Open Access

Enhanced performance of nitride-based ultraviolet vertical-injection light-emitting diodes by non-insulation current blocking layer and textured surface

Nanoscale Research Letters20149:699

https://doi.org/10.1186/1556-276X-9-699

Received: 22 August 2014

Accepted: 11 December 2014

Published: 29 December 2014

Abstract

For the purpose of light extraction and efficiency enhancement, the nitride-based ultraviolet vertical-injection light-emitting diodes (UV-VLEDs) with non-insulation current blocking layer (n-CBL) and optimized textured surface were fabricated. The optical and electrical characteristics were investigated in this n-CBL UV-VLED. Furthermore, the efficiency of optimized structure was improved by 5 ~ 6 times compared to our reference.

Keywords

Gallium nitride Light-emitting diode Vertical injection Ultraviolet Current blocking layer Textured surface

Background

The importance of high-power nitride-based light-emitting diodes (LEDs) has been rising since the last 20 years, and they are extensively used in outdoor displays, vehicle lightings, and backlights. With current trends in the consumer market, they are on the pace to replace incandescent bulbs and fluorescent lamps in the next decade [1]. While blue LEDs take the biggest part of GaN-based devices, ultraviolet (UV) emitters are also very crucial for chemical ink curing, flame detection, optical storage, water purification, and phosphor excitation [2]. In the mean time, the high-quality blue/green LEDs are commercially available, but the UV-LEDs are still low in efficiency and difficult to manufacture. Many reasons contribute to this situation: one of them is due to the inherent absorption from the GaN layer. Another one is caused by bad thermal conduction of the sapphire substrate. Third, low light extraction due to total internal reflection also plays a certain role [3]. Finally, the notorious efficiency droop that exhibits at high current condition can also deteriorate the performance of UV-LEDs [4, 5].

The reduction of efficiency is a direct loss of output power and thus leads to the increase of cost. How to solve this droop issue and improve external quantum efficiency (EQE) become important for both industrial companies and academic labs, and many previous efforts have been demonstrated [610]. Other than the fundamental droop issue, recently, laser lift-off (LLO) LEDs were demonstrated to eliminate the bad thermal dissipation of sapphire. Some scholars are using very high thermal conductivity material to solve this problem, such as electroplated copper (400 W/k-m) or metal base substrate [11, 12]. Because of difficulties in metal cutting, silicon substrate becomes an alternative to replace sapphire substrate as well [1315]. Great enhancements at high current efficiency and output power via wafer bonding technology and LLO have been shown [1618]. Other non-epitaxial improvements, such as dielectric current blocking layer (CBL) and surface textures, can also provide significant results in the past [1930].

In this article, different technologies such as a non-insulation current blocking layer, wafer bonding, LLO, and surface treatment processes were implemented to fabricate the nitride-based ultraviolet vertical-injection light-emitting diodes (UV-VLEDs). The optical and electrical property enhancement of UV-VLEDs will be reported in the following.

Methods

The LED structures were grown on (0001) sapphire substrates by a metal-organic chemical vapor deposition (MOCVD) system. The epitaxial structure of the 365-nm UV-VLED is composed of a 2.2-μm-thick Si-doped n-GaN layer, a 2.0-μm-thick Si-doped n-AlGaN cladding layer, six-period AlGaN/InGaN multiple quantum wells (MQWs), a 0.2-μm-thick Mg-doped p-AlGaN electron blocking layer, and a 0.3-μm-thick Mg-doped p-GaN layer. A 20-nm non-insulating current blocking layer (n-CBL) can be patterned on the top of the p-GaN surface after the growth. This n-CBL layer can be used as a current aperture layer, which regulates the current flow via different resistivities among different current paths [31, 32].

Regular semiconductor processes are used to fabricate the devices. The multilayer metal systems Ni(5 Å)/Ag(1,000 Å)/Ti(300 Å)/Pt(500 Å)/Ti(300 Å)/Pt(500 Å) and Cr(300 Å)/Pt(500 Å)/Au(12,000 Å) were deposited by electron beam evaporation at a pressure of 1 × 10-6 Torr to serve as the p-contact and bonding metal. After the metal deposition, the specimen was bonded to the Si substrate with Cr(300 Å)/Pt(500 Å)/Au(12,000 Å) at 220°C for 30 min. Through a wafer bonding technique, the substrate is transferred into a highly thermal conductive silicon substrate to provide great thermal dissipation, and this new substrate can potentially provide a platform for light-emitting devices to achieve high brightness operation. Then, the sapphire substrate was removed by an LLO process. A KrF excimer laser at a wavelength of 248 nm with a pulse width of 25 ns was used for the LLO process. The laser with a beam size of 0.3 mm × 0.3 mm was incident from the backside of the substrate onto the sapphire/n-GaN interface to decompose GaN into Ga and N. After the sapphire substrate removal, the specimen was dipped into a HCl solution to get rid of the residual Ga on the n-GaN. The details of the LLO process are described in [16]. To eliminate the possible UV absorption caused by laser damage in the n-GaN target layer, this layer was removed by inductively coupled plasma (ICP) dry etching. Additionally, in order to enhance light extraction, a 40% KOH solution at 90°C was used to create the surface roughness of the n-GaN epilayer under different time durations: (a) 1 min and (b) 2 min. As shown in Figure 1b,c, a multiple-layer structure of Ti(300 Å)/Al(1,500 Å)/Ni(1,000 Å)/Au(1.2 μm) was deposited on the surface of the n-GaN epilayer to serve as the n-contact. Finally, the UV-VLED chip was cut into square pieces with a dimension of 1.15 mm × 1.15 mm. In addition, a similar UV-VLED structure without the n-CBL and textured surface was also fabricated for comparison, denoted as conventional vertical LEDs (C-VLED). Note that the depth of the etched n-GaN of C-VLED is 2.2 μm. Figure 1 shows the schematic diagram of these UV-VLEDs.
Figure 1

Schematic diagram of UV-VLEDs. Schematic diagram of (a) C-VLED, (b) UV-VLED with n-CBL and larger pyramid textured surface (UV-VLED-1), (c) UV-VLED with n-CBL and smaller pyramid textured surface (UV-VLED-2), and (d) UV-VLED only with n-CBL (UV-VLED-3).

Figures 2 and 3 present typical scanning electron microscope (SEM) and atomic force microscope (AFM) images of the etched n-GaN surface appearance for these UV-VLED samples. The insets in Figure 2 are the cross-sectional views of the etched surfaces. Among the n-CBL samples, different degrees of surface roughness are also fabricated to test their effects on output power. By varying the KOH etching time, three different degrees of surface morphology and surface roughness (Rms) can be achieved, as shown in Table 1. We noticed that the pyramid dimensions and pyramidal distribution density is inversely proportional with the etching time. With this result, we will find the relationship of luminous intensity and efficiency corresponds to different degrees of roughness.
Figure 2

SEM images of emission area. (a) C-VLED after ICP 22-kÅ-deep dry etching, (b) UV-VLED-1 after ICP 5-kÅ-deep dry etching and KOH dipped for 120 s, (c) UV-VLED-2 after ICP 15-kÅ-deep dry etching and KOH dipped for 60 s, and (d) UV-VLED-3 after ICP 22-kÅ-deep dry etching.

Figure 3

AFM images of emission area. (a) C-VLED after ICP 22-kÅ-deep dry etching, (b) UV-VLED-1 after ICP 5-kÅ-deep dry etching and KOH dipped for 120 s, (c) UV-VLED-2 after ICP 15-kÅ-deep dry etching and KOH dipped for 60 s, and (d) UV-VLED-3 after ICP 22-kÅ-deep dry etching.

Table 1

Pyramid size and surface roughness in each UV-VLED sample

Type of device

Pyramid size

Surface roughness (Rms)

UV-VLED-1

0.8 to 1.5 μm

377 nm

UV-VLED-2

0.3 to 1.0 μm

163 nm

UV-VLED-3

Flat

17 nm

Results and discussion

Figure 4 shows the room-temperature electroluminescence (EL) spectra of these UV-VLEDs under a forward injection current of 350 mA. The luminescent properties of the fabricated UV-VLEDs were measured by a calibrated integrating sphere at room temperature. The emission dominant wavelength for these UV-VLEDs was about 366 ~ 371 nm. As shown in Figure 4, UV-VLED-2 displays superior emission intensity. In addition to discuss changes in intensity, we also observed full width at half maximum (FWHM) and wavelength shift problem. The FWHM of spectrum in these samples is as follows: C-VLED (about 14.8 nm), UV-VLED-1 (about 14.4 nm), UV-VLED-2 (about 13.3 nm), and UV-VLED-3 (about 12.5 nm). From the EL results, the dominant mechanisms for the wavelength shifts and spectral width changes can be attributed to two major reasons: (1) The different stresses on the epitaxial structure due to the thinned GaN substrate. This change of stress can relieve some of the quantum-confined Stark effect (QCSE) on the MQW, which can move the EL peak to shorter wavelength and narrower linewidth [3336]. (2) With better current spreading or reduced current crowding effect, the local heating of the chip can be greatly improved and such a longer wavelength can be observed in the C-LED, which does not have an n-CBL layer [3739].In Figure 5, the correlation between the surface morphology and output power is presented. A positive correlation could be observed from the plot. The rougher the surface is, the higher the output power of the device becomes, which is a clear indication of better light extraction. Following this result, we pick some marked devices to execute a detailed optical and electrical characteristic comparison.
Figure 4

Room-temperature EL spectra of UV-VLEDs at 350 mA.

Figure 5

Light output power corresponds to different roughness Rms conditions.

Figure 6 illustrates the light-current-voltage (L-I-V) characteristics of these UV-VLEDs. With an injection current of 350 mA, the forward voltages were about 3.3 ~ 3.6 V for these UV-VLEDs. However, the forward voltage of UV-VLED-2 is decreased by 0.15 V from the value of C-VLED. We believe the reason is due to the damages from various etching processes. Great enhancement in output power can be observed from the UV-VLED-1, UV-VLED-2, and UV-VLED-3 when compared to the C-VLED result. Among these three cases, the UV-VLED-2 posts the best increment, and this great output power improvement (approximately 522%) could be mainly attributed to the n-CBL and textured surface. From cross-examination among our samples, different enhancement mechanisms can be identified comparatively. Assuming the epitaxial qualities of these samples are the same, the different fabrication processes distinguished the performances of the chips. First, between the C-VLED and UV-VLED-3, the only difference is the insertion of the n-CBL layer, and the power enhancement is 214% at 350 mA. Second, among the three samples of UV-VLED-1, UV-VLED-2, and UV-VLED-3, the only difference among them is the surface texture and thus the light extraction efficiency. The L-I comparison shows 98% and 61% of increase between UV-VLED-1 vs. UV-VLED-3 and between UV-VLED-2 vs. UV-VLED-3, respectively. Combining these two effects (n-CBL and texture), the overall enhancement factor is calculated as [(1 + 98%) × (1 + 214%) - 1] = 521%, which is close to the observed value between UV-VLED-2 and C-VLED (522%). The near-field images of these UV-VLEDs are shown in Figure 7. It can be seen that the light emission is more uniform in the UV-VLED-2 than that of the C-VLED. Between C-VLED and UV-VLED-3, the uniformity improves greatly due to the introduction of the n-CBL which can distribute current more evenly and avoid current crowding.
Figure 6

Light-current-voltage ( L - I - V ) characteristics of UV-VLEDs.

Figure 7

Near-field images of UV-VLEDs. (a) C-VLED, (b) UV-VLED with n-CBL and larger pyramid textured surface (UV-VLED-1), (c) UV-VLED with n-CBL and smaller pyramid textured surface (UV-VLED-2), and (d) UV-VLED only with n-CBL (UV-VLED-3).

Figure 8 shows the relative EQE as a function of current for these UV-VLEDs, measured under room temperature pulse mode operation. A significant difference in device efficiency was observed at an injection current of 350 mA. The efficiency values of the C-VLED, UV-VLED-1, UV-VLED-2, and UV-VLED-3 were 11%, 60%, 72%, and 34%, respectively. Compared with the C-VLED, the efficiency of UV-VLED-2 was therefore increased by 6.5 times. The details of comparison data are shown in Tables 2, 3, and 4. This influence from adopted n-CBL and various textured surfaces was as below: Comparing with or without n-CBL, the efficiency of the UV-VLEDs was also increased by 3.1 times. In addition, the efficiencies of various textured surfaces were increased by 1.8 and 2.1 times, respectively.
Figure 8

Relative EQE curves of UV-VLEDs.

Table 2

Key parameters and measured data in different sample conditions

 

Sample ID

Adopted n-CBL

GaN removed depth (kÅ)

KOH etching time (min)

Rms (nm)

LOP

EQE (%)

(a)

C-VLED

No

22

0

20

138

11

(b)

UV-VLED-1

Yes

5

2

163

698

60

(c)

UV-VLED-2

Yes

15

1

377

858

72

(d)

UV-VLED-3

Yes

22

0

17

433

34

Table 3

Improvement of light output power correspond to different conditions

 

Sample ID

LOP

LOP improvement

n-CBL influence correspond to (a)

Roughness influence correspond to (d)

Influence of combination correspond to (a)

(a)

C-VLED

138

   

(b)

UV-VLED-1

698

 

61%

 

(c)

UV-VLED-2

858

 

98%

522%

(d)

UV-VLED-3

433

214%

  
Table 4

Improvement of external quantum efficiency correspond to different conditions

 

Sample ID

EQE (%)

EQE improvement

n-CBL influence correspond to (a)

Roughness influence correspond to (d)

Influence of combination correspond to (a)

(a)

C-VLED

11

   

(b)

UV-VLED-1

60

 

1.8

 

(c)

UV-VLED-2

72

 

2.1

6.5

(d)

UV-VLED-3

34

3.1

  

Conclusions

In conclusion, the UV-VLEDs were demonstrated and investigated - including the significance of the existence of n-CBL and discussion on the influence from extents of textured surface. The n-CBL influence is mentioned in the previous text very clearly. Furthermore, the results comprised n-CBL and textured surface; these two processes that indicated the output power intensity and relative external quantum efficiency of the better UV-VLED-2 increased approximately 525% and 6.5 times compared to the C-VLED, respectively. Consequently, we believe that the n-CBL and optimized textured surface should be promising for the future applications of solid-state lighting.

Abbreviations

AFM: 

atomic force microscope

CBL: 

current blocking layer

C-VLED: 

conventional vertical LEDs

EL: 

electroluminescence

EQE: 

external quantum efficiency

FWHM: 

full width at half maximum

LEDs: 

light-emitting diodes

L-I-V

light-current-voltage

LLO: 

laser lift-off

MOCVD: 

metal-organic chemical vapor deposition

n-CBL: 

non-insulation current blocking layer

SEM: 

scanning electron microscope

UV: 

ultraviolet

UV-VLEDs: 

ultraviolet vertical-injection light-emitting diodes.

Declarations

Acknowledgements

This work was funded by the National Science Council in Taiwan under grant number. This work was supported in part by the National Science Council in Taiwan under grant numbers NSC-103-3113-E-009-001-CC2 and NSC101-2221-E-009-046-MY3.

Authors’ Affiliations

(1)
Institute of Lighting and Energy Photonics, National Chiao Tung University
(2)
Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University
(3)
Institute of Photonics System, National Chiao Tung University

References

  1. Schubert EF, Kim JK: Solid-state light sources getting smart. Science 2005, 308: 1274–1278. 10.1126/science.1108712View ArticleGoogle Scholar
  2. Narukawa Y, NiKi I, Izuno K, Yamada M, Murazaki Y, Mukai T: Phosphor-conversion white light emitting diode using InGaN near-ultraviolet chip. Jpn J Appl Phys 2002, Part 2(4A):L371-L373.View ArticleGoogle Scholar
  3. Broditsky M, Yablonovitch E: Light-emitting-diode extraction efficiency. Proc SPIE 1997, 3002: 119–122. 10.1117/12.271033View ArticleGoogle Scholar
  4. Meyaard DS, Shan QF, Cho JH, Schubert EF, Han SH, Kim MH, Sone CS, Oh SJ, Kim JK: Temperature dependent efficiency droop in GaInN light-emitting diodes with different current densities. Appl Phys Lett 2012, 100: 081106–1–081106–3.View ArticleGoogle Scholar
  5. Zhao HP, Liu GY, Ronald AA, Nelson T: Current injection efficiency induced efficiency-droop in InGaN quantum well light-emitting diodes. Solid State Electron 2010, 54: 1119–1124. 10.1016/j.sse.2010.05.019View ArticleGoogle Scholar
  6. Kim MH, Schubert MF, Dai Q, Kim JK, Schubert EF, Piprek J, Park Y: Origin of efficiency droop in GaN-based light-emitting diodes. Appl Phys Lett 2007, 91: 183507–1–183507–3.Google Scholar
  7. Hader J, Moloney JV, Pasenow B, Koch SW, Sabathil M, Linder N, Lutgen S: On the importance of radiative and Auger losses in GaN-based quantum wells. Appl Phys Lett 2008, 92: 261103–1–261103–3.View ArticleGoogle Scholar
  8. Li SB, Ware ME, Wu J, Paul M, Wang ZM, Wu ZM, Jiang YD, Salamo GJ: Polarization induced pn-junction without dopant in graded AlGaN coherently strained on GaN. Appl Phys Lett 2012, 101: 122103. 10.1063/1.4753993View ArticleGoogle Scholar
  9. Li SB, Zhang T, Wu J, Yang YJ, Wang ZM, Wu ZM, Chen Z, Jiang YD: Polarization induced hole doping in graded AlxGa1-xN (x = 0.71) layer grown by molecular beam epitaxy. Appl Phys Lett 2013, 102: 062108. 10.1063/1.4792685View ArticleGoogle Scholar
  10. Li SB, Ware ME, Wu J, Kunets VP, Hawkridge M, Minor P, Wang ZM, Wu ZM, Jiang YD, Salamo GJ: Polarization doping: reservoir effects of the substrate in AlGaN graded layers. J Appl Phys 2012, 112: 053711. 10.1063/1.4750039View ArticleGoogle Scholar
  11. Lin WY, Wuu DS, Pan KF, Huang SH, Lee CE, Wang WK, Hsu SC, Su YY, Huang SY, Horng RH: High-power GaN–mirror–Cu light-emitting diodes for vertical current injection using laser liftoff and electroplating techniques. IEEE Photon Technol Lett 2005, 17(9):1809–1811.View ArticleGoogle Scholar
  12. Chu JT, Huang HW, Kao CC, Liang WD, Lai FI, Chu CF, Kuo HC, Wang SC: Fabrication of large-area GaN-based light-emitting diodes on Cu substrate. Jpn J Appl Phys 2005, 44(4B):2509–2511. 10.1143/JJAP.44.2509View ArticleGoogle Scholar
  13. Hsu SC, Liu CY: Fabrication of thin-GaN LED structures by Au–Si wafer bonding. Electrochem Solid-State Lett 2006, 9(5):G171-G173. 10.1149/1.2181293View ArticleGoogle Scholar
  14. Xiong CB, Jiang FY, Fang WQ, Wang L, Mo CN, Liu HC: The characteristics of GaN-based blue LED on Si substrate. J Lumin 2007, 122–123: 185–187.View ArticleGoogle Scholar
  15. Chang CL, Chuang YC, Liu CY: Ag/Au diffusion wafer bonding for thin-GaN LED fabrication electrochemical and solid-state letters. Electrochem Solid-State Lett 2007, 10(11):H344. 10.1149/1.2777875View ArticleGoogle Scholar
  16. Ha JS, Lee SW, Lee HJ, Lee HJ, Lee SH, Goto H, Kato T, Katsushi F, Cho MW, Yao T: The fabrication of vertical light-emitting diodes using chemical lift-off process. IEEE Photon Technol Lett 2008, 20(3):175–177.View ArticleGoogle Scholar
  17. Lin CL, Wang SJ, Liu CY: High-thermal-stability and low-resistance p-GaN contact for thin-GaN light emitting diodes structure. Electrochem Solid-State Lett 2005, 8(10):G265-G267. 10.1149/1.2012203View ArticleGoogle Scholar
  18. Zhang JY, Liu WJ, Chen M, Hu XL, Lv XQ, Ying LY, Zhang BP: Performance enhancement of GaN-based light emitting diodes by transfer from sapphire to silicon substrate using double-transfer technique. Nanoscale Res Lett 2012, 7: 244. 10.1186/1556-276X-7-244View ArticleGoogle Scholar
  19. Keppens A, Ryckaert WR, Deconinck G, Hanselaer P: High power light-emitting diode junction temperature determination from current–voltage characteristics. J Appl Phys 2008, 104(9):093104. 10.1063/1.3009966View ArticleGoogle Scholar
  20. Chu CF, Lai FI, Chu JT, Yu CC, Lin CF, Kuo HC, Wang SC: Study of GaN light-emitting diodes fabricated by laser lift-off technique. J Appl Phys 2004, 95: 3916–3922. 10.1063/1.1651338View ArticleGoogle Scholar
  21. Kao CC, Kuo HC, Yeh KF, Chu JT, Peng WL, Huang HW, Lu TC, Wang SC: GaN laser lift-off light-emitting diodes formed by ICP dry etching. IEEE Photon Technol Lett 2007, 19(11):849–885.View ArticleGoogle Scholar
  22. Chang SJ, Shen CF, Chen WS, Ko TK, Kuo CT, Yu KH, Shei SC, Chioud YZ: Nitride-based LEDs with an insulating SiO2 layer underneath p-pad electrodes. Electrochem Solid-State Lett 2007, 10: 175–177.View ArticleGoogle Scholar
  23. Fujii T, Gao Y, Sharma R, Hu EL, DenBaars SP, Nakamura S: Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening. Appl Phys Lett 2004, 84: 855–857. 10.1063/1.1645992View ArticleGoogle Scholar
  24. Windisch R, Rooman C, Meinlschmidt S, Kiesel P, Zipperer D, Döhler GH, Dutta B, Kuijk M, Borghs G, Heremans P: Impact of texture-enhanced transmission on high-efficiency surface-textured light-emitting diodes. Appl Phys Lett 2001, 79: 2315–2317. 10.1063/1.1397758View ArticleGoogle Scholar
  25. Guo X, Schubert EF: Current crowding in GaN/InGaN light emitting diodes on insulating substrates. J Appl Phys 2001, 90(8):4191–4195. 10.1063/1.1403665View ArticleGoogle Scholar
  26. Ryu HY: Large enhancement of light extraction efficiency in AlGaN-based nanorod ultraviolet light-emitting diode structures. Nanoscale Res Lett 2014, 9: 58. 10.1186/1556-276X-9-58View ArticleGoogle Scholar
  27. You YH, Su VC, Ho TE, Lin BW, Lee ML, Atanu D, Hsu WC, Kuan CH, Lin RM: Influence of patterned sapphire substrates with different symmetry on the light output power of InGaN-based LEDs. Nanoscale Res Lett 2014, 9: 596. 10.1186/1556-276X-9-596View ArticleGoogle Scholar
  28. Du CF, Lee CH, Cheng CT, Lin KH, Sheu JK, Hsu HC: Ultraviolet/blue light-emitting diodes based on single horizontal ZnO microrod/GaN heterojunction. Nanoscale Res Lett 2014, 9: 446. 10.1186/1556-276X-9-446View ArticleGoogle Scholar
  29. Lai FI, Yang JF: Enhancement of light output power of GaN-based light-emitting diodes with photonic quasi-crystal patterned on p-GaN surface and n-side sidewall roughing. Nanoscale Res Lett 2013, 8: 244. 10.1186/1556-276X-8-244View ArticleGoogle Scholar
  30. Chen LC, Tsai WF: Properties of GaN-based light-emitting diodes on patterned sapphire substrate coated with silver nanoparticles prepared by mask-free chemical etching. Nanoscale Res Lett 2013, 8: 157. 10.1186/1556-276X-8-157View ArticleGoogle Scholar
  31. Lee CM, Chuo CC, Liu YC, Chen IL, Chyi JI: InGaN-GaN MQW LEDs with current blocking layer formed by selective activation. IEEE Electron Device Lett 2004, 25(6):384–386. 10.1109/LED.2004.829666View ArticleGoogle Scholar
  32. Huh C, Lee JM, Kim DJ, Park SJ: Improvement in light-output efficiency of InGaN/GaN multiple-quantum well light-emitting diodes by current blocking layer. J Appl Phys 2002, 92(5):2248–2250. 10.1063/1.1497467View ArticleGoogle Scholar
  33. Khan N, Li J: Effects of compressive strain on optical properties of InxGa1 - xN/GaN quantum wells. Appl Phys Lett 2006, 89: 151916–1–151916–3.Google Scholar
  34. Chiu CH, Tu PM, Lin CC, Lin DW, Li ZY, Chuang KL, Chang JR, Lu TC, Zan HW, Chen CY, Kuo HC, Wang SC, Chang CY: Highly efficient and bright LEDs overgrown on GaN nanopillar substrates. IEEE J Sel Top Quantum Electron 2011, 17(4):971–978.View ArticleGoogle Scholar
  35. Zang KY, Wang YD, Liu HF, Chua SJ: Structural and optical properties of InGaN/GaN multiple quantum wells grown on nano-air-bridged GaN template. Appl Phys Lett 2006, 89(17):171921--1–171921–3.View ArticleGoogle Scholar
  36. Kikuchi A, Tada M, Miwa K, Kishino K: Growth and characterization of InGaN/GaN nanocolumn LED. Proc SPIE 2006, 6129: 36–43.Google Scholar
  37. Mukai T, Yamada M, Nakamura S: Characteristics of InGaN-based UV/blue/green/amber/red light-emitting diodes. Jpn J Appl Phys 1999, 38(7A):3976–3981.View ArticleGoogle Scholar
  38. Cao XA, LeBoeuf SF, Stecher TE: Temperature-dependent electroluminescence of AlGaN-based UV LEDs. IEEE Electron Device Lett 2006, 27(5):329–331.View ArticleGoogle Scholar
  39. Chitnis A, Sun J, Mandavilli V, Pachipulusu R, Wu S, Gaevski M, Adivarahan V, Zhang JP, Asif Khan M, Sarua A, Kuball M: Self-heating effects at high pump currents in deep ultraviolet light-emitting diodes at 324 nm. Appl Phys Lett 2002, 81(18):3491–3493. 10.1063/1.1518155View ArticleGoogle Scholar

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© Chiang et al.; licensee Springer. 2014

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.