1132 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 4, JULY/AUGUST 2009

Improved Output Power of 380 nm InGaN-BasedLEDs Using a Heavily Mg-Doped GaN Insertion

Layer TechniqueShih-Cheng Huang, Dong-Sing Wuu, Peng-Yi Wu, and Shih-Hsiung Chan

Abstract—High-performance InGaN-based 380 nm UV LEDsare fabricated by using a heavily Mg-doped GaN insertion layer(HD-IL) technique. Based on the transmission electron microscopy,etch pit density, and cathodoluminescence results, the HD-IL tech-nique can substantially reduce the defect density of GaN layer. Thedouble-crystal X-ray diffraction results are in good agreement withthose observations. The internal quantum efficiency of LED samplewith an HD-IL shows around 40% improvement compared withthe LED sample without the use of HD-IL. When the vertical-typeLED chips (size: 1 mm × 1 mm) are driven by a 350 mA current,the output powers of the LEDs with and without an HD-IL aremeasured to be 203.4 and 158.9 mW, respectively. As much as 28%increased light output power is achieved.

Index Terms—Metalorganic chemical vapor deposition, thread-ing dislocation, UV LEDs.

I. INTRODUCTION

R ECENTLY, LEDs in the UV region have attracted more at-tention because of their large potential in applications such

as white-color lamps [1], [2], environmental cleaning, and med-ical equipments. Unfortunately, the performance of UV LEDsis always limited by the very high density (109 − 1010 cm−2)of threading dislocations (TDs) that form when the nitride ma-terials are grown on lattice-mismatched substrates. Therefore,how to further reduce the dislocation density is an important is-sue for fabricating high-performance UV LEDs. Many differentgrowth approaches have been proposed for TD density reduc-tion such as epitaxial lateral overgrowth and patterned sapphiresubstrate [3]–[6]. However, both of these cases require addi-tional etching process to generate a template for the subsequentmetal–organic chemical vapor deposition (MOCVD) growth ofGaN epilayers. To some extent, these complicated proceduresdo not avoid some negative effects on the as-grown samples.In this study, through a single MOCVD process, we proposea heavily Mg-doped GaN insertion layer (HD-IL) technique toimprove crystalline quality of the GaN layer, which followed byrest of the required GaN-based LED structure. The schematicdiagram of dislocation reduction by HD-IL is shown in Fig. 1.The figure shows an overgrowth of GaN and blocking mecha-

Manuscript received December 4, 2008; revised January 8, 2009. Firstpublished May 19, 2009; current version published August 5, 2009. This workwas supported in part by MOE ATU Program and in part by MOEA TechnologyDevelopment for Academic Project 97-EC-17-A-07-S1-097.

S.-C. Huang and D.-S. Wuu are with the Department of Materials Engi-neering, National Chung Hsing University, Taichung 40227, Taiwan (e-mail:dsw@dragon.nchu.edu.tw).

P.-Y. Wu and S.-H. Chan are with the Advanced Optoelectronic Technology,Inc., Hsinchu 30351, Taiwan.

Digital Object Identifier 10.1109/JSTQE.2009.2014778

Fig. 1. Schematic diagram of dislocation reduction by a heavily Mg-dopedGaN insertion layer technique.

nism for TDs. The improvement in crystal quality of GaN couldgenerate high-performance 380 nm GaN-based LEDs. Detailsof electrical and optical properties of LED samples with andwithout an HD-IL will be described.

II. EXPERIMENT

All samples used in this study were grown on 2-inch c-planesapphire substrates using atmosphere-pressure MOCVD sys-tem. For the growth of group-III nitride layers, trimethylgal-lium, trimethylaluminum, trimethylindium, and ammonia areused as precursors. Silane and bis-cyclopentadienyl magnesium(Cp2Mg) are used as n-type and p-type dopants, respectively.The 380 nm UV LED structure consists of a low-temperature(500 ◦C) 30-nm-thick GaN nucleation layer, a 2-µm-thick Si-doped GaN buffer layer, a 2.5-nm-thick HD-IL, a 2.5-µm-thick Si-doped GaN cladding layer, an 8-period InGaN/AlGaNmultiple-quantum-wells (MQWs) active region, a 30-nm-thickMg-doped AlGaN cladding layer, and a 0.2-µm-thick Mg-dopedGaN contact layer. The Mg doping concentration of the HD-ILis two orders of magnitude higher than that of the conventionalMg-doped GaN contact layer, which is controlled by the Cp2Mgflow rate. For comparison, another similar structure includingGaN and LED active region without HD-IL was also grown. Thesurface morphology was examined using an optical image pro-filer (Sensofar PLµ 2300) to understand how the surface changedduring the GaN overgrowth on the HD-IL. The double-crystalX-ray diffraction (DCXRD) and photoluminescence (PL) wereused to identify the crystalline and optical properties of LEDepilayers, respectively. These samples were also characterizedby TEM (JEOL-2100F) to reveal the microstructure. The SEMsof the Si-doped GaN films etched by molten KOH were used

1077-260X/$25.00 © 2009 IEEE

HUANG et al.: IMPROVED OUTPUT POWER OF 380 nm InGaN-BASED LEDs USING A HD-IL TECHNIQUE 1133

Fig. 2. Three-dimensional optical images of sample surfaces under variousGaN overgrowth times after the HD-IL deposition. (a) 1 min. (b) 3 min.(c) 8 min. (d) 10 min. (e) 15 min. (f) 25 min.

to determine the etch-pit density (EPD). Cathodoluminescence(CL) measurement was performed using a CL system attachedto SEM system. Finally, the LED epi wafers were processed intovertical conducting chips (size: 1 mm × 1 mm) [7] and mountedon epoxy-free metal can (TO-66). It is worth mentioning thatan additional etching process was employed for the LED sam-ple with the HD-IL structure after laser lifted off the sapphiresubstrate. The n-GaN buffer as well as the HD-IL have beenetched out using the inductively coupled plasma (ICP) until theGaN cladding layer is exposed. The output powers of the LEDsamples were measured using an integrated sphere detector, andthe measured deviation is around 5% (CAS 140B, InstrumentSystem).

III. RESULTS AND DISCUSSION

Fig. 2 shows the 3-D optical images of sample surfaces at var-ious GaN overgrowth times after the HD-IL deposition. ClearGaN islands showing the 3-D growth are observed. By increas-ing the overgrowth time, the GaN islands grow up and start tocoalesce each other. After a 25-min growth on HD-IL, the GaNcompletely coalesces and leads to a flat surface. The growthevolution with time indicates that the dislocations have chanceto bend or terminate because of the epitaxial lateral overgrowthduring the 3-D growth mode. Details of the coral-like texturedsurface of the HD-IL caused by the heavily doped Mg has beenpublished elsewhere [8]. Based on the observation shown inFig. 2, the HD-IL was not fully covered the GaN surface andbecame a naturally patterned mask. Then the GaN overgrewon the area without the HD-IL cover and formed the GaN is-lands in the early stage of overgrowth. The 3-D to 2-D growth

Fig. 3. Cross-sectional TEM micrographs of GaN epitaxial structures grown(a) without and (b) with a heavily Mg-doped GaN insertion layer.

mechanism for the HD-IL technique could be similar to that re-ported for the method-facet assisted epitaxial lateral overgrowth(FACELO) [9], [10]. The difference between the FACELO andHD-IL is the distribution of the mask pattern. In the latter case,we can change the growth temperature and reactor pressure ofthe HD-IL to control the facet of lateral overgrowth and find outthe optimum condition in the near future.

Cross-sectional TEM measurements were performed to inves-tigate the dislocation distribution of the GaN film with HD-ILtemplate. For comparison, the TEM micrograph of the GaN epi-layer grown without an HD-IL is also illustrated in Fig. 3. It canbe clearly seen that a large number of extended TDs propagatethroughout the GaN epilayer, originating from GaN/sapphireinterface. The generation of these dislocations is caused by thelarge lattice mismatch between GaN and sapphire. For the sam-ple with an HD-IL, some of the dislocations bend and mergetogether and these dislocations do not subsequently propagateto the overgrown GaN surface. It is contributed to the lateralgrowth of GaN epilayer on the top of the HD-IL. A prime con-cern about the as-grown GaN epilayer with the HD-IL is theirdefect reduction revealed by the EPD measurement. Fig. 4 showsthe SEMs of the EPD of the GaN epilayers without and withan HD-IL, respectively. The etching process was carried out inmolten KOH for 10 min. These etch pits might be producedby the TDs propagation to the GaN top surface, which origi-nates from the GaN/sapphire interface. It is found that the EPDdecreases from 2.5 × 108 to 3.5 × 107 cm−2 , using an HD-ILtechnique.

High-resolution CL images were examined at 300 K for bothGaN samples with and without an HD-IL. As shown in Fig. 5(a)and (b), dark spots are clearly observed in both images, which

1134 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 4, JULY/AUGUST 2009

Fig. 4. Typical plane-view SEMs of etch pits on the GaN surface after etchingin KOH solution (a) without and (b) with a heavily Mg-doped GaN insertionlayer.

Fig. 5. Cathodoluminescence images of GaN epilayers (a) without and(b) with a heavily Mg-doped GaN insertion layer.

have been reported to be associated with nonradiative natureof dislocations in the case of GaN films [11]. However, it isnoted that the area of dark spots in Fig. 5(a) is obviously largerthan that observed in Fig. 5(b). These indicate that the dislo-cation density has been reduced when the HD-IL is employed,although it is difficult to quantify relationship between the CL

images and dislocation density due to the diffuse nature of CLemission.

The improvements in crystalline and optical quality of GaNusing the HD-IL technique are summarized in Table I, wherethe DCXRD, EPD, and PL data were tabulated. By using anHD-IL, the DCXRD full-width at half-maximum (FWHM) isreduced by 77 and 187 arcsec for (002) and (302) planes, re-spectively. It has been reported that the densities of screw TDsand edge TDs is related to DCXRD FWHMs of (002) and (302)planes, respectively [12], [13]. Additionally, the PL integratedintensity ratios of near-band-edge emission to the yellow lu-minescence band (IBE/IYL ) represent the optical quality ofGaN epilayer. From the DCXRD and PL measurement resultsshown in Table I, we can infer that the improvements in crys-talline and optical quality of the sample with an HD-IL couldbe due to the efficient reduction of defect densities. One shouldbe noting that the TDs’ densities extrapolated by DCXRD andEPD methods are slightly different. This could relate to their re-spective metrology principle, which is beyond the study scopehere.

To clarify the influence of dislocation reduction on the corre-sponding LED performance, we estimate the internal quantumefficiency (ηint) of the InGaN/AlGaN LED samples using thetemperature dependence of integrated PL intensity. The thermalquench of the luminescence of an ideal InGaN MQW is mainlydue to the thermal emission of carriers out of the MQW statesinto the barrier states, which can be attributed to thermal carriersto escape from localized stated and/or to capture at nonradiativerecombination centers in the MQW. In general, the ηint valueat low temperature (∼10 K) can be regarded as 100% whenneglecting the nonradiative recombination process. As shownin Fig. 6, the integrated PL intensities of both the LED sampleswith and without an HD-IL keep nearly constant below 100 Kand decline gradually with further increase in temperature. Atroom temperature, the ηint value is about 30.72% and 21.66%for the LED samples with and without an HD-IL, respectively.The significant reduction in defect density could contribute tothe present improvements in the ηint value.

Fig. 7 shows the light output power versus injection current(L–I) characteristics of these LED samples. Here, the LEDchips were mounted on the epoxy-free TO-66 metal can. Asshown in the insert of Fig. 7, the electroluminescence (EL)peak positions were located at 380 nm for both the samples.It can be seen from this figure that under a 350 mA for-ward injection current, the output power data of the LED sam-ples with and without the HD-IL were estimated to be 203.4and 158.9 mW, respectively. A 28% enhancement in outputpower was achieved in the LED sample with the HD-IL. Weattribute the enhanced output power to the reduction of dis-location density by incorporating the HD-IL structure. Nev-ertheless, the difference in efficiency improvement betweenthe IQE and output power could be due to the additionalICP etching process for the LED samples with the HD-ILstructure. Since the high-resistivity HD-IL was etched out bythe ICP process, the plasma-induced damage of the MQWsmight have occurred and reduced the output power of the LEDchip.

HUANG et al.: IMPROVED OUTPUT POWER OF 380 nm InGaN-BASED LEDs USING A HD-IL TECHNIQUE 1135

TABLE ICRYSTALLINE AND OPTICAL PERFORMANCE OF InGaN/AlGaN MQW LEDs GROWN WITHOUT AND WITH AN HD-IL

Fig. 6. Temperature dependence of Arrhenius plots for InGaN/AlGaN LEDsamples grown with and without a heavily Mg-doped GaN insertion layer.

Fig. 7. Output powers of 380 nm InGaN/AlGaN LED samples with and with-out a heavily Mg-doped GaN insertion layer. The insert shows the EL spectraof these LEDs.

IV. CONCLUSION

In this paper, we have reported a high-performance 380 nmInGaN-based UV LED on a sapphire substrate using an HD-IL technique, which can greatly reduce the dislocation density.Experimental results indicated that the LED sample with HD-ILexhibited a 28% enhancement in light output power as comparedwith that of the conventional LED sample. The improvementcan be attributed to the reduction of nonradiative recombinationcenters from a reduced dislocation density in the active layer. In

view of the fact that no MOCVD regrowth process is needed, thepresented HD-IL technique has high potential for high-qualityUV emitters.

REFERENCES

[1] Y. Narukawa, I. Niki, K. Izuno, M. Yamada, Y. Murazaki, and T. Mukai,“Phosphor-conversion white light emitting diode using InGaN near-ultraviolet chip,” Jpn. J. Appl. Phys., vol. 41, no. 4A, pp. L371–L373,2002.

[2] D. Morita, M. Sano, M. Yamamoto, T. Murayama, S. Nagahama, andT. Mukai, “High output power 365nm ultraviolet light emitting diode ofGaN-free structure,” Jpn. J. Appl. Phys., vol. 41, no. 12B, pp. L1434–L1436, 2002.

[3] C. H. Chiu, H. H. Yen, C. L. Chao, Z. Y. Li, P. C. Yu, H. C. Kuo, T. C. Lu,S. C. Wang, K. M. Lau, and S. J. Cheng, “Nanoscale epitaxial lateralovergrowth of GaN-based light-emitting diodes on a SiO2 nanorod-arraypatterned sapphire template,” Appl. Phys. Lett., vol. 93, pp. 081108-1–081108-3, 2008.

[4] R. H. Horng, W. K. Wang, S. C. Huang, S. Y. Huang, S. H. Lin, C. F. Lin,and D. S. Wuu, “Growth and characterization of 380-nm InGaN/AlGaNLEDs grown on patterned sapphire substrates,” J. Cryst. Growth, vol. 298,pp. 219–222, 2007.

[5] S. Bohyama, H. Miyake, K. Hiramatsu, Y. Tsuchida, and T. Maeda, “Free-standing GaN substrate by advanced facet-controlled epitaxial lateral over-growth technique with masking side facets,” Jpn. J. Appl. Phys., vol. 44,no. 1, pp. L24–L26, 2005.

[6] Y. D. Wang, K. Y. Zang, S. J. Chua, S. Tripathy, H. L. Zhou, andC. G. Fonstad, “Improvement of microstructural and optical properties ofGaN layer on sapphire by nanoscale lateral epitaxial overgrowth,” Appl.Phys. Lett., vol. 88, pp. 211908-1–211908-3, 2006.

[7] W. Y. Lin, D. S. Wuu, K. F. Pan, S. H. Huang, C. E. Lee, W. K. Wang,S. C. Hsu, Y. Y. Su, S. Y. Huang, and R. H. Horng, “High-power GaN-mirror-Cu light-emitting diodes for vertical current injection using laserliftoff and electroplating techniques,” IEEE Photon. Tech. Lett., vol. 17,no. 9, pp. 1809–1811, Sep. 2005.

[8] S. C. Huang, D. S. Wuu, P. Y. Wu, W. Y. Lin, P. M. Tu, Y. C. Yeh, C. P. Hsu,and S. H. Chan, “Improved output power of 400-nm InGaN/AlGaN LEDsusing a novel surface roughening technique,” J. Cryst. Growth, vol. 311,pp. 867–870, 2009.

[9] F. Habel, P. Bruckner, and F. Scholz, “Marker layers for the development ofa multistep GaN FACELO process,” J. Cryst. Growth, vol. 272, pp. 515–519, 2004.

[10] K. Hiramatsu, K. Nishiyama, M. Onishi, H. Mizutani, M. Narukawa,A. Motogaito, H. Miyake, Y. Iyechika, and T. Maeda, “Fabrication andcharacterization of low defect density GaN using facet-controlled epitaxiallateral overgrowth (FACELO),” J. Cryst. Growth, vol. 221, pp. 316–326,2000.

[11] K. Yamamoto, H. Ishikawa, T. Egawa, T. Jimbo, and M. Umeno, “EBICobservation of n-GaN grown on sapphire substrates by MOCVD,” J.Cryst. Growth, vol. 189/190, pp. 575–579, 1998.

[12] H. Heinke, V. Kirchner, S. Einfeldt, and D. Hommel, “X-ray diffractionanalysis of the defect structure in epitaxial GaN,” Appl. Phys. Lett., vol. 77,no. 14, pp. 2145–2147, 2000.

[13] B. Heying, X. H. Wu, S. Keller, Y. Li, D. Kapolnek, B. P. Keller,S. P. DenBaars, and J. S. Speck, “Role of threading dislocation struc-ture on the x-ray diffraction peak widths in epitaxial GaN films,” Appl.Phys. Lett., vol. 68, no. 5, pp. 643–645, 1996.

1136 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 4, JULY/AUGUST 2009

Shih-Cheng Huang received the M.S. degree inmaterials science and engineering from Feng ChiaUniversity, Taichung, Taiwan, in 1998. He is cur-rently a Ph.D. candidate of the Department of Ma-terials Science and Engineering of National ChungHsing University, Taichung, Taiwan.

His research focuses on metalorganic chemical va-por deposition (MOCVD) growth of III-nitride-basedoptoelectronic devices. He worked in the field ofIII-nitride compound materials for ten years. Hehas been a Senior Manager of Advanced Optoelec-

tronic Technology (AOT), Inc. and in charge of high-performance UV-LEDsdevelopment.

Dong-Sing Wuu received the B.S., M.S., and Ph.D.degrees in electrical engineering from the NationalSun Yat-Sen University, Kaohsiung, Taiwan, in 1985,1987, and 1991, respectively.

He has done work in the field of optoelectronicdevices [LEDs, laser diodes (LDs), and photodiodes(PDs)] and ink-jet printheads at the Industry Technol-ogy Research Institute, Hsinchu, Taiwan, from 1991to 1995. In 1995, he joined Da-Yeh University as anAssociate Professor in the Department of ElectricalEngineering. Since 2001, he has been a Professor in

the Department of Materials Science and Engineering, National Chung-HsingUniversity, Taichung, Taiwan. He has authored or coauthored more than 100technical papers in international scientific journals and holds more than 60patents in his fields of expertise. His main interests are solid-state optoelec-tronic devices and thin-film processing.

Prof. Wuu has been awarded by the Ministry of Education of Taiwan for In-dustry/University Corporation Project in 2004, by the National Science Councilof Taiwan for the excellent technology transfer of high-power LEDs in 2006,and by Chi Mei Optoelectronics for the First Prize of Chi Mei Award in 2008.

Peng-Yi Wu received the B.S. degree from the De-partment of Physics, Tamkang University, Taipei,Taiwan, in 2002, and the M.S. degree from the Grad-uate Institute of Electro-Optical Engineering, ChangGung University, Taoyuan, Taiwan, in 2005.

His research interests include development ofGaN-based optoelectronic semiconductors, espe-cially in UV-LEDs.

Shih-Hsiung Chan received the B.S. degree inelectrical engineering from the National ChengKung University, Tainan, Taiwan, in 1987, and theM.S. and Ph.D. degrees in electronics engineeringfrom the National Chiao Tung University, Hsinchu,Taiwan, in 1989 and 1994, respectively.

He founded Advanced Optoelectronic Technology(AOT), Inc., in 1999, where he has been the Chief Ex-ecutive Officer (CEO). His current research interestsinclude III–V compound semiconductor growth andprocess.