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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 15 — Jul. 28, 2014
  • pp: 17600–17606
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Self-textured oxide structure for improved performance of 365 nm ultraviolet vertical-type light-emitting diodes

Kun-Ching Shen, Wen-Yu Lin, Han-Yu Lin, Ken-Yen Chen, and Dong-Sing Wuu  »View Author Affiliations


Optics Express, Vol. 22, Issue 15, pp. 17600-17606 (2014)
http://dx.doi.org/10.1364/OE.22.017600


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Abstract

High performance 365 nm vertical-type ultraviolet light-emitting diodes (LEDs) are demonstrated by the insertion of a self-textured oxide mask (STOM) structure using metal-organic chemical vapor deposition. The dislocation densities were reduced significantly via the STOM by the observation of the transmission electron microcopy image. Under an injection current of 20 mA, a 50% light output power enhancement was achieved, representing an enhancement of 35.4% in light extraction efficiency and injected electron efficiency of the LED with STOM in comparison to that without STOM. At 350 mA, the light output power of the STOM-LEDs was approximately 24.4% higher. Measurements of the optical and electrical properties of the LED showed that the corrugated STOM structure improved the light scattering and reflection which increased the light output, and also enhanced the current spreading to intensify radiative recombination.

© 2014 Optical Society of America

1. Introduction

Recently, ultraviolet light-emitting diodes (UV-LEDs) have received renewed attention due of their potential for many applications, such as a pumping source for Hg-free lamps, water purification and biochemical detection [1

1. T. Nishida, N. Kobayashi, and T. Ban, “GaN-free transparent ultraviolet light-emitting diodes,” Appl. Phys. Lett. 82(1), 1–3 (2003). [CrossRef]

3

3. B. J. Kim, G. Yang, H. Y. Kim, K. H. Baik, M. A. Mastro, J. K. Hite, C. R. Eddy Jr, F. Ren, S. J. Pearton, and J. Kim, “GaN-based ultraviolet light-emitting diodes with AuCl₃-doped graphene electrodes,” Opt. Express 21(23), 29025–29030 (2013). [CrossRef] [PubMed]

]. In general, for blue- and green-LEDs, the indium fluctuation in the InGaN/GaN quantum wells (QWs) provides a localized state. This state enhances the LED radiative efficiency. Therefore, the indium-free QWs of UV-LEDs are expected to generate a lower radiative efficiency compared to blue- and green-LEDs. Moreover, a wide bandgap of AlGaN is usually employed in a UV-LED structure to avoid the absorption of the emitted UV light (λ ≦ 365 nm) by itself. The mismatch in the lattice constant and thermal expansion coefficient between the epilayers can easily create a poor quality of AlGaN with large dislocation densities. These dislocations serve as non-radiative recombination centers and hence degraded the UV-LED radiative efficiency. There have been several studies aimed at improving the AlGaN quality of UV-LED, including the use of a low-temperature AlN interlayer, a strain-released high-temperature AlN buffer layer, and AlN/AlGaN superlattices [4

4. M. Iwaya, S. Terao, N. Hayashi, T. Kashima, T. Takeuchi, H. Amano, and I. Akasaki, “Realization of crack-free and high-quality thick AlxGa1−xN for UV optoelectronics using low-temperature interlayer,” Appl. Surf. Sci. 159–160, 405–413 (2000). [CrossRef]

6

6. J. P. Zhang, H. M. Wang, M. E. Gaevski, C. Q. Chen, Q. Fareed, J. W. Yang, G. Simin, and M. Asif Khan, “Crack-free thick AlGaN grown on sapphire using AlN/AlGaN superlattices for strain management,” Appl. Phys. Lett. 80(19), 3542–3544 (2002). [CrossRef]

]. However, these preceding researches only focused on the improvement of the LED epilayer quality, and an increase in the light extraction efficiency (LEE) of LED is rarely discussed. Improving LEE is one of the most promising approaches of enhanced LED light output efficiency.

On the other hand, due to the geometry of horizontal-type sapphire-based LEDs, the finite resistance of n- and p-type materials causes poor lateral current spreading, which results in the current crowding near the metal contacts. As the injected current increases, the non-uniform current distribution cannot effectively contribute to the increase in the LED emission efficiency, and induces a simultaneous self-heating effect that accumulates near the electrodes. Vertical-type LED configurations were proposed to diminish the current crowding effect via a vertical current flow principle [7

7. H. Y. Ryu, K. S. Jeon, M. G. Kang, Y. Choi, and J. S. Lee, “Dependence of efficiencies in GaN-based vertical blue light-emitting diodes on the thickness and doping concentration of the n-GaN layer,” Opt. Express 21(S1Suppl 1), A190–A200 (2013). [CrossRef] [PubMed]

,8

8. C. F. Chu, F. I. Lai, J. T. Chu, C. C. Yu, C. F. Lin, H. C. Kuo, and S. C. Wang, “Study of GaN light-emitting diodes fabricated by laser lift-off technique,” J. Appl. Phys. 95(8), 3916–3922 (2004). [CrossRef]

]. Although the vertical current flow from the anode to the cathode shows promise for improving current crowding, the lack of a good lateral current spreading mechanism for the vertical LEDs results in the majority of the light being emitted from the QWs underneath the metal electrodes. The emitted light is shaded and absorbed by the electrodes, and therefore lowers the LED output intensity [9

9. R. H. Horng, K. C. Shen, Y. W. Kuo, and D. S. Wuu, “GaN light emitting diodes with wing-type imbedded contacts,” Opt. Express 21(S1Suppl 1), A1–A6 (2013). [CrossRef] [PubMed]

,10

10. O. B. Shchekin, J. E. Epler, T. A. Trottier, T. Margalith, D. A. Steigerwald, M. O. Holcomb, P. S. Martin, and M. R. Krames, “High performance thin-film flip-chip InGaN–GaN light-emitting diodes,” Appl. Phys. Lett. 89(7), 071109 (2006). [CrossRef]

]. To enhance the current spreading, the addition of injected electron efficiency (EE) structure, like current blocking layers (CBL) [11

11. H. Son, J. K. Lee, and S. M. Kim, “Effect of SiO2 nanoextractor on far-field radiation pattern of vertical light-emitting diodes,” Appl. Phys. Express 6(10), 102102 (2013). [CrossRef]

13

13. T. M. Chen, S. J. Wang, K. M. Uang, H. Y. Kuo, C. C. Tsai, W. C. Lee, and H. Kuan, “Current spreading and blocking designs for improving light output power from the vertical-structured GaN-based light-emitting diodes,” IEEE Photon. Technol. Lett. 20(9), 703–705 (2008). [CrossRef]

], is necessary.

2. Experimental

3. Results and discussion

Figure 4(a) shows the output power as a function of the injection current for both the STOM- and C-LEDs.
Fig. 4 (a) LED output power as functions of injection current of STOM- and C-LEDs. In the inset, forward I-V characteristics of the STOM-LEDs and C-LEDs. (b) LED emission patterns of the STOM- and C-LEDs at an injection current of 350 mA.
One can see that when the injected current was increased from 20 to 350 mA, the output power increased from 12 to 56 mW for the STOM-LEDs and from 8 to 45 mW for the C-LEDs respectively, corresponding to an increase in output power of 50% at 20 mA and 24.4% at 350 mA compared to the C-LEDs. The current-voltage characteristics of the two LEDs were measured and were shown in the inset of Fig. 4(a). The turn-on voltages for these LEDs were approximately 2.8 V at 20 mA and 3.4 V at 350 mA, respectively. The reverse-bias case with a reverse voltage of 5 V was 0.35 and 0.34 μA for the STOM-LEDs and C-LEDs, respectively. The lower turn-on voltage and the larger leakage current may be attributed to the imperfect LT-AlGaN etching process since it is more difficult to etch the AlGaN than the GaN case. Moreover, the series resistance of the STOM-LEDs was a little larger than that of C-LEDs since the STOM structure led to the reduction of current flow area. The external quantum efficiency (EQE) of the two LEDs at 20 mA was 17.6% for the STOM-LEDs and 11.7% for the C-LEDs, respectively. Generally, the EQE is expressed simply as multiplication of the IQE by LEE. This EQE estimation method involves a premise that the EE is set to 100%. However, in fact the EE value is usually overestimated since the strong dependence of the injected efficiency on the lateral spreading mechanism. The injected efficiency can be included in the EQE express using the following equation:
ηEQE=ηEE×ηIQE×ηLEE.
(1)
However, it is also not easy to distinguish the contribution in the enhanced output power by LEE or by EE. In our case, the product between the LEE and EE represented an enhancement of 35.4% for the STOM-LEDs (77.2%) in comparison to the C-LEDs (57%). It was suggested that a high light scattering and reflection for LEE and a good lateral current spreading for EE were induced by the corrugated STOM structure and further enhanced the LED intensity. The radiation patterns of STOM- and C-LEDs at the injection current of 350 mA are shown in Fig. 4(b). The divergence angles (50% light emission intensity of the full one) of 135° and 115° for the STOM-LEDs and C-LEDs were measured, respectively. The C-LEDs exhibited a smaller divergence angle than that of the STOM-LEDs, proving that the embedded STOM structure served as the scattering center to boost LED light extraction. This explained the more extensive integrated light emission intensity that was extracted in the case of the STOM-LEDs.

Furthermore, the SpeCLED software was used to simulate the current density distribution of the two LEDs. The chip size and n-pad region was 100 × 100 μm2 and 10 × 80 μm2, respectively. The gap between the two n-pads was 50 and 70 μm for the C-LEDs and the STOM-LEDs, respectively. At the injected current of 350 mA, the results of the current distribution simulation in the LT-AlGaN, n-AlGaN, and MQWs layers of the STOM- and C-LEDs are shown in Fig. 5.
Fig. 5 (a)–(f) Current density distribution in the LT-AlGaN, n-AlGaN, and MQW layer of the STOM- and C-LEDs at 350 mA injected current
Obviously, in Figs. 5(a) and 5(d), the injected current was driven into the n-pad, and then gradually spread to the center of the chip. The effect of the STOM in current spreading is more pronounced when the current diffused in the n-AlGaN and MQWs layer. Compared to the current distribution of C-LEDs in Fig. 5(c), a more uniform current density distribution was seen for the STOM-LEDs, which clearly indicated that the current crowding was suppressed via the STOM structure (see Fig. 5(f)). The STOM-LEDs exhibited a higher average current density (2499 A/cm2) in the MQWs layer, in comparison to the C-LEDs (2274 A/cm2). The simulation results verified that the hexagonal STOM arrangement presented a uniform lateral current spreading as a CBL for improved the current crowding effect.

4. Conclusion

In this paper, a high-quality 365 nm vertical-type UV-LEDs was successfully fabricated through the insertion of a STOM structure using an MOCVD system. The cross-sectional TEM observations revealed that dislocations in the n-AlGaN layer were effectively reduced by the incorporation of the STOM. The STOM plays the role of the CBL by enhancing current spreading in addition to the role of a reflector by extracting photons after multiple scattering events. Concurrently, the relative light output power was found to be enhanced by a factor of approximately 24.4% at an injection current of 350 mA. These results suggest that the use of the STOM is effective in at elevating the performance of InGaN-free UV-LEDs.

Acknowledgments

The authors would like to thank the Ministry of Economic Affairs under Grant No.102-E0605, and National Science Council of the Republic of China, Taiwan, NSC 101-2221-E-005-023-MY3, 102-2221-E-005-072-MY3, and 102-2622-E-005-006.

References and links

1.

T. Nishida, N. Kobayashi, and T. Ban, “GaN-free transparent ultraviolet light-emitting diodes,” Appl. Phys. Lett. 82(1), 1–3 (2003). [CrossRef]

2.

H. Zhu, C. X. Shan, L. K. Wang, Y. Yang, J. Y. Zhang, B. Yao, D. Z. Shen, and X. W. Fan, “A route to improved extraction efficiency of light-emitting diodes,” Appl. Phys. Lett. 96(4), 041110 (2010). [CrossRef]

3.

B. J. Kim, G. Yang, H. Y. Kim, K. H. Baik, M. A. Mastro, J. K. Hite, C. R. Eddy Jr, F. Ren, S. J. Pearton, and J. Kim, “GaN-based ultraviolet light-emitting diodes with AuCl₃-doped graphene electrodes,” Opt. Express 21(23), 29025–29030 (2013). [CrossRef] [PubMed]

4.

M. Iwaya, S. Terao, N. Hayashi, T. Kashima, T. Takeuchi, H. Amano, and I. Akasaki, “Realization of crack-free and high-quality thick AlxGa1−xN for UV optoelectronics using low-temperature interlayer,” Appl. Surf. Sci. 159–160, 405–413 (2000). [CrossRef]

5.

Y. Kida, T. Shibata, H. Naoi, H. Miyake, K. Hiramatsu, and M. Tanaka, “Growth of crack-free and high-quality AlGaN with high Al content using epitaxial AlN (0001) films on sapphire,” Phys. Status Solidi A 194(2), 498–501 (2002). [CrossRef]

6.

J. P. Zhang, H. M. Wang, M. E. Gaevski, C. Q. Chen, Q. Fareed, J. W. Yang, G. Simin, and M. Asif Khan, “Crack-free thick AlGaN grown on sapphire using AlN/AlGaN superlattices for strain management,” Appl. Phys. Lett. 80(19), 3542–3544 (2002). [CrossRef]

7.

H. Y. Ryu, K. S. Jeon, M. G. Kang, Y. Choi, and J. S. Lee, “Dependence of efficiencies in GaN-based vertical blue light-emitting diodes on the thickness and doping concentration of the n-GaN layer,” Opt. Express 21(S1Suppl 1), A190–A200 (2013). [CrossRef] [PubMed]

8.

C. F. Chu, F. I. Lai, J. T. Chu, C. C. Yu, C. F. Lin, H. C. Kuo, and S. C. Wang, “Study of GaN light-emitting diodes fabricated by laser lift-off technique,” J. Appl. Phys. 95(8), 3916–3922 (2004). [CrossRef]

9.

R. H. Horng, K. C. Shen, Y. W. Kuo, and D. S. Wuu, “GaN light emitting diodes with wing-type imbedded contacts,” Opt. Express 21(S1Suppl 1), A1–A6 (2013). [CrossRef] [PubMed]

10.

O. B. Shchekin, J. E. Epler, T. A. Trottier, T. Margalith, D. A. Steigerwald, M. O. Holcomb, P. S. Martin, and M. R. Krames, “High performance thin-film flip-chip InGaN–GaN light-emitting diodes,” Appl. Phys. Lett. 89(7), 071109 (2006). [CrossRef]

11.

H. Son, J. K. Lee, and S. M. Kim, “Effect of SiO2 nanoextractor on far-field radiation pattern of vertical light-emitting diodes,” Appl. Phys. Express 6(10), 102102 (2013). [CrossRef]

12.

S. Zhou, S. Liu, and H. Ding, “Enhancement in light extraction of LEDs with SiO2 current blocking layer deposited on naturally textured p-GaN surface,” Opt. Laser Technol. 47, 127–130 (2013). [CrossRef]

13.

T. M. Chen, S. J. Wang, K. M. Uang, H. Y. Kuo, C. C. Tsai, W. C. Lee, and H. Kuan, “Current spreading and blocking designs for improving light output power from the vertical-structured GaN-based light-emitting diodes,” IEEE Photon. Technol. Lett. 20(9), 703–705 (2008). [CrossRef]

14.

K. C. Shen, W. Y. Lin, D. S. Wuu, S. Y. Huang, K. S. Wen, S. F. Pai, L. W. Wu, and R. H. Horng, “An 83% enhancement in the external quantum efficiency of ultraviolet flip-chip light-emitting diodes with the incorporation of a self-textured oxide mask,” IEEE Electron Device Lett. 34(2), 274–276 (2013). [CrossRef]

15.

W. Y. Lin, K. C. Shen, R. H. Horng, and D. S. Wuu, “Enhancing light output power of InGaN-based light-emitting diodes with an embedded self-textured oxide mask structure,” J. Electrochem. Soc. 158(12), H1242–H1246 (2011). [CrossRef]

16.

K. C. Shen, D. S. Wuu, C. C. Shen, S. L. Ou, and R. H. Horng, “Surface modification on wet-etched patterned sapphire substrates using plasma treatments for improved GaN crystal quality and LED performance,” J. Electrochem. Soc. 158(10), H988–H993 (2011). [CrossRef]

17.

M. L. Nakarmi, N. Nepal, J. Y. Lin, and H. X. Jiang, “Photoluminescence studies of impurity transitions in Mg-doped AlGaN alloys,” Appl. Phys. Lett. 94(9), 091903 (2009). [CrossRef]

18.

N. I. Bochkareva, V. V. Voronenkov, R. I. Gorbunov, A. S. Zubrilov, Y. S. Lelikov, P. E. Latyshev, Y. T. Rebane, A. I. Tsyuk, and Y. G. Shreter, “Defect-related tunneling mechanism of efficiency droop in III-nitride light-emitting diodes,” Appl. Phys. Lett. 96(13), 133502 (2010). [CrossRef]

19.

S. C. Huang, K. C. Shen, D. S. Wuu, P. M. Tu, H. C. Kuo, C. C. Tu, and R. H. Horng, “Study of 375nm ultraviolet InGaN/AlGaN light-emitting diodes with heavily Si-doped GaN transition layer in growth mode, internal quantum efficiency, and device performance,” J. Appl. Phys. 110(12), 123102 (2011). [CrossRef]

20.

J. H. Ryou, P. D. Yoder, J. Liu, Z. Lochner, H. Kim, S. Choi, H. J. Kim, and R. D. Dupuis, “Control of quantum-confined stark effect in InGaN-based quantum wells,” IEEE J. Sel. Top. Quantum Electron. 15(4), 1080–1091 (2009). [CrossRef]

21.

C. C. Liao, S. W. Feng, C. C. Yang, Y. S. Lin, K. J. Ma, C. C. Chuo, C. M. Lee, and J. I. Chyi, “Stimulated emission study of InGaN/GaN multiple quantum well structures,” Appl. Phys. Lett. 76(3), 318–320 (2000). [CrossRef]

OCIS Codes
(160.2100) Materials : Electro-optical materials
(230.3670) Optical devices : Light-emitting diodes

ToC Category:
Optoelectronics

History
Original Manuscript: January 2, 2014
Revised Manuscript: April 7, 2014
Manuscript Accepted: June 27, 2014
Published: July 14, 2014

Citation
Kun-Ching Shen, Wen-Yu Lin, Han-Yu Lin, Ken-Yen Chen, and Dong-Sing Wuu, "Self-textured oxide structure for improved performance of 365 nm ultraviolet vertical-type light-emitting diodes," Opt. Express 22, 17600-17606 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-15-17600


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References

  1. T. Nishida, N. Kobayashi, and T. Ban, “GaN-free transparent ultraviolet light-emitting diodes,” Appl. Phys. Lett.82(1), 1–3 (2003). [CrossRef]
  2. H. Zhu, C. X. Shan, L. K. Wang, Y. Yang, J. Y. Zhang, B. Yao, D. Z. Shen, and X. W. Fan, “A route to improved extraction efficiency of light-emitting diodes,” Appl. Phys. Lett.96(4), 041110 (2010). [CrossRef]
  3. B. J. Kim, G. Yang, H. Y. Kim, K. H. Baik, M. A. Mastro, J. K. Hite, C. R. Eddy, F. Ren, S. J. Pearton, and J. Kim, “GaN-based ultraviolet light-emitting diodes with AuCl₃-doped graphene electrodes,” Opt. Express21(23), 29025–29030 (2013). [CrossRef] [PubMed]
  4. M. Iwaya, S. Terao, N. Hayashi, T. Kashima, T. Takeuchi, H. Amano, and I. Akasaki, “Realization of crack-free and high-quality thick AlxGa1−xN for UV optoelectronics using low-temperature interlayer,” Appl. Surf. Sci.159–160, 405–413 (2000). [CrossRef]
  5. Y. Kida, T. Shibata, H. Naoi, H. Miyake, K. Hiramatsu, and M. Tanaka, “Growth of crack-free and high-quality AlGaN with high Al content using epitaxial AlN (0001) films on sapphire,” Phys. Status Solidi A194(2), 498–501 (2002). [CrossRef]
  6. J. P. Zhang, H. M. Wang, M. E. Gaevski, C. Q. Chen, Q. Fareed, J. W. Yang, G. Simin, and M. Asif Khan, “Crack-free thick AlGaN grown on sapphire using AlN/AlGaN superlattices for strain management,” Appl. Phys. Lett.80(19), 3542–3544 (2002). [CrossRef]
  7. H. Y. Ryu, K. S. Jeon, M. G. Kang, Y. Choi, and J. S. Lee, “Dependence of efficiencies in GaN-based vertical blue light-emitting diodes on the thickness and doping concentration of the n-GaN layer,” Opt. Express21(S1Suppl 1), A190–A200 (2013). [CrossRef] [PubMed]
  8. C. F. Chu, F. I. Lai, J. T. Chu, C. C. Yu, C. F. Lin, H. C. Kuo, and S. C. Wang, “Study of GaN light-emitting diodes fabricated by laser lift-off technique,” J. Appl. Phys.95(8), 3916–3922 (2004). [CrossRef]
  9. R. H. Horng, K. C. Shen, Y. W. Kuo, and D. S. Wuu, “GaN light emitting diodes with wing-type imbedded contacts,” Opt. Express21(S1Suppl 1), A1–A6 (2013). [CrossRef] [PubMed]
  10. O. B. Shchekin, J. E. Epler, T. A. Trottier, T. Margalith, D. A. Steigerwald, M. O. Holcomb, P. S. Martin, and M. R. Krames, “High performance thin-film flip-chip InGaN–GaN light-emitting diodes,” Appl. Phys. Lett.89(7), 071109 (2006). [CrossRef]
  11. H. Son, J. K. Lee, and S. M. Kim, “Effect of SiO2 nanoextractor on far-field radiation pattern of vertical light-emitting diodes,” Appl. Phys. Express6(10), 102102 (2013). [CrossRef]
  12. S. Zhou, S. Liu, and H. Ding, “Enhancement in light extraction of LEDs with SiO2 current blocking layer deposited on naturally textured p-GaN surface,” Opt. Laser Technol.47, 127–130 (2013). [CrossRef]
  13. T. M. Chen, S. J. Wang, K. M. Uang, H. Y. Kuo, C. C. Tsai, W. C. Lee, and H. Kuan, “Current spreading and blocking designs for improving light output power from the vertical-structured GaN-based light-emitting diodes,” IEEE Photon. Technol. Lett.20(9), 703–705 (2008). [CrossRef]
  14. K. C. Shen, W. Y. Lin, D. S. Wuu, S. Y. Huang, K. S. Wen, S. F. Pai, L. W. Wu, and R. H. Horng, “An 83% enhancement in the external quantum efficiency of ultraviolet flip-chip light-emitting diodes with the incorporation of a self-textured oxide mask,” IEEE Electron Device Lett.34(2), 274–276 (2013). [CrossRef]
  15. W. Y. Lin, K. C. Shen, R. H. Horng, and D. S. Wuu, “Enhancing light output power of InGaN-based light-emitting diodes with an embedded self-textured oxide mask structure,” J. Electrochem. Soc.158(12), H1242–H1246 (2011). [CrossRef]
  16. K. C. Shen, D. S. Wuu, C. C. Shen, S. L. Ou, and R. H. Horng, “Surface modification on wet-etched patterned sapphire substrates using plasma treatments for improved GaN crystal quality and LED performance,” J. Electrochem. Soc.158(10), H988–H993 (2011). [CrossRef]
  17. M. L. Nakarmi, N. Nepal, J. Y. Lin, and H. X. Jiang, “Photoluminescence studies of impurity transitions in Mg-doped AlGaN alloys,” Appl. Phys. Lett.94(9), 091903 (2009). [CrossRef]
  18. N. I. Bochkareva, V. V. Voronenkov, R. I. Gorbunov, A. S. Zubrilov, Y. S. Lelikov, P. E. Latyshev, Y. T. Rebane, A. I. Tsyuk, and Y. G. Shreter, “Defect-related tunneling mechanism of efficiency droop in III-nitride light-emitting diodes,” Appl. Phys. Lett.96(13), 133502 (2010). [CrossRef]
  19. S. C. Huang, K. C. Shen, D. S. Wuu, P. M. Tu, H. C. Kuo, C. C. Tu, and R. H. Horng, “Study of 375nm ultraviolet InGaN/AlGaN light-emitting diodes with heavily Si-doped GaN transition layer in growth mode, internal quantum efficiency, and device performance,” J. Appl. Phys.110(12), 123102 (2011). [CrossRef]
  20. J. H. Ryou, P. D. Yoder, J. Liu, Z. Lochner, H. Kim, S. Choi, H. J. Kim, and R. D. Dupuis, “Control of quantum-confined stark effect in InGaN-based quantum wells,” IEEE J. Sel. Top. Quantum Electron.15(4), 1080–1091 (2009). [CrossRef]
  21. C. C. Liao, S. W. Feng, C. C. Yang, Y. S. Lin, K. J. Ma, C. C. Chuo, C. M. Lee, and J. I. Chyi, “Stimulated emission study of InGaN/GaN multiple quantum well structures,” Appl. Phys. Lett.76(3), 318–320 (2000). [CrossRef]

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