IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 11, NOVEMBER 2005 1403

Efficiency Improvement of Near-Ultraviolet InGaNLEDs 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 sub-strates (PSSs) to fabricate InGaN-based near-ultraviolet (410 nm)light-emitting diodes (LEDs) was demonstrated. The PSS was pre-pared using a periodic hole pattern (diameter: 3 m; spacing: 3

m) on the (0001) sapphire with different etching depths. Fromtransmission-electron-microscopy and etch-pit-density studies, thePSS with an optimum pattern depth ( = 1 5 m) was con-firmed to be an efficient way to reduce the thread dislocations in theGaN microstructure. It was found that the output power increasedfrom 8.6 to 10.4 mW, corresponding to about 29% increases inthe external quantum efficiency. However, the internal quantumefficiency (@ 20 mA) was about 36% and 38% for the conven-tional and PSS LEDs, respectively. The achieved improvement ofthe output power is not only due to the improvement of the in-ternal quantum efficiency upon decreasing the dislocation density,but also due to the enhancement of the extraction efficiency usingthe PSS. Finally, better long-time reliability of the PSS LED per-formance was observed.

Index Terms—GaN, InGaN, light-emitting diode (LED), nearultraviolet (UV), patterned sapphire substrate (PSS).

I. INTRODUCTION

L IGHT-EMITTING DIODES (LEDs) based on InGaNsemiconductor materials have become commercialized

products in recent years [1], [2]. These devices have alreadybeen extensively used in outdoor displays, traffic lights andgeneral lighting and show a greater potential to replace in-candescent bulbs and fluorescent lamps. Nevertheless, furtherprogress is still strongly desired in order for these devices toreach efficiency levels achievable with other III–V materials’systems. Currently, the most commonly used method to achievewhite-light LEDs is to combine an yttrium-aluminum-garnetphosphor 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, 2005. This work wassupported by National Science Council under Contract NSC-92-2622-E-005-015.

W.-K. Wang and D.-S. Wuu are with the Department of Materials Engi-neering, National Chung Hsing University, Taichung 402, Taiwan, R.O.C.(e-mail: dsw@dragon.nchu.edu.tw).

S.-H. Lin is with the Institute of Electro Optics and Materials Science, Na-tional 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. (e-mail:huahorng@dragon.nchu.edu.tw).

T.-C. Hsu, D. T.-C. Huo, and M.-J. Jou are with Epistar Corporation, Hsinchu300, Taiwan, R.O.C.

Y.-H. Yu and A. Lin are with Wafer Works Corporation, Taoyuan 326, Taiwan,R.O.C.

Digital Object Identifier 10.1109/JQE.2005.857057

for developing white-light LEDs to solve the low color-ren-dering-index problem [3], [4]. However, UV LEDs are moresensitive to dislocation than blue GaN-based LEDs, as indicatedfrom previous studies [5]. It is well known that a dislocationdensity in the order of 10 10 cm is inherent in the epi-taxial GaN films on sapphire substrates due to the large latticemismatch. High dislocation density will influence the devicecharacteristics, such as device lifetime, electron mobility, andthe quantum efficiency of radiative recombination. Therefore,how to further reduce the dislocation density is an importantissue for fabricating high-performance UV LEDs.

Many different growth approaches have been proposed forthreading dislocation density reduction [6], [7]. Lateral epitaxialovergrowth is a commonly used technique utilizing metal–or-ganic chemical vapor deposition (MOCVD) and hydride vaporphase epitaxy to reduce the threading dislocation density to the10 cm range. In this method, a GaN epilayer with sev-eral m in thickness is first grown onto a sapphire substrate[8], [9]. Subsequently, a SiN or SiO strip type mask is pro-duced, followed by epitaxial growth. Recently, several groupshave recently demonstrated direct lateral epitaxy growth onto astripe-type patterned sapphire substrate (PSS). Tadatomo. et al.have adopted models of parallel grooves along the (11 2 0) sap-phire to fabricate nitride LEDs [10], [11]. Bell. et al. have usedthe trenches of sapphire (11 2 0) direction to grow the Mg-dopedAlGaN epilayers [12]. A hexagon pattern parallel with the a axisof the sapphire was reported by Yamada. et al. [13]. Chang etal. [14] have described the use of parallel stripes along the sap-phire (1 00) direction for their blue LED growth. In our pre-viously study [15], a considerably improved output power ofInGaN-based UV LEDs on PSSs was obtained. The PSS wasprepared using a periodic hole pattern (diameter: 3 m; spacing:3 m) on the (0001) sapphire with different etching depths. Theproposed method can reduce the TDs via a single growth processwithout any interruption or deposition onto the SiO mask. Italso eliminates the need for a precise photolithography processto transfer a special pattern axis and prevents the induced con-tamination. We also found the enhancement of optical reflec-tion from the GaN/sapphire interface. To understand and applythe PSS to GaN device fabrication, it is important to know theinfluence of the optical scattering and dislocations in the ma-terial. In this paper, we report on the microstructure, electricaland optical properties of near-UV InGaN-based LEDs grown onboth PSS and conventional sapphire substrates. The light emis-sions and physical characteristics of the fabricated LEDs wereinvestigated. The reliability of these fabricated LEDs will alsobe described.

0018-9197/$20.00 © 2005 IEEE

1404 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 11, NOVEMBER 2005

Fig. 1. (a) Schematic diagram of near-UV InGaN PSS LED. (b) SEMmicrograph 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 schematicdiagram of the InGaN PSS-LED structure is shown in Fig. 1(a).The PSS was prepared using a periodic hole pattern, where thehole etching depths ranged from 0.5 to 1.5 m. The holearray (diameter: 3 m; spacing: 3 m) was generated using astandard photolithography process. The sapphire substrate wasetched using BCl Cl in an inductively coupled-plasma etcherat a typical dc bias of 100 V. After etching, the correspondingmicrograph of the PSS was examined by scanning electronmicroscopy (SEM, (JEOL-JSM 6360) and shown in Fig. 1(b),where a 1.5- m etching depth with a sidewall angle of 75was achieved. During the MOCVD growth, trimethylgallium,trimethylindium and ammonia were used as the gallium, indiumand nitrogen precursors. Biscyclopentadienyl magnesium anddisilane were employed as the p- and n-type dopant sources,respectively. The carrier gas was hydrogen through the growthexcept that nitrogen was used for the InGaN growth. Thereactor pressure was maintained at 100 torr throughout thegrowth process. Prior to the growth, sapphire substrates werethermally baked at 1100 C in hydrogen gas to remove surfacecontamination. The LED structure consisted of a 30-nm-thickGaN low-temperature buffer layer, a 1.5- m-thick layer ofundoped GaN, a 30-nm-thick GaN low-temperature bufferlayer, a 1.5- m-thick layer of undoped GaN, a 4- m-thicklayer of n-type GaN: Si, a n-type Al Ga N–GaN layer,an multiple quantum-wells (MQWs) active layer, a p-typeAl Ga N–GaN layer and a 0.3- m-thick p-type GaN: Mglayer. The LED sample used in this research had a chip size of365 m 365 m, fabricated using standard photolithographyand dry etch techniques. The Ni–Au and Ti–Al–Ti–Au metalcontacts were deposited as p-type and n-type GaN, respectively.Note that the active layers in the conventional and PSS LEDswere grown under the same growth run.

The GaN sample was polished with a chemical mechanicalpolishing (Logitech MP-5) process to produce tilt angle GaNsurface and the microstructure of the sample was character-ized cross-sectional transmission electron microscopy (TEM,JEOL-JEM 2010). TEM samples were prepared using standardmechanical polishing (down to a thickness below 20 m) andAr ion-milling (Gatan 600-DIF, 1 mA@ 4 keV) techniques to

Fig. 2. Cross section TEM image of GaN epilayer grown on (a) conventionalsapphire substrate, (b) PSS with D = 0:5 �m, and (c) PSS with D =1:5 �m.

achieve electron transparency. Photoluminescence (PL) map-ping (ACC-PLM 100) is used to investigate optical properties ofthe samples at room temperature using a He–Cd laser (325 nm)as an excitation source. The current–voltage ( – ) characteris-tics of the LEDs were measured using an Agilent 4155B semi-conductor parameter analyzer. The output power of the LEDlamp was measured using an integrated sphere detector and themeasured deviation was around 5%.

III. RESULTS AND DISCUSSION

Cross-sectional TEM measurements were performed to in-vestigate the dislocation distribution of the GaN-on-PSS sam-ples with various values. For comparison, the TEM micro-graph of the GaN grown on a conventional sapphire substrateis also illustrated in Fig. 2(a). It can be seen clearly that a largenumber of extended TDs propagate throughout the GaN film,originating from the GaN–sapphire interface. The generation ofthese dislocations is caused by the large lattice mismatch be-tween GaN and sapphire. For the sample grown on the PSSwith m, the GaN epilayer buried the cavity incom-pletely and some small voids near the cavity edge were observed

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 ran-domly. For the sample grown on the PSS with m,the GaN epilayer grew laterally from the top of the sapphiresubstrate and some voids (0.5 m in size) that formed whentwo growth front boundaries coalesced were observed on thepattern sidewall. The undoped GaN also grew into the sidewallof the trench. Fig. 2(c) shows the representative distributionsof the 90 bending dislocations in the lateral growth region inthe GaN-on-PSS m heterostructure. The observedangle might be determined using the lateral/vertical growth rateratio. These dislocations did not subsequently propagate to thesurface of the overgrown GaN layer. Above these voids, the dis-locations do seldom observed. These voids are associated withthe relaxed morphologies of the GaN film side faces and usuallylead to threading dislocation bending in the direction of thesevoids. Hence, free standing laterally grown GaN films wereachieved. Evidence of dislocation reduction is also obtainedfrom the etch pit density (EPD) measurements, where the GaNsamples were chemically etched in a H SO –H PO (1:3) mix-ture at 250 C for 10 min. It was found that the EPD was around1.1 10 cm for the sample with m (i.e., conven-tional sapphire substrate) and decreased to 2.8 10 cm forthe sample with m. These results indicated that asignificant reduction in the dislocation density was achieved viathe lateral epitaxial overgrowth on the PSS without a SiO mask.Fig. 3 shows the crystallinity of the GaN samples examined bydouble-crystal X-ray diffractometry. The average full-width athalf maximum of the (0002) X-ray rocking curves of GaN filmsgrown on PSS and conventional sapphire substrate were foundto be 320 and 360 arcsec, respectively. The crystallinity im-provement examined by X-ray measurements also showed ingood agreement with the result revealed by TEM observations.

The growth evolution of the GaN-on-PSS can be examinedby plane-view SEM as shown in Fig. 4(a), where the samplewas polished with an inclined angle. A schematic diagram ofthe corresponding epi-structure is shown in Fig. 4(b). The GaNwas vertically grown from the bottom of the sapphire substrateand extended forming lateral epitaxial growth over the trencheswith (11 2 0) facets. The GaN was kept (0001) and (11 2 0) or(11 2 2) facets until coalescence. It can be observed that the voidin 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 sapphiresubstrate.

finally disappeared. Based on the above TEM and SEM observa-tions, the growth evolution of the GaN-on-PSS can be schemat-ically shown in Fig. 5. A thin (0001) GaN epilayer was firstgrown onto a (0001) PSS [Fig. 5(a)]. The GaN also grew into thebottom and sidewall of the trench. Then the GaN epilayer grewlaterally with a new shape (11 2 2) facets [Fig. 5(b)]. When thegrowth continued, the coalescence of the GaN occurred and theepitaxial lateral overgrowth was formed [Fig. 5(c)]. Finally, aflat GaN epilayer was obtained on the PSS [Fig. 5(d)].

Fig. 6 shows the room-temperature PL intensity mapping ob-tained from the near-UV InGaN LED sample grown on a 2-insapphire substrate where there exist four zones with various

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Fig. 6. Room temperature photoluminescence intensity mapping of PSS LEDswith different etching depths (D = 0, 0.5, 1.0 and 1.5 �m).

Fig. 7. Trace-Pro simulated EL intensity of PSS LED with variousD values.

etching depths (i.e., and m). It was foundthat the PL peak around 3.07 eV (403 nm) increased remarkablyas the increased. At the m zone, the near band-edge PL intensity is obviously stronger than that of the LEDwith m (conventional sapphire substrate). Such asignificant enhancement in PL intensity due to the decreasesof trap densities and the dislocation-induced nonradiative re-combination centers indicates that the PSS can significantly im-prove the quality of GaN epilayers. It has been reported thatthe radiative recombination efficiency increases as the EPD de-creases from 3 10 to 1.2 10 cm and saturates at less than1.2 10 cm [16]. Our experimental results indicated that theEPD of the PSS LED is about 2.8 10 cm . Hence, the im-provement in PL intensity might also be related to the enhance-ment of the light extraction efficiency through light scatteringfrom the nitride epilayer and the PSS interface. The value of

may play a more important role in the enhancement of op-tical properties of the near-UV InGaN–GaN LEDs.

Fig. 7 shows a Trace-Pro simulation profile of the ray extrac-tion ratio rate take from the PSS InGaN LED as a function of the

value. Here a 1-mW power (50 000 light rays) is assumed toemit randomly from the InGaN/GaN MQW active layer, i.e., thespontaneous emission process. The output power can be calcu-lated 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

Fig. 8. Internal quantum efficiency of conventional and PSS (D = 1:5 �m)InGaN LEDs measured at various temperatures (@20 mA).

m . Moreover, the ray extraction ratio rate increasedas the increased and saturated when the reached above1.0 m. These results indicate that the has a large effect onthe improvement of light extraction efficiency of the PSS LED.However, the present simulation did not consider the absorptionin the epi-structure and the reflector effect from the silver cup inthe epoxy lamp form. Thus, this simplified simulation can onlybe used to explain a rough trend of electroluminescence (EL)emission intensity versus the hole etching depth. Since the PSSLED with m shows the best performance as evi-denced by the TEM and PL mapping results, we only focus onthe m samples in the following work. Moreover,the forward I–V characteristics of the near-UV LEDs with andwithout PSS at room temperature were also investigated [15].The corresponding forward voltages at 20 mA were 3.83 and3.84 V, respectively. This indicates that the PSS LED has sim-ilar I–V characteristic as compared with that of the conventionalLED.

To clarify the influence of dislocation reduction on EL in-tensity, we estimated the internal quantum efficiency ofthe InGaN LED sample roughly using the temperature depen-dence of the integrated EL intensity. Generally, the valueat low temperatures can be regarded as 100% when neglectingthe nonradiative recombination process. As shown in Fig. 8,the integrated EL intensities of both the conventional and PSSLEDs were nearly constant below 200 K and declined gradu-ally with a further increase in temperature. At room tempera-ture, the value (@ 20 mA) was about 36% and 38% for theconventional and PSS LEDs, respectively. No significant dif-ference in the values was observed. These results suggestthat the enhanced output power could not only be attributed tothe improvement in from decreasing the dislocation density.In order to measure the LED output power, the chips were en-capsulated in conventional lamp form (5 mm in diameter). TheEL emission was measured from the LED top surface. Fig. 9(a)shows the light output power of the conventional and PSS LEDsas a function of injection current. The output intensity of bothLEDs initially increases linearly with the injection current. Witha 20-mA forward injection current, the output power of a lamp-form PSS LED and conventional LED were estimated to be 10.4

WANG et al.: EFFICIENCY IMPROVEMENT OF NEAR-ULTRAVIOLET InGaN LEDs USING PATTERNED SAPPHIRE SUBSTRATES 1407

Fig. 9. (a) Light output power of the conventional and PSS LEDs as a functionof 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 quantumefficiency of the InGaN LED sample with various for-ward injection currents up to 100 mA. It was found that theof the PSS LED reached a maximum value of (11.6% @20 mA) and then decreased significantly with a further increasein the forward bias current. Nearly the same trend was alsoobtained for the conventional LED sample except for a lowerpeak value of (11.6% @ 20 mA).The degradation in atthe higher injection level might be related to carrier saturationand/or the joule heating effect. The can be expressed as

(1)

where is the light extraction efficiency and the current in-jection efficiency is assumed to be 100%. The values of theconventional and PSS LEDs are estimated to be 32 and 37%, re-spectively. Details of the , , and data for both LEDsare summarized in Table I. Clearly, the enhancement inplays a more important role in obtaining the higher valueof the InGaN PSS LED.

A prime concern of the PSS LEDs is their reliability revealedby the lifetime test. Fig. 10 shows the relative EL intensity of theconventional and PSS InGaN LEDs under a forward current of20 mA at room temperature during the 1000-h test. The relativeEL intensity to the initial EL intensity is shown as a function

TABLE IESTIMATED INTERNAL QUANTUM EFFICIENCY AND LIGHT EXTRACTION

EFFICIENCY FOR PSS LED AND CONVENTIONAL LED AT A 20-mACURRENT INJECTION AT ROOM TEMPERATURE

Fig. 10. Room-temperature reliability test of relative luminous intensity ofconventional and PSS (D = 1:5 �m) InGaN LEDs driven at 20 mA.

of the aging time. It can be seen that the relative EL intensityexhibited the same degradation trend for both LEDs. The PSSLED presents a gradual degradation in the EL intensity with an8% decreases after 24 h of the test. After 1000 h, the EL inten-sity of PSS LED and conventional LED are decayed by 18% and23%, respectively. The smaller decrease in the intensity of ELwas observed in the LED sample having PSS, as compared withthe conventional LED. This result suggests indicated that im-provement of the EL intensity due to the decreases in trap densi-ties and the TD-induced nonradiative recombination centers viagrown on PSS. Even though, the PSS LEDs may still suffer theincomplete step coverage problem at the GaN–PSS interface,there is no evident difference in life time as indicated from ourmeasurement results between the conventional and PSS LEDsamples.

IV. CONCLUSION

We described the characteristics of the near-UV InGaN-basedLEDs grown on the conventional and PSSs. The PSS was pre-pared using a periodic hole pattern on the (0001) sapphire sub-strate with different etching depths ranged from 0.5 to 1.5 m.For the PSS LED, the GaN epilayer grew laterally from the topof the sapphire substrate and overhung the trench. From theTEM and EPD studies, the use of PSS with an optimum pat-tern depth m was confirmed to be an efficientway to reduce the TDs in the GaN microstructure. At roomtemperature, the value (@ 20 mA) was about 36% and38% for the conventional and PSS LEDs, respectively. It wasfound that the of the PSS LED reached a maximum valueof 14.1% (@ 20 mA) and then decreased significantly with afurther increase in the forward bias current. Nearly the same

1408 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 11, NOVEMBER 2005

trend was also obtained for the conventional LED sample ex-cept for a lower peak value of (11.6% @ 20 mA). We at-tributed the enhanced output power ( 21%) to a combination ofimproved light extraction efficiency and improvement the crys-talline quality by the reduction in dislocation density using thePSS. After 1000 h, the reliability test of PSS LED was also supe-rior to that of the conventional LED. These results demonstratethat the PSS has high potential for the development of high-ef-ficiency GaN-based UV emitters.

REFERENCES

[1] S. Nakamura and G. Fasol, The Blue Laser Diode. Berlin, Germany:Springer-Verlag, 1997, pp. 216–219.

[2] S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama, “High-brightnessInGaN blue, green and yellow light-emitting diodes with quantum wellstructures,” Jpn. J. Appl. Phys., vol. 34, p. L797, 1995.

[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 ultravioletlight emitting diodes,” Appl. Phys. Lett., vol. 73, pp. 1688–1690, 1998.

[4] Y. Narukawa, I. Niki, K. Izuno, M. Yamada, Y. Murazki, and T. Mukai,“Phosphor-conversion white light emitting diode using InGaN near-ul-traviolet chip,” Jpn. J. Appl. Phys. Lett., vol. 41, pp. L371–L373, Apr.2002.

[5] T. Wang, Y. H. Liu, Y. B. Lee, Y. Izumi, J. P. Ao, J. Bai, H. D. Li, andS. Sakai, “Fabrication of high performance of AlGaN/GaN-based UVlight-emitting diodes,” J. Cryst. Growth., vol. 235, pp. 177–182, 2002.

[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 coalescenceof laterally overgrown GaN stripes on GaN/sapphire seed layers,” Appl.Phys. Lett., vol. 75, pp. 1706–1708, 1999.

[7] T. S. Zheleva, O. H. Nam, M. D. Bremser, and R. F. Davis, “Disloca-tion density reduction via lateral epitaxy in selectively grown GaN struc-tures,” Appl. Phys. Lett., vol. 71, pp. 2472–2474, 1997.

[8] S. Kitamura, K. Hiramatsu, and N. Sawaki, “Fabrication of GaN hexag-onal pyramids on dot-patterned GaN/sapphire substrates via selectivemetalorganic vapor phase epitaxy,” Jpn. J. Appl. Phys., vol. 34, pp.L1184–L1186, 1995.

[9] S. Nakamura, M. Senoh, S. Nagahama, N. Isawa, T. Yamada, T.Matsushita, H. Kiyoku, Y. Sugimoto, H. Umemoto, M. Sano, andK. Chocho, “InGaN/GaN/AlGaN-based laser diodes with modula-tion-doped strained-layer superlattices grown on an epitaxially laterallyovergrown GaN substrate,” Appl. Phys. Lett., vol. 72, pp. 211–213,1998.

[10] K. Tadatomo, H. Okagawa, T. Tsunekawa, T. Jyouichi, Y. Imada, M.Kato, H. Kudo, and T. Taguchi, “High output power InGaN ultravioletlight-emitting diodes fabricated on patterned substrates using metalor-ganic vapor phase epitaxy,” Phys. Stat. Sol. (a), vol. 188, pp. 121–125,2001.

[11] H. Kudo, K. Murakami, R. Zheng, Y. Yamada, T. Taguchi, K. Tadatomo,H. Okagawa, Y. Ohuchi, T. Tsunekawa, Y. Imada, and K. Kata, “In-tense ultraviolet electroluminescence properties of the high-powerInGaN-based light-emitting diodes fabricated on patterned sapphiresubstrates,” Jpn. J. Appl. Phys., vol. 41, pp. 2484–2488, Apr. 2002.

[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 pat-terned sapphire,” Appl. Phys. Lett., vol. 82, pp. 349–351, 2003.

[13] M. Yamada, T. Mitani, Y. Narukawa, S. Shioji, I. Niki, S. Sonobe, K.Deguchi, M. Sano, and T. Mukai, “InGaN-based near-ultraviolet andblue-light-emitting diodes with high external quantum efficiency usinga patterned sapphire substrate and a mesh electrode,” Jpn. J. Appl. Phys.Lett., vol. 41, no. 12B, pp. L1431–L1433, Jun. 2002.

[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 LEDsfabricated on patterned sapphire substrates,” Solid-State Electron., vol.47, pp. 1539–1542, 2003.

[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-GaNLEDs grown on patterned sapphire substrates,” IEEE Photon. Technol.Lett., vol. 17, no. 2, pp. 288–290, Feb. 2005.

[16] T. Hino, S. Tomiya, T. Miyaima, K. Yanashima, S. Hashimoto, andM. Ikeda, “Characterization of threading dislocations in GaN epitaxiallayers,” Appl. Phys. Lett., vol. 76, pp. 3421–3423, 2000.

Woei-Kai Wang received the B.S. degree in man-ufacturing engineering from Yuan Ze University,ChungLi, Taiwan, R.O.C., in 2000, and the M.S.degree in electrical engineering from the Universityof Chung Hua, Hsinchu, Taiwan, R.O.C., in 2002.He is currently pursuing the Ph.D. degree in theDepartment of Materials Engineering at Universityof Chung Hsing, Taiwan, R.O.C.

His research interests include development ofGaN-based optoelectronic semiconductors andelectric devices.

Dong-Sing Wuu received the B.S., M.S., and Ph.D.degrees in electrical engineering from National SunYat-Sen University, Taiwan, R.O.C., in 1985, 1987,and 1991, respectively.

He has done work in the field of optoelectronicdevices (LEDs, LDs, PDs) and ink-jet printheadsat OES/ITRI, Taiwan, R.O.C., from 1991 to 1995.In 1995, he joined Da-Yeh University, Chang-Hua,Taiwan, R.O.C., as an Associate Professor in De-partment of Electrical Engineering. He is now aProfessor of the Department of Materials Engi-

neering at National Chung Hsing University, Taichung, Taiwan, R.O.C. since2001 and a Dean of College of Electrical Engineering and Computer Science ,National Formosa University, Hu-Wei, Taiwan, R.O.C., since 2004. His maininterests are solid-state optoelectronic devices and thin-film processing. Hehas authored or co-authored more than 80 technical papers in internationalscientific journals and holds over 40 patents in his fields of expertise.

Dr. Wuu was awarded by the Ministry of Education of Taiwan for the In-dustry/University Corporation Project in 2004

Shu-Hei Lin received the B.S. degree in materialsengineering from the National Fomosa University,Taiwan, R.O.C., in 2001, where she is currentlypursuing the M.S. degree in the Institute of ElectroOptics and Materials Science.

Her major research focuses on nitride-based light-emitting diodes.

Pin Han received the Ph.D. degree in physics fromArizona University, Tucson, in 1996 and the M.S.degree in electrooptics engineering from NationalChiao Tung University, Taiwan, R.O.C., in 1991.

He is currently an Associate Professor in theInstitute of Precision Engineering , National ChungHsing University, Taichung, Taiwan, R.O.C. Hismain research interests are optical engineering andcomputing physics.

WANG et al.: EFFICIENCY IMPROVEMENT OF NEAR-ULTRAVIOLET InGaN LEDs USING PATTERNED SAPPHIRE SUBSTRATES 1409

Ray-Hua Horng received the B.S. degree in elec-trical engineering from Cheng Kung University,Taiwan, R.O.C., in 1987 and the Ph.D. degree inelectrical engineering from Sun Yat-Sen University,Taiwan, R.O.C. in 1993.

She has done work in the field III–V compoundmaterials by MOCVD as an Associate Researcherat Telecommunication Laboratpries/MOTC, R.O.C.She has been a Professor of the Institute of PrecisionEngineering at National Chung-Hsing University,Taichung, Taiwan, R.O.C. since 2001. In November

2000, she vitalized her research on high-brightness LEDs with mirror substrateinto practical mass products that enable high-power and large-area LEDs. Shereceived 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 bynatural lithography and GaN nanowire growth. She is the author or coauthor ofover 50 technical papers and holds four U.S. patents.

Ta-Cheng Hsu received the Ph.D. degree in mate-rials science from the University of Utah, Salt LakeCity, in 1998.

From 1998 to 1999, he was a Postdoctoral Re-search Associate at the University of Utah, wherehe worked on device design and crystal growth oflong-wavelength laser diodes. In 2000, he was withProcomp Ltd., Taiwan, R.O.C., and worked on thedevelopment and production of p-HEMT and HBTdevices until he joined the research and developmentgroup at Epistar Corporation, Hsinchu, Taiwan,

R.O.C., in 2002. His present interests are the development of advanced processfor high-brightness GaN- and AlGaInP-based LEDs. He has authored orco-authored more than 20 technical papers in international scientific journals inthe field of crystal growth of compound semiconductor materials and devicesby MOCVD.

Donald Tai-Chan Hou received the B.S. degreein metallurgical engineering from National ChengKung University, Taiwan, R.O.C., in 1971 and thePh.D. degree in material science and engineeringfrom University of California, Berkeley, in 1980.

He worked for AT&T Bell Laboratories, MurrayHill, NJ, and Lucent Technologies Bell Laboratories,Sagamihara, Japan, in the field of VLSI technologiesfrom 1981 to 2001. In 2001, he joined Truelight Cor-poration, Taiwan, R.O.C., as Operation Vice Presi-dent. He is currently the Vice President of Epistar

Corporation, Hsinchu, Taiwan, R.O.C., responsible for yield improvement ofhigh-brightness LEDs. He has authored or co-authored more than 30 technicalpapers in scientific journals and holds over four patents in his fields of expertise.

Ming-Jiunn Jou received the B.S. degree in chem-ical engineering from National Taiwan University,Taiwan, R.O.C., in 1982 and the Ph.D. degree inmaterial science and engineering from University ofUtah, Salt Lake City, in 1990.

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 1996. In 1996, he joinedEpistar Corporation, Hsinchu, Taiwan, R.O.C.,as Vice President of Research and Development,responsible for AlGaInP and InGaN LEDs devel-

opment. He is currently the Executive Vice President and COO of EpistarCorporation. His main interests are metal–organic vapor-phase epitaxial(MOVPE) growth of optoelectronic devices and device processing. He hasauthored or co-authored more than 50 technical papers in scientific journalsand holds over 30 patents in his fields of expertise.

Yuan-Hsin Yu received the B.S., M.S., and Ph.D. de-grees 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 ofsurface science, thin-film processing, and vacuumdevices at Department of Materials Science andEngineering, Northwestern University, Evanston, IL.From 2002 to 2003, he was a Post Doctoral Fellow.He has been Project Manager of the Research andDevelopment Division at Wafer Works Corporation,

Taoyuan, Taiwan, R.O.C., since 2003.

Aikey Lin received the B.S. degree from NationalTaipei University, Taipei, Taiwan, R.O.C., andthe M.S. degree from the National Cheng-KungUniversity (NCKU), Taiwan, R.O.C., in 1999 and2001, respectively, both in materials science andengineering.

While at NCKU, her research was focused onoxide nanopowders synthesized by using the hy-drothermal process. From 2001 to 2003, she was anEngineer in the Research and Development Divisionof Wafer Works Corporation, Taoyuan, Taiwan,

R.O.C., and handled the projects related to crystal growth of silicon ingots andsilicon-based devices. She is currently the Marketing Manager. Within the pastfive years, she has published five scientific papers on international and/or localjournals and conferences in the field relative to materials science.