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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 10 — May. 20, 2013
  • pp: 11698–11704
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Improved electroluminescence from ZnO light-emitting diodes by p-type MgZnO electron blocking layer

Yong-Seok Choi, Jang-Won Kang, Byeong-Hyeok Kim, Dong-Keun Na, Sang-Jun Lee, and Seong-Ju Park  »View Author Affiliations


Optics Express, Vol. 21, Issue 10, pp. 11698-11704 (2013)
http://dx.doi.org/10.1364/OE.21.011698


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Abstract

We report on the effect of a p-type MgZnO electron blocking layer (EBL) on the electroluminescence from n-type ZnO/undoped ZnO/p-type ZnO light-emitting diodes (LEDs). The p-type Mg0.1Zn0.9O EBL was introduced between the undoped and p-type ZnO layers. The p-type Mg0.1Zn0.9O EBL increased the ultraviolet emission by 140% at 60 mA and decreased the broad deep-level emission from ZnO LEDs. The calculated band structures and carrier distribution in ZnO LEDs show that p-type Mg0.1Zn0.9O EBL effectively suppresses the electron overflow from undoped ZnO to p-type ZnO and increases the hole concentration in the undoped ZnO layer.

© 2013 OSA

1. Introduction

Ultraviolet (UV) light-emitting diodes (LEDs) and laser diodes are attractive for their potential use in solid-state white lighting, high-density information storage, secure communications, water and air sterilization, and chemical and biological detection systems [1

1. K. Watanabe, T. Taniguchi, and H. Kanda, “Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal,” Nat. Mater. 3(6), 404–409 (2004). [CrossRef] [PubMed]

6

6. J. P. Zhang, X. Hu, Y. Bilenko, J. Deng, A. Lunev, M. S. Shur, R. Gaska, M. Shatalov, J. W. Yang, and M. A. Khan, “AlGaN-based 280 nm light-emitting diodes with continuous-wave power exceeding 1 mW at 25 mA,” Appl. Phys. Lett. 85(23), 5532–5534 (2004). [CrossRef]

]. Recently, intensive research efforts have focused on finding materials to realize more efficient UV LEDs. Of the available wide-bandgap semiconductors, ZnO is a promising candidate for creating efficient UV-light emitters due to its large direct bandgap of 3.37 eV, low-power threshold for optical pumping, and large exciton binding energy of 60 meV [7

7. S. Chu, M. Olmedo, Z. Yang, J. Kong, and J. Liu, “Electrically pumped ultraviolet ZnO diode lasers on Si,” Appl. Phys. Lett. 93(18), 181106 (2008). [CrossRef]

9

9. D. C. Look, “Recent advances in ZnO materials and devices,” Mater. Sci. Eng. B 80(1–3), 383–387 (2001). [CrossRef]

]. These attractive properties have drawn much attention to ZnO-based homojunction LEDs [10

10. D. K. Hwang, M. S. Oh, J. H. Lim, and S. J. Park, “ZnO thin films and light-emitting diodes,” J. Phys. D Appl. Phys. 40(22), R387–R412 (2007). [CrossRef]

13

13. Y. J. Zeng, Z. Z. Ye, Y. F. Lu, W. Z. Xu, L. P. Zhu, J. Y. Huang, H. P. He, and B. H. Zhao, “Plasma-free nitrogen doping and homojunction light-emitting diodes based on ZnO,” J. Phys. D Appl. Phys. 41(16), 165104 (2008). [CrossRef]

]. However, the higher mobility of electrons than holes in ZnO LEDs causes electron overflow and increases recombination processes in the p-type ZnO region [11

11. Y. S. Choi, J. W. Kang, D. K. Hwang, and S. J. Park, “Recent advances in ZnO-based light-emitting diodes,” IEEE Trans. Electron. Dev. 57(1), 26–41 (2010). [CrossRef]

, 14

14. J. H. Lim, C. K. Kang, K. K. Kim, I. K. Park, D. K. Hwang, and S. J. Park, “UV electroluminescence emission from ZnO light-emitting diodes grown by high temperature radiofrequency sputtering,” Adv. Mater. 18(20), 2720–2724 (2006). [CrossRef]

]. To overcome this problem, undoped MgZnO was introduced as an energy barrier layer to confine the recombination processes to the active layer [14

14. J. H. Lim, C. K. Kang, K. K. Kim, I. K. Park, D. K. Hwang, and S. J. Park, “UV electroluminescence emission from ZnO light-emitting diodes grown by high temperature radiofrequency sputtering,” Adv. Mater. 18(20), 2720–2724 (2006). [CrossRef]

]. The ZnO LEDs with an undoped MgZnO energy barrier layer showed enhanced UV emission and suppressed deep-level emission. However, the undoped MgZnO layer also decreases the hole transport from p-type ZnO to the ZnO active layer because its large bandgap provides an energy barrier to hole injection. One way to improve hole injection efficiency and the recombination rate in the active layer of LEDs is to use a p-type electron blocking layer (EBL) [15

15. M. Hansen, J. Piprek, P. M. Pattison, J. S. Speck, S. Nakamura, and S. P. DenBaars, “Higher efficiency InGaN laser diodes with an improved quantum well capping configuration,” Appl. Phys. Lett. 81(22), 4275–4277 (2002). [CrossRef]

, 16

16. S. Grzanka, G. Franssen, G. Targowski, K. Krowicki, T. Suski, R. Czernecki, P. Perlin, and M. Leszczyński, “Role of the electron blocking layer in the low-temperature collapse of electroluminescence in nitride light-emitting diodes,” Appl. Phys. Lett. 90(10), 103507 (2007). [CrossRef]

]. The hole concentration in the undoped ZnO active layer can be increased because of the lower Fermi energy level in p-type MgZnO compared with undoped or n-type MgZnO. In this study, we have investigated the effect of a p-type MgZnO EBL on the performance of n-type ZnO/undoped ZnO/p-type ZnO LEDs. The p-type Mg0.1Zn0.9O EBL was introduced between the undoped ZnO and p-type ZnO layer to enhance UV emission by increasing the electron and hole concentrations in the undoped ZnO active layer.

2. Experiments

The ZnO LEDs were grown on the oxygen-polar face of n-type Ga-doped ZnO (ZnO:Ga) substrates by metalorganic chemical vapor deposition (MOCVD). Diethylzinc (DEZn), trimethylantimony (TMSb), bis(cyclopentadienyl)magnesium (Cp2Mg), and O2 gas (99.999% purity) were used as sources of Zn, Sb, Mg, and O, respectively. The metalorganic sources and O2 gas were introduced into the reactor separately, and the source gases were mixed 1 cm ahead of the substrate to minimize gas phase parasitic reactions [17

17. Y. S. Choi, D. K. Hwang, B. J. Kwon, J. W. Kang, Y. H. Cho, and S. J. Park, “Effect of VI/II gas ratio on the epitaxial growth of ZnO films by metalorganic chemical vapor deposition,” Jpn. J. Appl. Phys. 50(10), 105502 (2011). [CrossRef]

].

Figure 1
Fig. 1 Structure of the ZnO LED with a p-type Mg0.1Zn0.9O EBL.
shows the structure of the ZnO LED with a p-type Mg0.1Zn0.9O EBL. A 100 nm-thick undoped ZnO layer was grown on the n-type ZnO substrate at 650 °C. Then a 50 nm-thick p-type Mg0.1Zn0.9O EBL and a 600 nm-thick p-type ZnO layer were grown at 600 °C. As-grown Sb-doped layers exhibited semi-insulating electrical properties and these layers were converted to p-type ZnO by a rapid thermal annealing process at 500 °C under N2 ambient conditions for 1 min. Table 1

Table 1. Electrical properties of the n-type ZnO substrate, undoped ZnO, p-type Mg0.1Zn0.9O, and p-type ZnO layers measured using the van der Pauw method at room temperature.

table-icon
View This Table
shows the electrical properties of n-type ZnO:Ga substrate, undoped ZnO, p-type Mg0.1Zn0.9O, and p-type ZnO layers. Ti (30 nm)/Au (100 nm) and Ni (30 nm)/Au (100 nm) were deposited on the n-type ZnO substrate and p-type ZnO as n-type [18

18. A. A. Iliadis, R. D. Vispute, T. Venkatesan, and K. A. Jones, “Ohmic metallization technology for wide band-gap semiconductors,” Thin Solid Films 420–421(1), 478–486 (2002). [CrossRef]

] and p-type metal electrodes [10

10. D. K. Hwang, M. S. Oh, J. H. Lim, and S. J. Park, “ZnO thin films and light-emitting diodes,” J. Phys. D Appl. Phys. 40(22), R387–R412 (2007). [CrossRef]

], as shown in Fig. 1. The current-voltage (I–V) characteristics were measured at room temperature using an HP 4155 parameter analyzer. Electroluminescence (EL) spectra and integrated optical output power were measured using a UV-visible spectrometer (USB4000-UV-VIS Fiber Optic Spectrometer, Ocean Optics Inc.). The total output power of ZnO LEDs was measured by using an integrating sphere system.

3. Results and discussion

Figure 4(a)
Fig. 4 (a) Total output power of ZnO LEDs with and without the p-type Mg0.1Zn0.9O EBL as a function of injection current. (b) Integrated UV emission intensity of ZnO LEDs with and without the p-type Mg0.1Zn0.9O EBL as a function of injection current.
shows the total output power of ZnO LEDs with and without the p-type Mg0.1Zn0.9O EBL as a function of injection current. The total optical output power of ZnO LED without the p-type Mg0.1Zn0.9O EBL was 1.61 μW at 50 mA and it was increased to 1.94 μW with the addition of the p-type Mg0.1Zn0.9O EBL. The 20.2% increase in the total optical output power is attributed to the improved carrier recombination process in the undoped ZnO layer. Furthermore, the ZnO LED with the p-type Mg0.1Zn0.9O EBL shows a peak output power at 58 mA which is 8 mA higher than that of the ZnO LED without the p-type Mg0.1Zn0.9O EBL. This indicates that the p-type Mg0.1Zn0.9O EBL effectively confines the carriers in the undoped ZnO layer of the ZnO LED at high injection currents, compared with ZnO LED without EBL. Figure 4(b) shows the integrated UV emission intensity of ZnO LEDs with and without a p-type Mg0.1Zn0.9O EBL as a function of injection current. The UV emission intensity of ZnO LED with p-type Mg0.1Zn0.9O EBL is increased by 140% at 60 mA compared with that of the ZnO LED without p-type Mg0.1Zn0.9O EBL because of the improved carrier recombination process in the undoped ZnO layer. Moreover, the ZnO LED with the p-type Mg0.1Zn0.9O EBL shows a peak UV emission at 60 mA, while the ZnO LED without the p-type Mg0.1Zn0.9O EBL shows a peak UV emission at 50 mA. The large current shift of 10 mA for the peak intensity of UV emission also indicates that the p-type Mg0.1Zn0.9O EBL effectively confines the electron and holes in the undoped ZnO layer of the ZnO LED at high injection currents. The improved characteristics were reproducible for all devices with the p-type Mg0.1Zn0.9O EBL.

To further understand the carrier recombination process in ZnO LEDs, we calculated the carrier distribution and energy band structures of ZnO LEDs using the LED simulator, SiLENSe 5.2.1 [19

19. See http://www.semitech.us/products/SiLENSe/ for details on the software package.

, 20

20. J. W. Mares, M. Falanga, A. V. Thompson, A. Osinsky, J. Q. Xie, B. Hertog, A. Dabiran, P. P. Chow, S. Karpov, and W. V. Schoenfeld, “Hybrid CdZnO/GaN quantum-well light emitting diodes,” J. Appl. Phys. 104(9), 093107 (2008). [CrossRef]

]. In the simulation of energy band diagrams and carrier distributions of ZnO LEDs, we used the ZnO LED structure shown in Fig. 1 and electrical properties of films listed in Table 1. Figures 5(a)
Fig. 5 Calculated carrier concentrations of ZnO LEDs (a) without and (b) with the p-type Mg0.1Zn0.9O EBL. (c) Calculated energy band diagram of ZnO LED with the p-type Mg0.1Zn0.9O EBL.
and 5(b) show the distribution of electron and hole concentrations in the n-type, undoped, and p-type regions of ZnO LEDs without and with the p-type Mg0.1Zn0.9O EBL at an input current of 20 mA, respectively. As shown in Fig. 5(b), the electron concentration is increased from 7.91 × 1016 cm−3 to 5.98 × 1017 cm−3 in the undoped ZnO region because of the blocking of electron overflow by the p-type Mg0.1Zn0.9O EBL. Therefore, the electron concentration in the p-type ZnO region is subsequently decreased from 7.82 × 1015 cm−3 to 8.05 × 1014 cm−3. It is noteworthy that the p-type Mg0.1Zn0.9O EBL also increases the hole concentration from 7.84 × 1016 cm−3 to 5.98 × 1017 cm−3 in the undoped ZnO region, as shown in Fig. 5(b). Figure 5(c) shows the energy band diagram of a ZnO LED with the p-type Mg0.1Zn0.9O EBL at 20 mA. The p-type Mg0.1Zn0.9O EBL shows a potential notch and spike in the valence band (circles in Fig. 5(c)) at the interfaces of the undoped ZnO/EBL/p-type ZnO due to the polarization-electric field in the layers [21

21. S. Heikman, S. Keller, Y. Wu, J. S. Speck, S. P. DenBaars, and U. K. Mishra, “Polarization effects in AlGaN/GaN and GaN/AlGaN/GaN heterostructures,” J. Appl. Phys. 93(12), 10114–10118 (2003). [CrossRef]

, 22

22. E. F. Schubert, “Electron-blocking layers” in Light-Emitting Diodes, 2nd ed. (Cambridge University, Cambridge, 2006), pp. 81–82.

]. The holes can be accumulated in the notch, leading to a high hole concentration in the undoped ZnO layer of the ZnO LED with the p-type Mg0.1Zn0.9O EBL, as shown in Fig. 5(b). Therefore, the improved EL property and optical output power of ZnO LEDs with a p-type Mg0.1Zn0.9O EBL are attributed to the increased electron and hole concentrations in the undoped ZnO active layer.

4. Summary

In summary, we have investigated the effect of a p-type MgZnO EBL on the properties of ZnO LEDs. The intensity of UV emission was enhanced and the deep-level emission was suppressed by the p-type Mg0.1Zn0.9O EBL. The intensity of UV emission of ZnO LEDs was increased by 140% at 60 mA by using p-type Mg0.1Zn0.9O EBL. The energy band structures and distribution of carrier concentrations show that p-type Mg0.1Zn0.9O EBL efficiently blocks electron overflow and increases the hole concentration in the active layer, increasing the UV output power of ZnO LEDs.

Acknowledgments

This work was supported by the World Class University program under Project R31-2008-000-10026-0, which was funded by the Ministry of Education, Science and Technology of Korea and by the Inter-ER Cooperation Projects (Grant No. R0000499) from the Ministry of Knowledge Economy.

References and links

1.

K. Watanabe, T. Taniguchi, and H. Kanda, “Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal,” Nat. Mater. 3(6), 404–409 (2004). [CrossRef] [PubMed]

2.

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. 41(Part 2, No. 4A4A), L371–L373 (2002). [CrossRef]

3.

D. E. Sunstein, “A scatter communications link at ultraviolet frequencies,” Thesis, Massachusetts Institute of Technology, (1968).

4.

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

5.

J. Shakya, K. H. Kim, J. Y. Lin, and H. X. Jiang, “Enhanced light extraction in III-nitride ultraviolet photonic crystal light-emitting diodes,” Appl. Phys. Lett. 85(1), 142–144 (2004). [CrossRef]

6.

J. P. Zhang, X. Hu, Y. Bilenko, J. Deng, A. Lunev, M. S. Shur, R. Gaska, M. Shatalov, J. W. Yang, and M. A. Khan, “AlGaN-based 280 nm light-emitting diodes with continuous-wave power exceeding 1 mW at 25 mA,” Appl. Phys. Lett. 85(23), 5532–5534 (2004). [CrossRef]

7.

S. Chu, M. Olmedo, Z. Yang, J. Kong, and J. Liu, “Electrically pumped ultraviolet ZnO diode lasers on Si,” Appl. Phys. Lett. 93(18), 181106 (2008). [CrossRef]

8.

C. Zhang, F. Zhang, T. Xia, N. Kumar, J. I. Hahm, J. Liu, Z. L. Wang, and J. Xu, “Low-threshold two-photon pumped ZnO nanowire lasers,” Opt. Express 17(10), 7893–7900 (2009). [CrossRef] [PubMed]

9.

D. C. Look, “Recent advances in ZnO materials and devices,” Mater. Sci. Eng. B 80(1–3), 383–387 (2001). [CrossRef]

10.

D. K. Hwang, M. S. Oh, J. H. Lim, and S. J. Park, “ZnO thin films and light-emitting diodes,” J. Phys. D Appl. Phys. 40(22), R387–R412 (2007). [CrossRef]

11.

Y. S. Choi, J. W. Kang, D. K. Hwang, and S. J. Park, “Recent advances in ZnO-based light-emitting diodes,” IEEE Trans. Electron. Dev. 57(1), 26–41 (2010). [CrossRef]

12.

J. Z. Zhao, H. W. Liang, J. C. Sun, J. M. Bian, Q. J. Feng, L. Z. Hu, H. Q. Zhang, X. P. Liang, Y. M. Luo, and G. T. Du, “Electroluminescence from n-ZnO/p-ZnO:Sb homojunction light emitting diode on sapphire substrate with metal–organic precursors doped p-type ZnO layer grown by MOCVD technology,” J. Phys. D Appl. Phys. 41(19), 195110 (2008). [CrossRef]

13.

Y. J. Zeng, Z. Z. Ye, Y. F. Lu, W. Z. Xu, L. P. Zhu, J. Y. Huang, H. P. He, and B. H. Zhao, “Plasma-free nitrogen doping and homojunction light-emitting diodes based on ZnO,” J. Phys. D Appl. Phys. 41(16), 165104 (2008). [CrossRef]

14.

J. H. Lim, C. K. Kang, K. K. Kim, I. K. Park, D. K. Hwang, and S. J. Park, “UV electroluminescence emission from ZnO light-emitting diodes grown by high temperature radiofrequency sputtering,” Adv. Mater. 18(20), 2720–2724 (2006). [CrossRef]

15.

M. Hansen, J. Piprek, P. M. Pattison, J. S. Speck, S. Nakamura, and S. P. DenBaars, “Higher efficiency InGaN laser diodes with an improved quantum well capping configuration,” Appl. Phys. Lett. 81(22), 4275–4277 (2002). [CrossRef]

16.

S. Grzanka, G. Franssen, G. Targowski, K. Krowicki, T. Suski, R. Czernecki, P. Perlin, and M. Leszczyński, “Role of the electron blocking layer in the low-temperature collapse of electroluminescence in nitride light-emitting diodes,” Appl. Phys. Lett. 90(10), 103507 (2007). [CrossRef]

17.

Y. S. Choi, D. K. Hwang, B. J. Kwon, J. W. Kang, Y. H. Cho, and S. J. Park, “Effect of VI/II gas ratio on the epitaxial growth of ZnO films by metalorganic chemical vapor deposition,” Jpn. J. Appl. Phys. 50(10), 105502 (2011). [CrossRef]

18.

A. A. Iliadis, R. D. Vispute, T. Venkatesan, and K. A. Jones, “Ohmic metallization technology for wide band-gap semiconductors,” Thin Solid Films 420–421(1), 478–486 (2002). [CrossRef]

19.

See http://www.semitech.us/products/SiLENSe/ for details on the software package.

20.

J. W. Mares, M. Falanga, A. V. Thompson, A. Osinsky, J. Q. Xie, B. Hertog, A. Dabiran, P. P. Chow, S. Karpov, and W. V. Schoenfeld, “Hybrid CdZnO/GaN quantum-well light emitting diodes,” J. Appl. Phys. 104(9), 093107 (2008). [CrossRef]

21.

S. Heikman, S. Keller, Y. Wu, J. S. Speck, S. P. DenBaars, and U. K. Mishra, “Polarization effects in AlGaN/GaN and GaN/AlGaN/GaN heterostructures,” J. Appl. Phys. 93(12), 10114–10118 (2003). [CrossRef]

22.

E. F. Schubert, “Electron-blocking layers” in Light-Emitting Diodes, 2nd ed. (Cambridge University, Cambridge, 2006), pp. 81–82.

OCIS Codes
(160.6000) Materials : Semiconductor materials
(230.0230) Optical devices : Optical devices
(230.3670) Optical devices : Light-emitting diodes

ToC Category:
Optical Devices

History
Original Manuscript: March 18, 2013
Revised Manuscript: April 30, 2013
Manuscript Accepted: April 30, 2013
Published: May 6, 2013

Citation
Yong-Seok Choi, Jang-Won Kang, Byeong-Hyeok Kim, Dong-Keun Na, Sang-Jun Lee, and Seong-Ju Park, "Improved electroluminescence from ZnO light-emitting diodes by p-type MgZnO electron blocking layer," Opt. Express 21, 11698-11704 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-10-11698


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References

  1. K. Watanabe, T. Taniguchi, and H. Kanda, “Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal,” Nat. Mater.3(6), 404–409 (2004). [CrossRef] [PubMed]
  2. 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.41(Part 2, No. 4A4A), L371–L373 (2002). [CrossRef]
  3. D. E. Sunstein, “A scatter communications link at ultraviolet frequencies,” Thesis, Massachusetts Institute of Technology, (1968).
  4. T. Nishida, N. Kobayashi, and T. Ban, “GaN-free transparent ultraviolet light-emitting diodes,” Appl. Phys. Lett.82(1), 1–3 (2003). [CrossRef]
  5. J. Shakya, K. H. Kim, J. Y. Lin, and H. X. Jiang, “Enhanced light extraction in III-nitride ultraviolet photonic crystal light-emitting diodes,” Appl. Phys. Lett.85(1), 142–144 (2004). [CrossRef]
  6. J. P. Zhang, X. Hu, Y. Bilenko, J. Deng, A. Lunev, M. S. Shur, R. Gaska, M. Shatalov, J. W. Yang, and M. A. Khan, “AlGaN-based 280 nm light-emitting diodes with continuous-wave power exceeding 1 mW at 25 mA,” Appl. Phys. Lett.85(23), 5532–5534 (2004). [CrossRef]
  7. S. Chu, M. Olmedo, Z. Yang, J. Kong, and J. Liu, “Electrically pumped ultraviolet ZnO diode lasers on Si,” Appl. Phys. Lett.93(18), 181106 (2008). [CrossRef]
  8. C. Zhang, F. Zhang, T. Xia, N. Kumar, J. I. Hahm, J. Liu, Z. L. Wang, and J. Xu, “Low-threshold two-photon pumped ZnO nanowire lasers,” Opt. Express17(10), 7893–7900 (2009). [CrossRef] [PubMed]
  9. D. C. Look, “Recent advances in ZnO materials and devices,” Mater. Sci. Eng. B80(1–3), 383–387 (2001). [CrossRef]
  10. D. K. Hwang, M. S. Oh, J. H. Lim, and S. J. Park, “ZnO thin films and light-emitting diodes,” J. Phys. D Appl. Phys.40(22), R387–R412 (2007). [CrossRef]
  11. Y. S. Choi, J. W. Kang, D. K. Hwang, and S. J. Park, “Recent advances in ZnO-based light-emitting diodes,” IEEE Trans. Electron. Dev.57(1), 26–41 (2010). [CrossRef]
  12. J. Z. Zhao, H. W. Liang, J. C. Sun, J. M. Bian, Q. J. Feng, L. Z. Hu, H. Q. Zhang, X. P. Liang, Y. M. Luo, and G. T. Du, “Electroluminescence from n-ZnO/p-ZnO:Sb homojunction light emitting diode on sapphire substrate with metal–organic precursors doped p-type ZnO layer grown by MOCVD technology,” J. Phys. D Appl. Phys.41(19), 195110 (2008). [CrossRef]
  13. Y. J. Zeng, Z. Z. Ye, Y. F. Lu, W. Z. Xu, L. P. Zhu, J. Y. Huang, H. P. He, and B. H. Zhao, “Plasma-free nitrogen doping and homojunction light-emitting diodes based on ZnO,” J. Phys. D Appl. Phys.41(16), 165104 (2008). [CrossRef]
  14. J. H. Lim, C. K. Kang, K. K. Kim, I. K. Park, D. K. Hwang, and S. J. Park, “UV electroluminescence emission from ZnO light-emitting diodes grown by high temperature radiofrequency sputtering,” Adv. Mater.18(20), 2720–2724 (2006). [CrossRef]
  15. M. Hansen, J. Piprek, P. M. Pattison, J. S. Speck, S. Nakamura, and S. P. DenBaars, “Higher efficiency InGaN laser diodes with an improved quantum well capping configuration,” Appl. Phys. Lett.81(22), 4275–4277 (2002). [CrossRef]
  16. S. Grzanka, G. Franssen, G. Targowski, K. Krowicki, T. Suski, R. Czernecki, P. Perlin, and M. Leszczyński, “Role of the electron blocking layer in the low-temperature collapse of electroluminescence in nitride light-emitting diodes,” Appl. Phys. Lett.90(10), 103507 (2007). [CrossRef]
  17. Y. S. Choi, D. K. Hwang, B. J. Kwon, J. W. Kang, Y. H. Cho, and S. J. Park, “Effect of VI/II gas ratio on the epitaxial growth of ZnO films by metalorganic chemical vapor deposition,” Jpn. J. Appl. Phys.50(10), 105502 (2011). [CrossRef]
  18. A. A. Iliadis, R. D. Vispute, T. Venkatesan, and K. A. Jones, “Ohmic metallization technology for wide band-gap semiconductors,” Thin Solid Films420–421(1), 478–486 (2002). [CrossRef]
  19. See http://www.semitech.us/products/SiLENSe/ for details on the software package.
  20. J. W. Mares, M. Falanga, A. V. Thompson, A. Osinsky, J. Q. Xie, B. Hertog, A. Dabiran, P. P. Chow, S. Karpov, and W. V. Schoenfeld, “Hybrid CdZnO/GaN quantum-well light emitting diodes,” J. Appl. Phys.104(9), 093107 (2008). [CrossRef]
  21. S. Heikman, S. Keller, Y. Wu, J. S. Speck, S. P. DenBaars, and U. K. Mishra, “Polarization effects in AlGaN/GaN and GaN/AlGaN/GaN heterostructures,” J. Appl. Phys.93(12), 10114–10118 (2003). [CrossRef]
  22. E. F. Schubert, “Electron-blocking layers” in Light-Emitting Diodes, 2nd ed. (Cambridge University, Cambridge, 2006), pp. 81–82.

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