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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 14 — Jul. 4, 2011
  • pp: 12919–12924
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Orange a-plane InGaN/GaN light-emitting diodes grown on r-plane sapphire substrates

Yong Gon Seo, Kwang Hyeon Baik, Hooyoung Song, Ji-Su Son, Kyunghwan Oh, and Sung-Min Hwang  »View Author Affiliations


Optics Express, Vol. 19, Issue 14, pp. 12919-12924 (2011)
http://dx.doi.org/10.1364/OE.19.012919


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Abstract

We report on orange a-plane light-emitting diodes (LEDs) with InGaN single quantum well (SQW) grown on r-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). The peak wavelength and the full-width at half maximum (FWHM) at a drive current of 20mA were 612.2 nm and 72 nm, respectively. The device demonstrated a blue shift in emission wavelength from 614.6 nm at 10 mA to 607.5 nm at 100 mA, representing a net shift of 7.1 nm over a 90 mA range, which is the longest wavelength compared with reported values in nonpolar LEDs. The polarization ratio values obtained from the orange LED varied between 0.36 and 0.44 from 10 to 100mA and a weak dependence of the polarization ratio on the injection current was observed.

© 2011 OSA

1. Introduction

Commercially available gallium nitride based light emitting diodes (LEDs) are grown along the polar c-axis direction and have strong built-in electric fields, such as spontaneous and piezoelectric polarization fields, which reduce the overlap of the electron and hole wave functions within quantum wells (QWs), resulting in lower device efficiencies [1

1. S. Chichibu, T. Azuhata, T. Sota, and S. Nakamura, “Spontaneous emission of localized excitons in InGaN single and multiquantum well structures,” Appl. Phys. Lett. 69(27), 4188–4190 (1996). [CrossRef]

,2

2. F. Bernardini, V. Fiorentini, and D. Vanderbilt, “Spontaneous polarization and piezoelectric constants of III-V nitrides,” Phys. Rev. B 56(16), R10024–R10027 (1997). [CrossRef]

]. In the LEDs, the carrier-induced screening by the polarization-induced electric fields presents a blueshift in peak emission wavelength with increasing injection current [3

3. P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, and K. H. Ploog, “Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes,” Nature 406(6798), 865–868 (2000). [CrossRef] [PubMed]

]. To eliminate these polarization effects in InGaN QWs, several groups have tried to fabricate LEDs grown on nonpolar planes such as the (10-10) m-plane or the (11-20) a-plane [4

4. A. Chakraborty, B. A. Haskell, S. Keller, J. S. Speck, S. P. DenBaars, S. Nakamura, and U. K. Mishra, “Demonstration of nonpolar m-plane InGaN/GaN light-emitting diodes on free-standing m-plane GaN substrates,” Jpn. J. Appl. Phys. 44(5), L173–L175 (2005). [CrossRef]

7

7. S.-M. Hwang, Y. G. Seo, K. H. Baik, I.-S. Cho, J. H. Baek, S. Jung, T. G. Kim, and M. Cho, “Demonstration of nonpolar a-plane InGaN/GaN light emitting diode on r-plane sapphire substrate,” Appl. Phys. Lett. 95(7), 071101 (2009). [CrossRef]

]. Another approach to reduce the polarization effects is to use the inclined planes with respect to c-direction, i.e., semipolar planes, (10-1-1), (11-22), (10-1-3), and (20-21) [8

8. H. Masui, T. J. Baker, R. Sharma, P. M. Pattison, M. Iza, H. Zhong, S. Nakamura, and S. P. DenBaars, “First-moment analysis of polarized light emission from InGaN/GaN light-emitting diodes prepared semipolar planes,” Jpn. J. Appl. Phys. 45(34), L904–L906 (2006). [CrossRef]

10

10. R. B. Chung, Y. Lin, I. Koslow, N. Pfaff, H. Ohta, J. Ha, S. P. DenBaars, and S. Nakamura, “Electroluminescence characterization of (20-21) InGaN/GaN light emitting diodes with various wavelengths,” Jpn. J. Appl. Phys. 49(7), 070203 (2010). [CrossRef]

]. Besides circumventing the polarization effects, the LEDs grown on the nonpolar and the semipolar planes also emit the polarized light due to optical gain anisotropy [11

11. N. F. Gardner, J. C. Kim, J. J. Wierer, Y. C. Shen, and M. R. Krames, “Polarization anisotropy in the electroluminescence of m-plane InGaN–GaN multiple-quantum-well light-emitting diodes,” Appl. Phys. Lett. 86(11), 111101 (2005). [CrossRef]

14

14. H. Masui, T. J. Baker, M. Iza, H. Zhong, S. Nakamura, and S. P. DenBaars, “Light-polarization characteristics of electroluminescence from InGaN/GaN light-emitting diodes prepared on (11-22) plane GaN,” J. Appl. Phys. 100(11), 113109 (2006). [CrossRef]

].

The research on the longer wavelength InGaN LEDs with high indium composition is required to expand the LED applications, for example monolithic white light LEDs, full color displays, and so on. Although the InGaN LEDs with various indium compositions fabricated on polar, semipolar and nonpolar planes have been widely reported [9

9. M. Funato, M. Ueda, Y. Kawakami, Y. Narukawa, T. Kosugi, M. Takahashi, and T. Mukai, “Blue, green, and amber InGaN/GaN light-emitting diodes on semipolar {11-22} GaN bulk substrates,” Jpn. J. Appl. Phys. 45(26), L659–L662 (2006). [CrossRef]

,15

15. S. Nakamura, M. Senoh, N. Iwasa, and S.-I. Nagahama, “High-brightness InGaN blue, green and yellow light-emitting diodes with quantum well structures,” Jpn. J. Appl. Phys. 34(Part 2, No. 7A), L797–L799 (1995). [CrossRef]

17

17. H. Yamada, K. Iso, M. Saito, H. Masui, K. Fujito, S. P. DenBaars, and S. Nakamura, “Compositional dependence of nonpolar m-plane InxGa1-xN/GaN light emitting diodes,” Appl. Phys. Express 1, 041101 (2008). [CrossRef]

], there are few reports on the nonpolar a-plane LEDs with long wavelength because high indium InGaN growth is very difficult. In this paper, we demonstrate the orange a-plane InGaN/GaN single quantum well (SQW) LEDs grown on r-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD).

2. Experimental details

High crystalline quality a-plane (11-20) GaN film was directly grown on r-plane (1-102) sapphire substrates by MOCVD. Trimethylgallium (TMGa), trimethylindium (TMIn), bis-magnesium (CP2Mg), monosilane (SiH4), and ammonia (NH3) were used as the Ga, In, Mg, Si, and N precursors, respectively. Hydrogen was used as the carrier gas. Figure 1
Fig. 1 Schematic view of orange a-plane InGaN/GaN SQW LED structure.
shows the schematic structure of a-plane SQW LED with high indium composition used in this study. A 150-nm thick GaN buffer layer was grown in mixed atmosphere of N2 and H2 at high temperature to obtain a minimum surface roughness and dislocation density. Subsequently, a 4.5-μm thick a-plane GaN template with a high crystalline quality surface was obtained by the conventional two-step growth method. Next, we grew a 2-μm Si-doped n-type GaN layer, followed by an active region consisting of SQW with 20-nm thick GaN barriers and 18 nm InGaN well, followed by a 120-nm thick p-type GaN:Mg layer. The more detailed growth conditions and the properties of the a-plane GaN films were reported previously [18

18. H. Song, J. Suh, E. Kyu Kim, K. Hyeon Baik, and S.-M. Hwang, “Growth of high quality a-plane GaN epi-layer on r-plane sapphire substrates with optimization of multi-buffer layer,” J. Cryst. Growth 312(21), 3122–3126 (2010). [CrossRef]

]. The growth temperature, the reactor pressure, and the V/III ratio of the InGaN layer were 740 °C, 200 mbar, and 230, respectively. To obtain the high indium composition, we maintained the low growth temperature and the low V/III ratio. We could estimate the indium composition to be 42% from the simulation using SiLENSe software. The doping concentration of the n-type GaN layer was ~4.4 × 1018 cm−3, and the hole concentration was estimated to be ~1.4 × 1018 cm−3. For a p-electrode, Ni/Au (3 nm/3 nm) was deposited on the p-GaN layer using an e-beam evaporator, followed by annealing in ambient air to form ohmic contacts. The mesa pattern with a dimension of 200 µm × 500 µm was transferred into the n-GaN layer down to 1 µm by inductively coupled plasma etching. Ti/Al/Ni/Au (20/150/30/100nm) was deposited as the n-electrode.

All measurements of orange a-plane LEDs were carried out on wafer-level under direct current operation at room temperature.

3. Results and discussion

Figure 2(a)
Fig. 2 (a) EL spectra of orange a-plane InGaN/GaN SQW LED. (b) I-V characteristic of orange LED.
shows the change in the electroluminescence (EL) spectra as the driving current is increased from 10 to 100 mA. The LED emitted in the orange spectral range and the EL spectra have a single peak up to 20 mA despite the high indium content in the InGaN layer. Above the 30 mA, there is a shoulder on the shorter wavelength side due to inhomogeneous InGaN layer. The current-voltage (I-V) characteristic of the orange a-plane LED is shown in Fig. 2(b). The forward voltage was 4 V at a forward current of 20 mA. This relatively high operating voltage in comparison to the c-plane LEDs is probably attributed to the unoptimized InGaN SQW, p-GaN layer, and Ni/Au contact.

4. Conclusion

Acknowledgments

This work was supported by the IT R&D programs of the Ministry of Knowledge Economy at Korea Electronics Technology Institute (Project No. K1002099, K1002079, and 10032325).

References and links

1.

S. Chichibu, T. Azuhata, T. Sota, and S. Nakamura, “Spontaneous emission of localized excitons in InGaN single and multiquantum well structures,” Appl. Phys. Lett. 69(27), 4188–4190 (1996). [CrossRef]

2.

F. Bernardini, V. Fiorentini, and D. Vanderbilt, “Spontaneous polarization and piezoelectric constants of III-V nitrides,” Phys. Rev. B 56(16), R10024–R10027 (1997). [CrossRef]

3.

P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, and K. H. Ploog, “Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes,” Nature 406(6798), 865–868 (2000). [CrossRef] [PubMed]

4.

A. Chakraborty, B. A. Haskell, S. Keller, J. S. Speck, S. P. DenBaars, S. Nakamura, and U. K. Mishra, “Demonstration of nonpolar m-plane InGaN/GaN light-emitting diodes on free-standing m-plane GaN substrates,” Jpn. J. Appl. Phys. 44(5), L173–L175 (2005). [CrossRef]

5.

Y. Saito, K. Okuno, S. Boyama, N. Nakada, S. Nitta, Y. Ushida, and N. Shibata, “m-plane GaInN light emitting diodes grown on patterned a-plane sapphire substrates,” Appl. Phys. Express 2, 041001 (2009). [CrossRef]

6.

T. Detchprohm, M. Zhu, Y. Li, Y. Xia, C. Wetzel, E. A. Preble, L. Liu, T. Paskova, and D. Hanser, “Green light emitting diodes on a-plane GaN bulk substrates,” Appl. Phys. Lett. 92(24), 241109 (2008). [CrossRef]

7.

S.-M. Hwang, Y. G. Seo, K. H. Baik, I.-S. Cho, J. H. Baek, S. Jung, T. G. Kim, and M. Cho, “Demonstration of nonpolar a-plane InGaN/GaN light emitting diode on r-plane sapphire substrate,” Appl. Phys. Lett. 95(7), 071101 (2009). [CrossRef]

8.

H. Masui, T. J. Baker, R. Sharma, P. M. Pattison, M. Iza, H. Zhong, S. Nakamura, and S. P. DenBaars, “First-moment analysis of polarized light emission from InGaN/GaN light-emitting diodes prepared semipolar planes,” Jpn. J. Appl. Phys. 45(34), L904–L906 (2006). [CrossRef]

9.

M. Funato, M. Ueda, Y. Kawakami, Y. Narukawa, T. Kosugi, M. Takahashi, and T. Mukai, “Blue, green, and amber InGaN/GaN light-emitting diodes on semipolar {11-22} GaN bulk substrates,” Jpn. J. Appl. Phys. 45(26), L659–L662 (2006). [CrossRef]

10.

R. B. Chung, Y. Lin, I. Koslow, N. Pfaff, H. Ohta, J. Ha, S. P. DenBaars, and S. Nakamura, “Electroluminescence characterization of (20-21) InGaN/GaN light emitting diodes with various wavelengths,” Jpn. J. Appl. Phys. 49(7), 070203 (2010). [CrossRef]

11.

N. F. Gardner, J. C. Kim, J. J. Wierer, Y. C. Shen, and M. R. Krames, “Polarization anisotropy in the electroluminescence of m-plane InGaN–GaN multiple-quantum-well light-emitting diodes,” Appl. Phys. Lett. 86(11), 111101 (2005). [CrossRef]

12.

H. Masui, A. Chakraborty, B. A. Haskell, U. K. Mishra, J. S. Speck, S. Nakamura, and S. P. DenBaars, “Polarized light emission from nonpolar InGaN light-emitting diodes grown on a bulk m-plane GaN substrate,” Jpn. J. Appl. Phys. 44(43), L1329–L1332 (2005). [CrossRef]

13.

R. Sharma, P. M. Pattison, H. Masui, R. M. Farrell, T. J. Baker, B. A. Haskell, F. Wu, S. P. DenBaars, J. S. Speck, and S. Nakamura, “Demonstration of a semipolar (10-1-3) InGaN/GaN green light emitting diode,” Appl. Phys. Lett. 87(23), 231110 (2005). [CrossRef]

14.

H. Masui, T. J. Baker, M. Iza, H. Zhong, S. Nakamura, and S. P. DenBaars, “Light-polarization characteristics of electroluminescence from InGaN/GaN light-emitting diodes prepared on (11-22) plane GaN,” J. Appl. Phys. 100(11), 113109 (2006). [CrossRef]

15.

S. Nakamura, M. Senoh, N. Iwasa, and S.-I. Nagahama, “High-brightness InGaN blue, green and yellow light-emitting diodes with quantum well structures,” Jpn. J. Appl. Phys. 34(Part 2, No. 7A), L797–L799 (1995). [CrossRef]

16.

K. P. O'Donnell, T. Breitkopf, H. Kalt, W. Van der Stricht, I. Moerman, P. Demeester, and P. G. Middleton, “Optical linewidths of InGaN light emitting diodes and epilayers,” Appl. Phys. Lett. 70(14), 1843–1845 (1997). [CrossRef]

17.

H. Yamada, K. Iso, M. Saito, H. Masui, K. Fujito, S. P. DenBaars, and S. Nakamura, “Compositional dependence of nonpolar m-plane InxGa1-xN/GaN light emitting diodes,” Appl. Phys. Express 1, 041101 (2008). [CrossRef]

18.

H. Song, J. Suh, E. Kyu Kim, K. Hyeon Baik, and S.-M. Hwang, “Growth of high quality a-plane GaN epi-layer on r-plane sapphire substrates with optimization of multi-buffer layer,” J. Cryst. Growth 312(21), 3122–3126 (2010). [CrossRef]

19.

S. You, T. Detchprohm, M. Zhu, W. Hou, E. A. Preble, D. Hanser, T. Paskova, and C. Wetzel, “Highly polarized green light emitting diode in m-axis GaInN/GaN,” Appl. Phys. Express 3(10), 102103 (2010). [CrossRef]

20.

K. Iso, H. Yamada, H. Hirasawa, N. Fellows, M. Saito, K. Fujito, S. P. DenBaars, J. S. Speck, and S. Nakamura, “High brightness blue InGaN/GaN light emitting diode on nonpolar m-plane bulk GaN substrate,” Jpn. J. Appl. Phys. 46(40), L960–L962 (2007). [CrossRef]

21.

T. Koyama, T. Onuma, H. Masui, A. Chakraborty, M. A. Haskell, S. Keller, U. K. Mishra, J. S. Speck, S. Nakamura, S. P. DenBaars, T. Sota, and S. F. Chichibu, “Prospective emission efficiency and in-plane light polarization of nonpolar m-plane InxGa1-xN/ GaN blue light emitting diodes fabricated on freestanding GaN substrates,” Appl. Phys. Lett. 89(9), 091906 (2006). [CrossRef]

22.

H.-H. Huang and Y.-R. Wu, “Study of polarization properties of light emitted from a-plane InGaN/GaN quantum well-based light emitting diodes,” J. Appl. Phys. 106(2), 023106 (2009). [CrossRef]

23.

N. Fellows, H. Sato, H. Masui, S. P. DenBaars, and S. Nakamura, “Increased polarization ratio on semipolar (11–22) InGaN/GaN light-emitting diodes with increasing indium composition,” Jpn. J. Appl. Phys. 47(10), 7854–7856 (2008). [CrossRef]

24.

H. Masui, H. Yamada, K. Iso, S. Nakamura, and S. P. DenBaars, “Optical polarization characteristics of m-oriented InGaN/GaN light-emitting diodes with various indium compositions in single-quantum-well structure,” J. Phys. D 41(22), 225104 (2008). [CrossRef]

25.

Y. G. Seo, K. H. Baik, K.-M. Song, S. Lee, H. Yoon, J.-H. Park, K. Oh, and S.-M. Hwang, “Milliwatt-class non-polar a-plane InGaN/GaN light-emitting diodes grown directly on r-plane sapphire substrates,” Curr. Appl. Phys. 10(6), 1407–1410 (2010). [CrossRef]

26.

C. H. Chiu, S. Y. Kuo, M. H. Lo, C. C. Ke, T. C. Wang, Y. T. Lee, H. C. Kuo, T. C. Lu, and S. C. Wang, “Optical properties of a-plane InGaN/GaN multiple quantum wells on r-plane sapphire substrates with different indium compositions,” J. Appl. Phys. 105(6), 063105 (2009). [CrossRef]

27.

A. M. Fischer, Z. Wu, K. Sun, Q. Wei, Y. Huang, R. Senda, D. Iida, M. Iwaya, H. Amano, and F. A. Ponce, “Misfit strain relaxation by stacking fault generation in InGaN quantum wells grown on m-plane GaN,” Appl. Phys. Express 2, 041002 (2009). [CrossRef]

OCIS Codes
(230.0230) Optical devices : Optical devices
(230.3670) Optical devices : Light-emitting diodes

ToC Category:
Optical Devices

History
Original Manuscript: May 9, 2011
Revised Manuscript: June 7, 2011
Manuscript Accepted: June 10, 2011
Published: June 20, 2011

Citation
Yong Gon Seo, Kwang Hyeon Baik, Hooyoung Song, Ji-Su Son, Kyunghwan Oh, and Sung-Min Hwang, "Orange a-plane InGaN/GaN light-emitting diodes grown on r-plane sapphire substrates," Opt. Express 19, 12919-12924 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-14-12919


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References

  1. S. Chichibu, T. Azuhata, T. Sota, and S. Nakamura, “Spontaneous emission of localized excitons in InGaN single and multiquantum well structures,” Appl. Phys. Lett. 69(27), 4188–4190 (1996). [CrossRef]
  2. F. Bernardini, V. Fiorentini, and D. Vanderbilt, “Spontaneous polarization and piezoelectric constants of III-V nitrides,” Phys. Rev. B 56(16), R10024–R10027 (1997). [CrossRef]
  3. P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, and K. H. Ploog, “Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes,” Nature 406(6798), 865–868 (2000). [CrossRef] [PubMed]
  4. A. Chakraborty, B. A. Haskell, S. Keller, J. S. Speck, S. P. DenBaars, S. Nakamura, and U. K. Mishra, “Demonstration of nonpolar m-plane InGaN/GaN light-emitting diodes on free-standing m-plane GaN substrates,” Jpn. J. Appl. Phys. 44(5), L173–L175 (2005). [CrossRef]
  5. Y. Saito, K. Okuno, S. Boyama, N. Nakada, S. Nitta, Y. Ushida, and N. Shibata, “m-plane GaInN light emitting diodes grown on patterned a-plane sapphire substrates,” Appl. Phys. Express 2, 041001 (2009). [CrossRef]
  6. T. Detchprohm, M. Zhu, Y. Li, Y. Xia, C. Wetzel, E. A. Preble, L. Liu, T. Paskova, and D. Hanser, “Green light emitting diodes on a-plane GaN bulk substrates,” Appl. Phys. Lett. 92(24), 241109 (2008). [CrossRef]
  7. S.-M. Hwang, Y. G. Seo, K. H. Baik, I.-S. Cho, J. H. Baek, S. Jung, T. G. Kim, and M. Cho, “Demonstration of nonpolar a-plane InGaN/GaN light emitting diode on r-plane sapphire substrate,” Appl. Phys. Lett. 95(7), 071101 (2009). [CrossRef]
  8. H. Masui, T. J. Baker, R. Sharma, P. M. Pattison, M. Iza, H. Zhong, S. Nakamura, and S. P. DenBaars, “First-moment analysis of polarized light emission from InGaN/GaN light-emitting diodes prepared semipolar planes,” Jpn. J. Appl. Phys. 45(34), L904–L906 (2006). [CrossRef]
  9. M. Funato, M. Ueda, Y. Kawakami, Y. Narukawa, T. Kosugi, M. Takahashi, and T. Mukai, “Blue, green, and amber InGaN/GaN light-emitting diodes on semipolar {11-22} GaN bulk substrates,” Jpn. J. Appl. Phys. 45(26), L659–L662 (2006). [CrossRef]
  10. R. B. Chung, Y. Lin, I. Koslow, N. Pfaff, H. Ohta, J. Ha, S. P. DenBaars, and S. Nakamura, “Electroluminescence characterization of (20-21) InGaN/GaN light emitting diodes with various wavelengths,” Jpn. J. Appl. Phys. 49(7), 070203 (2010). [CrossRef]
  11. N. F. Gardner, J. C. Kim, J. J. Wierer, Y. C. Shen, and M. R. Krames, “Polarization anisotropy in the electroluminescence of m-plane InGaN–GaN multiple-quantum-well light-emitting diodes,” Appl. Phys. Lett. 86(11), 111101 (2005). [CrossRef]
  12. H. Masui, A. Chakraborty, B. A. Haskell, U. K. Mishra, J. S. Speck, S. Nakamura, and S. P. DenBaars, “Polarized light emission from nonpolar InGaN light-emitting diodes grown on a bulk m-plane GaN substrate,” Jpn. J. Appl. Phys. 44(43), L1329–L1332 (2005). [CrossRef]
  13. R. Sharma, P. M. Pattison, H. Masui, R. M. Farrell, T. J. Baker, B. A. Haskell, F. Wu, S. P. DenBaars, J. S. Speck, and S. Nakamura, “Demonstration of a semipolar (10-1-3) InGaN/GaN green light emitting diode,” Appl. Phys. Lett. 87(23), 231110 (2005). [CrossRef]
  14. H. Masui, T. J. Baker, M. Iza, H. Zhong, S. Nakamura, and S. P. DenBaars, “Light-polarization characteristics of electroluminescence from InGaN/GaN light-emitting diodes prepared on (11-22) plane GaN,” J. Appl. Phys. 100(11), 113109 (2006). [CrossRef]
  15. S. Nakamura, M. Senoh, N. Iwasa, and S.-I. Nagahama, “High-brightness InGaN blue, green and yellow light-emitting diodes with quantum well structures,” Jpn. J. Appl. Phys. 34(Part 2, No. 7A), L797–L799 (1995). [CrossRef]
  16. K. P. O'Donnell, T. Breitkopf, H. Kalt, W. Van der Stricht, I. Moerman, P. Demeester, and P. G. Middleton, “Optical linewidths of InGaN light emitting diodes and epilayers,” Appl. Phys. Lett. 70(14), 1843–1845 (1997). [CrossRef]
  17. H. Yamada, K. Iso, M. Saito, H. Masui, K. Fujito, S. P. DenBaars, and S. Nakamura, “Compositional dependence of nonpolar m-plane InxGa1-xN/GaN light emitting diodes,” Appl. Phys. Express 1, 041101 (2008). [CrossRef]
  18. H. Song, J. Suh, E. Kyu Kim, K. Hyeon Baik, and S.-M. Hwang, “Growth of high quality a-plane GaN epi-layer on r-plane sapphire substrates with optimization of multi-buffer layer,” J. Cryst. Growth 312(21), 3122–3126 (2010). [CrossRef]
  19. S. You, T. Detchprohm, M. Zhu, W. Hou, E. A. Preble, D. Hanser, T. Paskova, and C. Wetzel, “Highly polarized green light emitting diode in m-axis GaInN/GaN,” Appl. Phys. Express 3(10), 102103 (2010). [CrossRef]
  20. K. Iso, H. Yamada, H. Hirasawa, N. Fellows, M. Saito, K. Fujito, S. P. DenBaars, J. S. Speck, and S. Nakamura, “High brightness blue InGaN/GaN light emitting diode on nonpolar m-plane bulk GaN substrate,” Jpn. J. Appl. Phys. 46(40), L960–L962 (2007). [CrossRef]
  21. T. Koyama, T. Onuma, H. Masui, A. Chakraborty, M. A. Haskell, S. Keller, U. K. Mishra, J. S. Speck, S. Nakamura, S. P. DenBaars, T. Sota, and S. F. Chichibu, “Prospective emission efficiency and in-plane light polarization of nonpolar m-plane InxGa1-xN/ GaN blue light emitting diodes fabricated on freestanding GaN substrates,” Appl. Phys. Lett. 89(9), 091906 (2006). [CrossRef]
  22. H.-H. Huang and Y.-R. Wu, “Study of polarization properties of light emitted from a-plane InGaN/GaN quantum well-based light emitting diodes,” J. Appl. Phys. 106(2), 023106 (2009). [CrossRef]
  23. N. Fellows, H. Sato, H. Masui, S. P. DenBaars, and S. Nakamura, “Increased polarization ratio on semipolar (11–22) InGaN/GaN light-emitting diodes with increasing indium composition,” Jpn. J. Appl. Phys. 47(10), 7854–7856 (2008). [CrossRef]
  24. H. Masui, H. Yamada, K. Iso, S. Nakamura, and S. P. DenBaars, “Optical polarization characteristics of m-oriented InGaN/GaN light-emitting diodes with various indium compositions in single-quantum-well structure,” J. Phys. D 41(22), 225104 (2008). [CrossRef]
  25. Y. G. Seo, K. H. Baik, K.-M. Song, S. Lee, H. Yoon, J.-H. Park, K. Oh, and S.-M. Hwang, “Milliwatt-class non-polar a-plane InGaN/GaN light-emitting diodes grown directly on r-plane sapphire substrates,” Curr. Appl. Phys. 10(6), 1407–1410 (2010). [CrossRef]
  26. C. H. Chiu, S. Y. Kuo, M. H. Lo, C. C. Ke, T. C. Wang, Y. T. Lee, H. C. Kuo, T. C. Lu, and S. C. Wang, “Optical properties of a-plane InGaN/GaN multiple quantum wells on r-plane sapphire substrates with different indium compositions,” J. Appl. Phys. 105(6), 063105 (2009). [CrossRef]
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