported that the radiative recombination efficiency increases as the EPD decreases from 3 10 to cm and saturates at less than cm [16]. Our experimental results indicated that the EPD of the PSS LED is about cm. Hence, the improvement in PL intensity might also be related to the enhancement of the light extraction efficiency through light scattering from the nitride epilayer and the PSS interface. The value of may play a more important role in the enhancement of optical properties of the near-uv InGaN GaN LEDs. Fig. 7 shows a Trace-Pro simulation profile of the ray extraction ratio rate take from the PSS InGaN LED as a function of the value. Here a 1-mW power ( light rays) is assumed to emit randomly from the InGaN/GaN MQW active layer, i.e., the spontaneous emission process. The output power can be calculated by collecting the light rays which hit the observation plane. It was found that there is up to about 70% higher light output at m as compared with that of the unpatterned structure m. Moreover, the ray extraction ratio rate increased as the increased and saturated when the reached above 1.0 m. These results indicate that the has a large effect on the improvement of light extraction efficiency of the PSS LED. However, the present simulation did not consider the absorption in the epi-structure and the reflector effect from the silver cup in the epoxy lamp form. Thus, this simplified simulation can only be used to explain a rough trend of electroluminescence (EL) emission intensity versus the hole etching depth. Since the PSS LED with m shows the best performance as evidenced by the TEM and PL mapping results, we only focus on the m samples in the following work. Moreover, the forward I V characteristics of the near-uv LEDs with and without PSS at room temperature were also investigated [15]. The corresponding forward voltages at 20 ma were 3.83 and 3.84 V, respectively. This indicates that the PSS LED has similar I V characteristic as compared with that of the conventional LED. To clarify the influence of dislocation reduction on EL intensity, we estimated the internal quantum efficiency of the InGaN LED sample roughly using the temperature dependence of the integrated EL intensity. Generally, the value at low temperatures can be regarded as 100% when neglecting the nonradiative recombination process. As shown in Fig. 8, the integrated EL intensities of both the conventional and PSS LEDs were nearly constant below 200 K and declined gradually with a further increase in temperature. At room temperature, the value (@ 20 ma) was about 36% and 38% for the conventional and PSS LEDs, respectively. No significant difference in the values was observed. These results suggest that the enhanced output power could not only be attributed to the improvement in from decreasing the dislocation density. In order to measure the LED output power, the chips were encapsulated in conventional lamp form (5 mm in diameter). The EL emission was measured from the LED top surface. Fig. 9(a) shows the light output power of the conventional and PSS LEDs as a function of injection current. The output intensity of both LEDs initially increases linearly with the injection current. With a 20-mA forward injection current, the output power of a lampform PSS LED and conventional LED were estimated to be 10.4

5 WANG et al.: EFFICIENCY IMPROVEMENT OF NEAR-ULTRAVIOLET InGaN LEDs USING PATTERNED SAPPHIRE SUBSTRATES 1407 TABLE I ESTIMATED INTERNAL QUANTUM EFFICIENCY AND LIGHT EXTRACTION EFFICIENCY FOR PSS LED AND CONVENTIONAL LED AT A 20-mA CURRENT INJECTION AT ROOM TEMPERATURE Fig. 10. Room-temperature reliability test of relative luminous intensity of conventional and PSS (D =1:5 m) InGaN LEDs driven at 20 ma. Fig. 9. (a) Light output power of the conventional and PSS LEDs as a function of injection current. (b) External quantum efficiency of conventional and PSS (D =1:5 m) InGaN LEDs measured at various forward current injections. and 8.6 mw, respectively. Fig. 9(b) shows the external quantum efficiency of the InGaN LED sample with various forward injection currents up to 100 ma. It was found that the of the PSS LED reached a maximum value of 20 ma) and then decreased significantly with a further increase in the forward bias current. Nearly the same trend was also obtained for the conventional LED sample except for a lower peak value of 20 ma).the degradation in at the higher injection level might be related to carrier saturation and/or the joule heating effect. The can be expressed as where is the light extraction efficiency and the current injection efficiency is assumed to be 100%. The values of the conventional and PSS LEDs are estimated to be 32 and 37%, respectively. Details of the,, and data for both LEDs are summarized in Table I. Clearly, the enhancement in plays a more important role in obtaining the higher value of the InGaN PSS LED. A prime concern of the PSS LEDs is their reliability revealed by the lifetime test. Fig. 10 shows the relative EL intensity of the conventional and PSS InGaN LEDs under a forward current of 20 ma at room temperature during the 1000-h test. The relative EL intensity to the initial EL intensity is shown as a function (1) of the aging time. It can be seen that the relative EL intensity exhibited the same degradation trend for both LEDs. The PSS LED presents a gradual degradation in the EL intensity with an 8% decreases after 24 h of the test. After 1000 h, the EL intensity of PSS LED and conventional LED are decayed by 18% and 23%, respectively. The smaller decrease in the intensity of EL was observed in the LED sample having PSS, as compared with the conventional LED. This result suggests indicated that improvement of the EL intensity due to the decreases in trap densities and the TD-induced nonradiative recombination centers via grown on PSS. Even though, the PSS LEDs may still suffer the incomplete step coverage problem at the GaN PSS interface, there is no evident difference in life time as indicated from our measurement results between the conventional and PSS LED samples. IV. CONCLUSION We described the characteristics of the near-uv InGaN-based LEDs grown on the conventional and PSSs. The PSS was prepared using a periodic hole pattern on the (0001) sapphire substrate with different etching depths ranged from 0.5 to 1.5 m. For the PSS LED, the GaN epilayer grew laterally from the top of the sapphire substrate and overhung the trench. From the TEM and EPD studies, the use of PSS with an optimum pattern depth m was confirmed to be an efficient way to reduce the TDs in the GaN microstructure. At room temperature, the value (@ 20 ma) was about 36% and 38% for the conventional and PSS LEDs, respectively. It was found that the of the PSS LED reached a maximum value of 14.1% (@ 20 ma) and then decreased significantly with a further increase in the forward bias current. Nearly the same

6 1408 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 11, NOVEMBER 2005 trend was also obtained for the conventional LED sample except for a lower peak value of 20 ma). We attributed the enhanced output power ( 21%) to a combination of improved light extraction efficiency and improvement the crystalline quality by the reduction in dislocation density using the PSS. After 1000 h, the reliability test of PSS LED was also superior to that of the conventional LED. These results demonstrate that the PSS has high potential for the development of high-efficiency GaN-based UV emitters. Woei-Kai Wang received the B.S. degree in manufacturing engineering from Yuan Ze University, ChungLi, Taiwan, R.O.C., in 2000, and the M.S. degree in electrical engineering from the University of Chung Hua, Hsinchu, Taiwan, R.O.C., in He is currently pursuing the Ph.D. degree in the Department of Materials Engineering at University of Chung Hsing, Taiwan, R.O.C. His research interests include development of GaN-based optoelectronic semiconductors and electric devices. REFERENCES [1] S. Nakamura and G. Fasol, The Blue Laser Diode. Berlin, Germany: Springer-Verlag, 1997, pp [2] S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama, High-brightness InGaN blue, green and yellow light-emitting diodes with quantum well structures, Jpn. J. Appl. Phys., vol. 34, p. L797, [3] J. Han, M. H. Crawford, R. J. Shul, J. J. Figiel, L. Zhang, Y. K. Song, H. Zhou, and A. V. Nurmikko, AlGaN/GaN quantum well ultraviolet light emitting diodes, Appl. Phys. Lett., vol. 73, pp , [4] Y. Narukawa, I. Niki, K. Izuno, M. Yamada, Y. Murazki, and T. Mukai, Phosphor-conversion white light emitting diode using InGaN near-ultraviolet chip, Jpn. J. Appl. Phys. Lett., vol. 41, pp. L371 L373, Apr [5] T. Wang, Y. H. Liu, Y. B. Lee, Y. Izumi, J. P. Ao, J. Bai, H. D. Li, and S. Sakai, Fabrication of high performance of AlGaN/GaN-based UV light-emitting diodes, J. Cryst. Growth., vol. 235, pp , [6] P. Fini, L. Zhao, B. Moran, M. Hansen, H. Marchand, J. P. Ibbetson, S. P. DenBaars, U. K. Mishra, and J. S. Speck, High-quality coalescence of laterally overgrown GaN stripes on GaN/sapphire seed layers, Appl. Phys. Lett., vol. 75, pp , [7] T. S. Zheleva, O. H. Nam, M. D. Bremser, and R. F. Davis, Dislocation density reduction via lateral epitaxy in selectively grown GaN structures, Appl. Phys. Lett., vol. 71, pp , [8] S. Kitamura, K. Hiramatsu, and N. Sawaki, Fabrication of GaN hexagonal pyramids on dot-patterned GaN/sapphire substrates via selective metalorganic vapor phase epitaxy, Jpn. J. Appl. Phys., vol. 34, pp. L1184 L1186, [9] S. Nakamura, M. Senoh, S. Nagahama, N. Isawa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, H. Umemoto, M. Sano, and K. Chocho, InGaN/GaN/AlGaN-based laser diodes with modulation-doped strained-layer superlattices grown on an epitaxially laterally overgrown GaN substrate, Appl. Phys. Lett., vol. 72, pp , [10] K. Tadatomo, H. Okagawa, T. Tsunekawa, T. Jyouichi, Y. Imada, M. Kato, H. Kudo, and T. Taguchi, High output power InGaN ultraviolet light-emitting diodes fabricated on patterned substrates using metalorganic vapor phase epitaxy, Phys. Stat. Sol. (a), vol. 188, pp , [11] H. Kudo, K. Murakami, R. Zheng, Y. Yamada, T. Taguchi, K. Tadatomo, H. Okagawa, Y. Ohuchi, T. Tsunekawa, Y. Imada, and K. Kata, Intense ultraviolet electroluminescence properties of the high-power InGaN-based light-emitting diodes fabricated on patterned sapphire substrates, Jpn. J. Appl. Phys., vol. 41, pp , Apr [12] A. Bell, R. Liu, F. A. Ponce, H. Amano, I. Akasaki, and D. Cherns, Light emission and microstructure of Mg-doped AlGaN grown on patterned sapphire, Appl. Phys. Lett., vol. 82, pp , [13] M. Yamada, T. Mitani, Y. Narukawa, S. Shioji, I. Niki, S. Sonobe, K. Deguchi, M. Sano, and T. Mukai, InGaN-based near-ultraviolet and blue-light-emitting diodes with high external quantum efficiency using a patterned sapphire substrate and a mesh electrode, Jpn. J. Appl. Phys. Lett., vol. 41, no. 12B, pp. L1431 L1433, Jun [14] S. J. Chang, Y. C. Lin, Y. K. Su, C. S. Chang, T. C. Wen, S. C. Shei, J. C. Ke, C. W. Kuo, S. C. Chen, and C. H. Liu, Nitride-based LEDs fabricated on patterned sapphire substrates, Solid-State Electron., vol. 47, pp , [15] D. S. Wuu, W. K. Wang, W. C. Shin, R. H. Hrong, C. E. Lee, Y. W. Lin, and J. S. Fang, Enhanced output power of near-ultraviolet InGaN-GaN LEDs grown on patterned sapphire substrates, IEEE Photon. Technol. Lett., vol. 17, no. 2, pp , Feb [16] T. Hino, S. Tomiya, T. Miyaima, K. Yanashima, S. Hashimoto, and M. Ikeda, Characterization of threading dislocations in GaN epitaxial layers, Appl. Phys. Lett., vol. 76, pp , Dong-Sing Wuu received the B.S., M.S., and Ph.D. degrees in electrical engineering from National Sun Yat-Sen University, Taiwan, R.O.C., in 1985, 1987, and 1991, respectively. He has done work in the field of optoelectronic devices (LEDs, LDs, PDs) and ink-jet printheads at OES/ITRI, Taiwan, R.O.C., from 1991 to In 1995, he joined Da-Yeh University, Chang-Hua, Taiwan, R.O.C., as an Associate Professor in Department of Electrical Engineering. He is now a Professor of the Department of Materials Engineering at National Chung Hsing University, Taichung, Taiwan, R.O.C. since 2001 and a Dean of College of Electrical Engineering and Computer Science, National Formosa University, Hu-Wei, Taiwan, R.O.C., since His main interests are solid-state optoelectronic devices and thin-film processing. He has authored or co-authored more than 80 technical papers in international scientific journals and holds over 40 patents in his fields of expertise. Dr. Wuu was awarded by the Ministry of Education of Taiwan for the Industry/University Corporation Project in 2004 Shu-Hei Lin received the B.S. degree in materials engineering from the National Fomosa University, Taiwan, R.O.C., in 2001, where she is currently pursuing the M.S. degree in the Institute of Electro Optics and Materials Science. Her major research focuses on nitride-based lightemitting diodes. Pin Han received the Ph.D. degree in physics from Arizona University, Tucson, in 1996 and the M.S. degree in electrooptics engineering from National Chiao Tung University, Taiwan, R.O.C., in He is currently an Associate Professor in the Institute of Precision Engineering, National Chung Hsing University, Taichung, Taiwan, R.O.C. His main research interests are optical engineering and computing physics.

7 WANG et al.: EFFICIENCY IMPROVEMENT OF NEAR-ULTRAVIOLET InGaN LEDs USING PATTERNED SAPPHIRE SUBSTRATES 1409 Ray-Hua Horng received the B.S. degree in electrical engineering from Cheng Kung University, Taiwan, R.O.C., in 1987 and the Ph.D. degree in electrical engineering from Sun Yat-Sen University, Taiwan, R.O.C. in She has done work in the field III V compound materials by MOCVD as an Associate Researcher at Telecommunication Laboratpries/MOTC, R.O.C. She has been a Professor of the Institute of Precision Engineering at National Chung-Hsing University, Taichung, Taiwan, R.O.C. since In November 2000, she vitalized her research on high-brightness LEDs with mirror substrate into practical mass products that enable high-power and large-area LEDs. She received numerous awards recognizing her work on high-brighness LEDs. Her main interests are solid-state EL devices, III V optoelectronic devices, high-dielectric materials for DRAM applications, nanosurface treatment by natural lithography and GaN nanowire growth. She is the author or coauthor of over 50 technical papers and holds four U.S. patents. Ming-Jiunn Jou received the B.S. degree in chemical engineering from National Taiwan University, Taiwan, R.O.C., in 1982 and the Ph.D. degree in material science and engineering from University of Utah, Salt Lake City, in He worked for MRL/ITRI and OES/ITRI, Taiwan, R.O.C., in the field of optoelectronic devices (LEDs, LDs, and PDs) from 1990 to In 1996, he joined Epistar Corporation, Hsinchu, Taiwan, R.O.C., as Vice President of Research and Development, responsible for AlGaInP and InGaN LEDs development. He is currently the Executive Vice President and COO of Epistar Corporation. His main interests are metal organic vapor-phase epitaxial (MOVPE) growth of optoelectronic devices and device processing. He has authored or co-authored more than 50 technical papers in scientific journals and holds over 30 patents in his fields of expertise. Ta-Cheng Hsu received the Ph.D. degree in materials science from the University of Utah, Salt Lake City, in From 1998 to 1999, he was a Postdoctoral Research Associate at the University of Utah, where he worked on device design and crystal growth of long-wavelength laser diodes. In 2000, he was with Procomp Ltd., Taiwan, R.O.C., and worked on the development and production of p-hemt and HBT devices until he joined the research and development group at Epistar Corporation, Hsinchu, Taiwan, R.O.C., in His present interests are the development of advanced process for high-brightness GaN- and AlGaInP-based LEDs. He has authored or co-authored more than 20 technical papers in international scientific journals in the field of crystal growth of compound semiconductor materials and devices by MOCVD. Yuan-Hsin Yu received the B.S., M.S., and Ph.D. degrees from the Department of Materials Engineering, Tatung University, Taiwan, R.O.C., in 1995, 1997, and 2001, respectively. From 1998 to 1999, he worked in the field of surface science, thin-film processing, and vacuum devices at Department of Materials Science and Engineering, Northwestern University, Evanston, IL. From 2002 to 2003, he was a Post Doctoral Fellow. He has been Project Manager of the Research and Development Division at Wafer Works Corporation, Taoyuan, Taiwan, R.O.C., since Donald Tai-Chan Hou received the B.S. degree in metallurgical engineering from National Cheng Kung University, Taiwan, R.O.C., in 1971 and the Ph.D. degree in material science and engineering from University of California, Berkeley, in He worked for AT&T Bell Laboratories, Murray Hill, NJ, and Lucent Technologies Bell Laboratories, Sagamihara, Japan, in the field of VLSI technologies from 1981 to In 2001, he joined Truelight Corporation, Taiwan, R.O.C., as Operation Vice President. He is currently the Vice President of Epistar Corporation, Hsinchu, Taiwan, R.O.C., responsible for yield improvement of high-brightness LEDs. He has authored or co-authored more than 30 technical papers in scientific journals and holds over four patents in his fields of expertise. Aikey Lin received the B.S. degree from National Taipei University, Taipei, Taiwan, R.O.C., and the M.S. degree from the National Cheng-Kung University (NCKU), Taiwan, R.O.C., in 1999 and 2001, respectively, both in materials science and engineering. While at NCKU, her research was focused on oxide nanopowders synthesized by using the hydrothermal process. From 2001 to 2003, she was an Engineer in the Research and Development Division of Wafer Works Corporation, Taoyuan, Taiwan, R.O.C., and handled the projects related to crystal growth of silicon ingots and silicon-based devices. She is currently the Marketing Manager. Within the past five years, she has published five scientific papers on international and/or local journals and conferences in the field relative to materials science.