Enhanced ultraviolet emission of MgZnO/ZnO multiple quantum wells light-emitting diode by p-type MgZnO electron blocking layer Yong-Seok Choi,1 Jang-Won Kang,1 Byeong-Hyeok Kim,2 and Seong-Ju Park1,2,* 1
School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea 2 Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju 500712, South Korea * [email protected]
Abstract: We report on the effect of a p-type MgZnO electron blocking layer (EBL) on the optical and electrical properties of MgZnO/ZnO multiple quantum wells (MQWs) light-emitting diodes (LEDs). The p-type Mg0.15Zn0.85O EBL was introduced between the MQWs and p-type Mg0.1Zn0.9O layers. The p-type Mg0.15Zn0.85O EBL increased the ultraviolet emission by 111.2% 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 Mg0.15Zn0.85O EBL effectively suppresses the electron overflow from MQWs to p-type Mg0.1Zn0.9O and increases the hole concentration in the MQWs. ©2013 Optical Society of America OCIS codes: (160.6000) Semiconductor materials; (230.0230) Optical devices; (230.3670) Light-emitting diodes.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
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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). 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(4A), L371–L373 (2002). D. E. Sunstein, “A scatter communications link at ultraviolet frequencies,” Thesis, Massachusetts Institute of Technology, (1968). T. Nishida, N. Kobayashi, and T. Ban, “GaN-free transparent ultraviolet light-emitting diodes,” Appl. Phys. Lett. 82(1), 1–3 (2003). S. Shakya, K. H. Kim, and H. X. Jiang, “Enhanced light extraction in III-nitride ultraviolet photonic crystal lightemitting diodes,” Appl. Phys. Lett. 85(1), 142–144 (2004). 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,” Appl. Phys. Lett. 85(23), 5532–5534 (2004). 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). 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). D. C. Look, “Recent advances in ZnO materials and devices,” Mater. Sci. Eng. B 80(1–3), 383–387 (2001). 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). 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). 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). 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). Y. S. Choi, J. W. Kang, B. H. Kim, D. K. Na, S. J. Lee, and S. J. Park, “Improved electroluminescence from ZnO light-emitting diodes by p-type MgZnO electron blocking layer,” Opt. Express 21(10), 11698–11704 (2013).
#200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31560
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). 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 lightemitting diodes,” Appl. Phys. Lett. 90(10), 103507 (2007). 17. Y. S. Choi, J. W. Kang, B. H. Kim, and S. J. Park, “Optical Properties of ZnO/MgZnO Multiple Quantum Wells Grown by Metallorganic Chemical Vapor Deposition,” ECS J. Solid State Sci. Technol. 2(1), R21–R23 (2013). 18. 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). 19. Th. Gruber, C. Kirchner, R. Kling, F. Reuss, and A. Waag, “ZnMgO epilayers and ZnO–ZnMgO quantum wells for optoelectronic applications in the blue and UV spectral region,” Appl. Phys. Lett. 84(26), 5359–5361 (2004). 20. M. X. Qiu, Z. Z. Ye, H. P. He, Y. Z. Zhang, X. Q. Gu, L. P. Zhu, and B. H. Zhao, “Effect of Mg content on structural, electrical, and optical properties of Li-doped Zn1xMgxO thin films,” Appl. Phys. Lett. 90(18), 182116 (2007). 21. A. A. Iliadis, R. D. Vispute, T. Venkatesan, and K. A. Jones, “Ohmic metallization technology for wide bandgap semiconductors,” Thin Solid Films 420–421(1), 478–486 (2002). 22. K. Nakahara, S. Akasaka, H. Yuji, K. Tamura, T. Fujii, Y. Nishimoto, D. Takamizu, A. Sasaki, T. Tanabe, H. Takasu, H. Amaike, T. Onuma, S. F. Chichibu, A. Tsukazaki, A. Ohtomo, and M. Kawasaki, “Nitrogen doped MgxZn1xO/ZnO single heterostructure ultraviolet light-emitting diodes on ZnO substrates,” Appl. Phys. Lett. 97(1), 013501 (2010). 23. B. Lin, Z. Fu, and Y. Jia, “Green luminescent center in undoped zinc oxide films deposited on silicon substrates,” Appl. Phys. Lett. 79(7), 943–945 (2001). 24. A. A. Efremov, N. I. Bochkareva, R. I. Gorbunov, D. A. Lavrinovich, Y. T. Rebane, D. V. Tarkhin, and Y. G. Shreter, “Effect of the joule heating on the quantum efficiency and choice of thermal conditions for high-power blue InGaN/GaN LEDs,” Semiconductors 40(5), 605–610 (2006). 25. See http://www.semitech.us/products/SiLENSe/ for details on the software package. 26. 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). 27. 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). 28. E. F. Schubert, “Electron-blocking layers” in Light-Emitting Diodes, 2nd ed. (Cambridge University, 2006), pp. 81–82. 29. S. H. Han, D. Y. Lee, S. J. Lee, C. Y. Cho, M. K. Kwon, S. P. Lee, D. Y. Noh, D. J. Kim, Y. C. Kim, and S. J. Park, “Effect of electron blocking layer on efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes,” Appl. Phys. Lett. 94(23), 231123 (2009).
1. Introduction Ultraviolet (UV) light-emitting diodes (LEDs) and laser diodes are attractive for use in solidstate lighting, high-density information storage, secure communications, water and air sterilization, and chemical and biological detection systems [1–6]. Recently, intensive research efforts have focused on finding materials to realize more efficient UV LEDs. Among the available wide-bandgap semiconductors, ZnO is a promising candidate for creating efficient UV-light emitting devices due to its large direct bandgap of 3.37 eV, low-power threshold for optical pumping, and large exciton binding energy of 60 meV [7–9]. These attractive properties have drawn much attention to ZnO-based homojunction LEDs [10–13]. However, the progress on ZnO LEDs has slowed dramatically because of the low emission efficiency and inadequate design of carrier flow and injection efficiency. Recently, we reported the effect of a p-type electron blocking layer (EBL) on the optical and electrical properties of p-i-n ZnO LEDs [14]. The p-type EBL effectively increased carrier concentrations in the active
#200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31561
Fig. 1. Structure of the ZnO MQWs LED with a p-type Mg0.15Zn0.85O EBL.
layer and improved the emission intensity of ZnO LEDs [14–16]. The electron energy barrier and high hole accumulation region generated between the p-type MgZnO EBL and active layer improved electron and hole concentrations in the active layer [14]. To improve the efficiency of the ZnO LEDs, we have investigated the growth and optical properties of MgZnO/ZnO multiple quantum wells (MQWs). The MgZnO/ZnO MQWs were successfully grown by metalorganic chemical vapor deposition (MOCVD) and showed strong UV emissions at 370 nm [17]. In this study, we have investigated the effect of a p-type MgZnO EBL on the performance of ZnO MQWs LEDs. The p-type Mg0.15Zn0.85O EBL introduced between the Mg0.1Zn0.9O/ZnO MQWs and p-type Mg0.1Zn0.9O ZnO layer enhanced UV emission by increasing the electron and hole concentrations in the MgZnO/ZnO MQWs. 2. Experiments The ZnO LEDs were grown on the oxygen-polar face of n-type Ga-doped ZnO (ZnO:Ga) substrates by 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 O 2 gas were introduced into the reactor separately, and mixed 1 cm away from the substrate to minimize gas phase parasitic reactions [18]. Figure 1 shows the structure of the ZnO LED (300 × 300 μm2) with a p-type Mg0.15Zn0.85O EBL. A 400 nm-thick n-type Mg0.1Zn0.9O layer was grown on the n-type ZnO substrate at 650 °C. The Mg0.1Zn0.9O/ZnO MQWs with five pairs of undoped ZnO wells (2 nm) and Mg0.1Zn0.9O barriers (5 nm) were grown on the n-type Mg0.1Zn0.9O layer at 670 °C [17]. Then a 50 nm-thick p-type Mg0.15Zn0.85O EBL and a 600 nm-thick p-type Mg0.1Zn0.9O ZnO layer were grown at 600 °C. Table 1. Electrical properties of n-type ZnO substrate, n-type Mg0.1Zn0.9O, p-type Mg0.15Zn0.85O, and p-type Mg0.1Zn0.9O layers measured using the van der Pauw method at room temperature. Layers
Resistivity (Ωcm)
Mobility (cm2/Vs)
Concentration (/cm3)
n-type ZnO:Ga substrate
0.02
56.2
6.13 × 1018
n-type Mg0.1Zn0.9O layer
0.08
68.8
1.07 × 1018
p-type Mg0.15Zn0.85O EBL
123
0.4
1.27 × 1017
p-type Mg0.1Zn0.9O layer
62.14
0.5
2.01 × 1017
To estimate the Mg compositions of MgZnO in Table 1, the undoped Mg0.1Zn0.9O and Mg0.15Zn0.85ZnO films were grown on undoped ZnO templates and the Mg composition was
#200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31562
determined by PL measurement at 10 K. The PL peak position of Mg 0.1Zn0.9O and Mg0.15Zn0.85ZnO was 349.2 nm and 340.6 nm, respectively and the Mg compositions were estimated based on the PL data given in [19]. As-grown Sb-doped layers exhibited semiinsulating 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 shows the electrical properties of n-type ZnO:Ga substrate, n-type Mg0.1Zn0.9O, p-type Mg0.15Zn0.85O, and p-type Mg0.1Zn0.9O layers. In order to measure the conductivity of n-type and p-type MgZnO layers, Hall effect measurement was conducted in a van der Pauw configuration with a film thickness of ~1 μm grown on the undoped ZnO template (electrically semi-insulator) which was grown on c-sapphire substrate. The hole concentration of p-type MgZnO layer decreased with increasing the Mg composition due to the increase of acceptor activation energy in p-type MgZnO with higher Mg composition [20]. Ti (30 nm)/Au (100 nm) and Ni (30 nm)/Au (00 nm) were deposited on the n-type ZnO substrate and p-type Mg0.1Zn0.9O ZnO as n-type [21] and p-type metal electrodes [10], 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 2(a) shows I-V curves of ZnO MQWs LEDs with and without the p-type Mg0.15Zn0.85O EBL. The I-V curves show good rectification for both MQWs LEDs. The forward voltage at 20 mA increased from 4.8 V to 6.2 V upon addition of the p-type Mg0.15Zn0.85O EBL. The large decrease of current is due to the increase in the turn-on voltage and series resistance resulting from the higher barrier height and high resistivity of p-type Mg0.15Zn0.85O EBL, compared with the p-type Mg0.1Zn0.9O layer [22]. Figure 2(b) shows the EL spectra of LEDs with and without the p-type Mg0.15Zn0.85O EBL at an injection current of 50 mA. As shown in Fig. 2(b), the p-type Mg0.15Zn0.85O EBL enhances the UV emission intensity and decreases the deep-level emission. The improved UV emission is attributed to the increase in the radiative recombination rate in the MQWs. Moreover, the decrease of electron overflow to p-type region is responsible for the decrease of deep-level emission from the p-type ZnO layer. The deep level emissions are not observed on the photoluminescence (PL) spectra of ZnO/MgZnO MQWs and MgZnO, as shown in Fig. 2(c). The deep level emissions are observed only from Sb-doped p-type MgZnO layer. Therefore, it is believed that the deep level EL emissions in ZnO MQWs LEDs with p-type MgZnO EBL, as shown in Fig. 2(b) originate from the Sbrelated defects in p-type MgZnO EBL layer. The Sb-related defects and many point defects such as oxygen vacancies and zinc interstitials can be formed by doping the large-mismatched Sb into ZnO matrix [12, 14, 23].
Fig. 2. (a) I-V curves of ZnO MQWs LEDs with and without p-type Mg0.15Zn0.85O EBL. (b) EL spectra of the ZnO MQWs LEDs with and without the p-type Mg0.15Zn0.85O EBL, operating at a forward current of 50 mA. (c) PL spectra of p-type Mg0.15Zn0.85O EBL, p-type and n-type Mg0.1Zn0.9O and Mg0.1Zn0.9O/ZnO MQWs.
#200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31563
Figure 3(a) shows the total output power of ZnO LEDs with and without the p-type Mg0.15Zn0.85O EBL as a function of injection current. The total optical output power of ZnO MQWs LEDs was 2.14 μW at 60 mA and this increased by 21.5% to 2.60 μW with the p-type Mg0.15Zn0.85O EBL. The improved output power was attributed to the improved carrier recombination processes in the MQWs. Furthermore, the ZnO MQWs LEDs with the p-type Mg0.15Zn0.85O EBL shows a peak output power at 60 mA which is 10 mA higher than that of the ZnO MQWs LEDs without the p-type Mg0.15Zn0.85O EBL. This indicates that the p-type Mg0.15Zn0.85O EBL effectively confines the carriers in the ZnO quantum well layer of the MgZnO/ZnO MQWs LEDs at high injection currents, compared with ZnO MQWs LEDs without an EBL. Figure 3(b) shows the normalized integrated UV emission intensity of ZnO MQWs LEDs with and without a p-type Mg0.15Zn0.85O EBL as a function of injection current. The integrated UV emission intensity of ZnO MQWs LEDs with p-type Mg0.15Zn0.85O EBL is increased by 111.2% at 60 mA compared with that of the ZnO MQWs LEDs without p-type Mg0.15Zn0.85O EBL because of the improved carrier recombination process in the MQWs. Moreover, the ZnO MQWs LED with the p-type Mg0.15Zn0.85O EBL shows a peak UV emission intensity at 60 mA, while the ZnO LED without the p-type Mg0.15Zn0.85O 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 Mg0.15Zn0.85O EBL effectively confines the electrons and holes in the MQWs at high injection currents. In addition, the integrated output power and integrated UV emission intensity of the LEDs with EBL drop more pronouncedly after 60mA than that without EBL. This behavior in optical output power of LEDs with EBL is closely related to the joule heating effect due to the highly resistive EBL. The joule heating in the LEDs is reported to reduce the quantum yield and the output power of LEDs [24]. As shown in Figs. 4(a) and 4(b), the electron and hole concentrations in the MQWs are increased, while the electron concentrations in the p-type MgZnO EBL are reduced by the blocking effect of the p-type MgZnO EBL. As a result, the radiative recombination of the carriers occurs dominantly in MQWs, not in the p-type MgZnO EBL. This leads to the sharp improvement of UV emission from the MQWs (111.2%) and the slight reduction of broad deep-level emissions from the p-type MgZnO as shown in Fig. 2(b). Even though the UV emission is sharply increased by 111.2%, the overall optical output power is increased by 21.5% due to the narrow UV peak compared to the broad spectrum of ZnO LEDs as shown in Fig. 2(b).
Fig. 3. (a) Total output power of ZnO MQWs LEDs with and without the p-type Mg0.15Zn0.85O EBL as a function of injection current. (b) Normalized integrated UV emission intensity of ZnO MQWs LEDs with and without the p-type Mg0.15Zn0.85O EBL as a function of injection current.
#200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31564
Fig. 4. Calculated (a) electron and (b) hole concentrations of ZnO MQWs LEDs without and with the p-type Mg0.15Zn0.85O EBL. (c) Conduction and (d) valence energy band diagrams with and without p-type Mg0.15Zn0.85O EBL.
To further understand the carrier recombination process in the ZnO MQWs LEDs, we calculated the carrier distribution and energy band structures of ZnO MQWs LEDs using the #200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31565
LED simulator, SiLENSe 5.2.1 [25, 26]. In the simulation of ZnO MQWs LEDs, we used the LED structure shown in Fig. 1 and electrical properties of films listed in Table 1. Figures 4(a) and 4(b) show the distribution of electron and hole concentrations in each region of ZnO MQWs LEDs without and with the p-type Mg0.15Zn0.85O EBL at an input current of 20 mA, respectively. As shown in Fig. 4(a), the electron concentration increases by 52.0% in the 1st quantum well adjacent to the n-type MgZnO and 127.4% in the 5th quantum well adjacent to the p-type MgZnO due to the blocking of electron overflow from MQWs to p-type Mg0.1Zn0.9O by the p-type Mg0.15Zn0.85O EBL. Therefore, the electron concentration in the ptype Mg0.15Zn0.85O region decreases from 1.08 × 1016 cm3 to 6.24 × 1014 cm3. It is noteworthy that the p-type Mg0.15Zn0.85O EBL also increases the hole concentration by 146.9% in the 1st quantum well and 80.7% in the 5th quantum well region, as shown in Fig. 4(b). Figure 4(c) shows the conduction band energy diagram with and without p-type Mg0.15Zn0.85O EBL. The p-type Mg0.15Zn0.85O EBL produces an effective potential energy barrier of 102.9 meV for the electrons in the conduction band (marked with a circle in Fig. 4(c)). This potential energy barrier effectively blocks the overflow of electrons to the p-type MgZnO region and increases the electron concentration in the MQWs. Figure 4(d) shows the valence band diagram of ZnO LEDs with and without p-type Mg0.15Zn0.85O EBL at 20 mA. The p-type Mg0.15Zn0.85O EBL shows a potential notch and spike in the valence band (circles in Fig. 4(d)) at the interfaces of the 5th quantum barrier/EBL/p-type Mg0.1Zn0.9O layer due to the polarized-electric field in the layers [14, 27, 28]. The holes can be accumulated in the notch, leading to a high hole concentration in the MQWs of the ZnO LEDs with the p-type Mg0.15Zn0.85O EBL, as shown in Fig. 4(b) [14]. Although the hole concentration in the spike region is decreased by potential barrier, the hole concentration is rapidly recovered in the EBL region. This result shows that the holes can easily tunnel into EBL region through the spike [29]. Therefore, the improved EL property and optical output power of ZnO MQWs LEDs with a p-type Mg0.15Zn0.85O EBL are attributed to the increased electron and hole concentrations in the ZnO quantum well layer. 4. Summary We have investigated the effect of a p-type MgZnO EBL on the properties of ZnO MQWs LEDs. The UV emission intensity is enhanced and the deep-level emission is suppressed by the p-type Mg0.15Zn0.85O EBL. The total output power and integrated UV emission intensity of ZnO MQWs LEDs are improved by 21.5% and 111.2% at 60 mA with the p-type Mg0.15Zn0.85O EBL, respectively. The energy band structures and distribution of carrier concentrations show that p-type Mg0.15Zn0.85O EBL efficiently blocks electron overflow and increases the electron and hole concentrations in the MQWs, increasing the UV output power of ZnO MQWs LEDs. Acknowledgments This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant No. R15-2008-006-02001-0) and the Inter-ER Cooperation Project from the Ministry of Trade, Industry & Energy and Korea Institute for Advancement of Technology (KIAT) (Grant No. R0000499).
#200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31566
School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea 2 Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju 500712, South Korea * [email protected]
Abstract: We report on the effect of a p-type MgZnO electron blocking layer (EBL) on the optical and electrical properties of MgZnO/ZnO multiple quantum wells (MQWs) light-emitting diodes (LEDs). The p-type Mg0.15Zn0.85O EBL was introduced between the MQWs and p-type Mg0.1Zn0.9O layers. The p-type Mg0.15Zn0.85O EBL increased the ultraviolet emission by 111.2% 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 Mg0.15Zn0.85O EBL effectively suppresses the electron overflow from MQWs to p-type Mg0.1Zn0.9O and increases the hole concentration in the MQWs. ©2013 Optical Society of America OCIS codes: (160.6000) Semiconductor materials; (230.0230) Optical devices; (230.3670) Light-emitting diodes.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
13. 14.
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). 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(4A), L371–L373 (2002). D. E. Sunstein, “A scatter communications link at ultraviolet frequencies,” Thesis, Massachusetts Institute of Technology, (1968). T. Nishida, N. Kobayashi, and T. Ban, “GaN-free transparent ultraviolet light-emitting diodes,” Appl. Phys. Lett. 82(1), 1–3 (2003). S. Shakya, K. H. Kim, and H. X. Jiang, “Enhanced light extraction in III-nitride ultraviolet photonic crystal lightemitting diodes,” Appl. Phys. Lett. 85(1), 142–144 (2004). 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,” Appl. Phys. Lett. 85(23), 5532–5534 (2004). 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). 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). D. C. Look, “Recent advances in ZnO materials and devices,” Mater. Sci. Eng. B 80(1–3), 383–387 (2001). 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). 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). 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). 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). Y. S. Choi, J. W. Kang, B. H. Kim, D. K. Na, S. J. Lee, and S. J. Park, “Improved electroluminescence from ZnO light-emitting diodes by p-type MgZnO electron blocking layer,” Opt. Express 21(10), 11698–11704 (2013).
#200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31560
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). 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 lightemitting diodes,” Appl. Phys. Lett. 90(10), 103507 (2007). 17. Y. S. Choi, J. W. Kang, B. H. Kim, and S. J. Park, “Optical Properties of ZnO/MgZnO Multiple Quantum Wells Grown by Metallorganic Chemical Vapor Deposition,” ECS J. Solid State Sci. Technol. 2(1), R21–R23 (2013). 18. 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). 19. Th. Gruber, C. Kirchner, R. Kling, F. Reuss, and A. Waag, “ZnMgO epilayers and ZnO–ZnMgO quantum wells for optoelectronic applications in the blue and UV spectral region,” Appl. Phys. Lett. 84(26), 5359–5361 (2004). 20. M. X. Qiu, Z. Z. Ye, H. P. He, Y. Z. Zhang, X. Q. Gu, L. P. Zhu, and B. H. Zhao, “Effect of Mg content on structural, electrical, and optical properties of Li-doped Zn1xMgxO thin films,” Appl. Phys. Lett. 90(18), 182116 (2007). 21. A. A. Iliadis, R. D. Vispute, T. Venkatesan, and K. A. Jones, “Ohmic metallization technology for wide bandgap semiconductors,” Thin Solid Films 420–421(1), 478–486 (2002). 22. K. Nakahara, S. Akasaka, H. Yuji, K. Tamura, T. Fujii, Y. Nishimoto, D. Takamizu, A. Sasaki, T. Tanabe, H. Takasu, H. Amaike, T. Onuma, S. F. Chichibu, A. Tsukazaki, A. Ohtomo, and M. Kawasaki, “Nitrogen doped MgxZn1xO/ZnO single heterostructure ultraviolet light-emitting diodes on ZnO substrates,” Appl. Phys. Lett. 97(1), 013501 (2010). 23. B. Lin, Z. Fu, and Y. Jia, “Green luminescent center in undoped zinc oxide films deposited on silicon substrates,” Appl. Phys. Lett. 79(7), 943–945 (2001). 24. A. A. Efremov, N. I. Bochkareva, R. I. Gorbunov, D. A. Lavrinovich, Y. T. Rebane, D. V. Tarkhin, and Y. G. Shreter, “Effect of the joule heating on the quantum efficiency and choice of thermal conditions for high-power blue InGaN/GaN LEDs,” Semiconductors 40(5), 605–610 (2006). 25. See http://www.semitech.us/products/SiLENSe/ for details on the software package. 26. 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). 27. 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). 28. E. F. Schubert, “Electron-blocking layers” in Light-Emitting Diodes, 2nd ed. (Cambridge University, 2006), pp. 81–82. 29. S. H. Han, D. Y. Lee, S. J. Lee, C. Y. Cho, M. K. Kwon, S. P. Lee, D. Y. Noh, D. J. Kim, Y. C. Kim, and S. J. Park, “Effect of electron blocking layer on efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes,” Appl. Phys. Lett. 94(23), 231123 (2009).
1. Introduction Ultraviolet (UV) light-emitting diodes (LEDs) and laser diodes are attractive for use in solidstate lighting, high-density information storage, secure communications, water and air sterilization, and chemical and biological detection systems [1–6]. Recently, intensive research efforts have focused on finding materials to realize more efficient UV LEDs. Among the available wide-bandgap semiconductors, ZnO is a promising candidate for creating efficient UV-light emitting devices due to its large direct bandgap of 3.37 eV, low-power threshold for optical pumping, and large exciton binding energy of 60 meV [7–9]. These attractive properties have drawn much attention to ZnO-based homojunction LEDs [10–13]. However, the progress on ZnO LEDs has slowed dramatically because of the low emission efficiency and inadequate design of carrier flow and injection efficiency. Recently, we reported the effect of a p-type electron blocking layer (EBL) on the optical and electrical properties of p-i-n ZnO LEDs [14]. The p-type EBL effectively increased carrier concentrations in the active
#200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31561
Fig. 1. Structure of the ZnO MQWs LED with a p-type Mg0.15Zn0.85O EBL.
layer and improved the emission intensity of ZnO LEDs [14–16]. The electron energy barrier and high hole accumulation region generated between the p-type MgZnO EBL and active layer improved electron and hole concentrations in the active layer [14]. To improve the efficiency of the ZnO LEDs, we have investigated the growth and optical properties of MgZnO/ZnO multiple quantum wells (MQWs). The MgZnO/ZnO MQWs were successfully grown by metalorganic chemical vapor deposition (MOCVD) and showed strong UV emissions at 370 nm [17]. In this study, we have investigated the effect of a p-type MgZnO EBL on the performance of ZnO MQWs LEDs. The p-type Mg0.15Zn0.85O EBL introduced between the Mg0.1Zn0.9O/ZnO MQWs and p-type Mg0.1Zn0.9O ZnO layer enhanced UV emission by increasing the electron and hole concentrations in the MgZnO/ZnO MQWs. 2. Experiments The ZnO LEDs were grown on the oxygen-polar face of n-type Ga-doped ZnO (ZnO:Ga) substrates by 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 O 2 gas were introduced into the reactor separately, and mixed 1 cm away from the substrate to minimize gas phase parasitic reactions [18]. Figure 1 shows the structure of the ZnO LED (300 × 300 μm2) with a p-type Mg0.15Zn0.85O EBL. A 400 nm-thick n-type Mg0.1Zn0.9O layer was grown on the n-type ZnO substrate at 650 °C. The Mg0.1Zn0.9O/ZnO MQWs with five pairs of undoped ZnO wells (2 nm) and Mg0.1Zn0.9O barriers (5 nm) were grown on the n-type Mg0.1Zn0.9O layer at 670 °C [17]. Then a 50 nm-thick p-type Mg0.15Zn0.85O EBL and a 600 nm-thick p-type Mg0.1Zn0.9O ZnO layer were grown at 600 °C. Table 1. Electrical properties of n-type ZnO substrate, n-type Mg0.1Zn0.9O, p-type Mg0.15Zn0.85O, and p-type Mg0.1Zn0.9O layers measured using the van der Pauw method at room temperature. Layers
Resistivity (Ωcm)
Mobility (cm2/Vs)
Concentration (/cm3)
n-type ZnO:Ga substrate
0.02
56.2
6.13 × 1018
n-type Mg0.1Zn0.9O layer
0.08
68.8
1.07 × 1018
p-type Mg0.15Zn0.85O EBL
123
0.4
1.27 × 1017
p-type Mg0.1Zn0.9O layer
62.14
0.5
2.01 × 1017
To estimate the Mg compositions of MgZnO in Table 1, the undoped Mg0.1Zn0.9O and Mg0.15Zn0.85ZnO films were grown on undoped ZnO templates and the Mg composition was
#200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31562
determined by PL measurement at 10 K. The PL peak position of Mg 0.1Zn0.9O and Mg0.15Zn0.85ZnO was 349.2 nm and 340.6 nm, respectively and the Mg compositions were estimated based on the PL data given in [19]. As-grown Sb-doped layers exhibited semiinsulating 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 shows the electrical properties of n-type ZnO:Ga substrate, n-type Mg0.1Zn0.9O, p-type Mg0.15Zn0.85O, and p-type Mg0.1Zn0.9O layers. In order to measure the conductivity of n-type and p-type MgZnO layers, Hall effect measurement was conducted in a van der Pauw configuration with a film thickness of ~1 μm grown on the undoped ZnO template (electrically semi-insulator) which was grown on c-sapphire substrate. The hole concentration of p-type MgZnO layer decreased with increasing the Mg composition due to the increase of acceptor activation energy in p-type MgZnO with higher Mg composition [20]. Ti (30 nm)/Au (100 nm) and Ni (30 nm)/Au (00 nm) were deposited on the n-type ZnO substrate and p-type Mg0.1Zn0.9O ZnO as n-type [21] and p-type metal electrodes [10], 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 2(a) shows I-V curves of ZnO MQWs LEDs with and without the p-type Mg0.15Zn0.85O EBL. The I-V curves show good rectification for both MQWs LEDs. The forward voltage at 20 mA increased from 4.8 V to 6.2 V upon addition of the p-type Mg0.15Zn0.85O EBL. The large decrease of current is due to the increase in the turn-on voltage and series resistance resulting from the higher barrier height and high resistivity of p-type Mg0.15Zn0.85O EBL, compared with the p-type Mg0.1Zn0.9O layer [22]. Figure 2(b) shows the EL spectra of LEDs with and without the p-type Mg0.15Zn0.85O EBL at an injection current of 50 mA. As shown in Fig. 2(b), the p-type Mg0.15Zn0.85O EBL enhances the UV emission intensity and decreases the deep-level emission. The improved UV emission is attributed to the increase in the radiative recombination rate in the MQWs. Moreover, the decrease of electron overflow to p-type region is responsible for the decrease of deep-level emission from the p-type ZnO layer. The deep level emissions are not observed on the photoluminescence (PL) spectra of ZnO/MgZnO MQWs and MgZnO, as shown in Fig. 2(c). The deep level emissions are observed only from Sb-doped p-type MgZnO layer. Therefore, it is believed that the deep level EL emissions in ZnO MQWs LEDs with p-type MgZnO EBL, as shown in Fig. 2(b) originate from the Sbrelated defects in p-type MgZnO EBL layer. The Sb-related defects and many point defects such as oxygen vacancies and zinc interstitials can be formed by doping the large-mismatched Sb into ZnO matrix [12, 14, 23].
Fig. 2. (a) I-V curves of ZnO MQWs LEDs with and without p-type Mg0.15Zn0.85O EBL. (b) EL spectra of the ZnO MQWs LEDs with and without the p-type Mg0.15Zn0.85O EBL, operating at a forward current of 50 mA. (c) PL spectra of p-type Mg0.15Zn0.85O EBL, p-type and n-type Mg0.1Zn0.9O and Mg0.1Zn0.9O/ZnO MQWs.
#200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31563
Figure 3(a) shows the total output power of ZnO LEDs with and without the p-type Mg0.15Zn0.85O EBL as a function of injection current. The total optical output power of ZnO MQWs LEDs was 2.14 μW at 60 mA and this increased by 21.5% to 2.60 μW with the p-type Mg0.15Zn0.85O EBL. The improved output power was attributed to the improved carrier recombination processes in the MQWs. Furthermore, the ZnO MQWs LEDs with the p-type Mg0.15Zn0.85O EBL shows a peak output power at 60 mA which is 10 mA higher than that of the ZnO MQWs LEDs without the p-type Mg0.15Zn0.85O EBL. This indicates that the p-type Mg0.15Zn0.85O EBL effectively confines the carriers in the ZnO quantum well layer of the MgZnO/ZnO MQWs LEDs at high injection currents, compared with ZnO MQWs LEDs without an EBL. Figure 3(b) shows the normalized integrated UV emission intensity of ZnO MQWs LEDs with and without a p-type Mg0.15Zn0.85O EBL as a function of injection current. The integrated UV emission intensity of ZnO MQWs LEDs with p-type Mg0.15Zn0.85O EBL is increased by 111.2% at 60 mA compared with that of the ZnO MQWs LEDs without p-type Mg0.15Zn0.85O EBL because of the improved carrier recombination process in the MQWs. Moreover, the ZnO MQWs LED with the p-type Mg0.15Zn0.85O EBL shows a peak UV emission intensity at 60 mA, while the ZnO LED without the p-type Mg0.15Zn0.85O 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 Mg0.15Zn0.85O EBL effectively confines the electrons and holes in the MQWs at high injection currents. In addition, the integrated output power and integrated UV emission intensity of the LEDs with EBL drop more pronouncedly after 60mA than that without EBL. This behavior in optical output power of LEDs with EBL is closely related to the joule heating effect due to the highly resistive EBL. The joule heating in the LEDs is reported to reduce the quantum yield and the output power of LEDs [24]. As shown in Figs. 4(a) and 4(b), the electron and hole concentrations in the MQWs are increased, while the electron concentrations in the p-type MgZnO EBL are reduced by the blocking effect of the p-type MgZnO EBL. As a result, the radiative recombination of the carriers occurs dominantly in MQWs, not in the p-type MgZnO EBL. This leads to the sharp improvement of UV emission from the MQWs (111.2%) and the slight reduction of broad deep-level emissions from the p-type MgZnO as shown in Fig. 2(b). Even though the UV emission is sharply increased by 111.2%, the overall optical output power is increased by 21.5% due to the narrow UV peak compared to the broad spectrum of ZnO LEDs as shown in Fig. 2(b).
Fig. 3. (a) Total output power of ZnO MQWs LEDs with and without the p-type Mg0.15Zn0.85O EBL as a function of injection current. (b) Normalized integrated UV emission intensity of ZnO MQWs LEDs with and without the p-type Mg0.15Zn0.85O EBL as a function of injection current.
#200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31564
Fig. 4. Calculated (a) electron and (b) hole concentrations of ZnO MQWs LEDs without and with the p-type Mg0.15Zn0.85O EBL. (c) Conduction and (d) valence energy band diagrams with and without p-type Mg0.15Zn0.85O EBL.
To further understand the carrier recombination process in the ZnO MQWs LEDs, we calculated the carrier distribution and energy band structures of ZnO MQWs LEDs using the #200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31565
LED simulator, SiLENSe 5.2.1 [25, 26]. In the simulation of ZnO MQWs LEDs, we used the LED structure shown in Fig. 1 and electrical properties of films listed in Table 1. Figures 4(a) and 4(b) show the distribution of electron and hole concentrations in each region of ZnO MQWs LEDs without and with the p-type Mg0.15Zn0.85O EBL at an input current of 20 mA, respectively. As shown in Fig. 4(a), the electron concentration increases by 52.0% in the 1st quantum well adjacent to the n-type MgZnO and 127.4% in the 5th quantum well adjacent to the p-type MgZnO due to the blocking of electron overflow from MQWs to p-type Mg0.1Zn0.9O by the p-type Mg0.15Zn0.85O EBL. Therefore, the electron concentration in the ptype Mg0.15Zn0.85O region decreases from 1.08 × 1016 cm3 to 6.24 × 1014 cm3. It is noteworthy that the p-type Mg0.15Zn0.85O EBL also increases the hole concentration by 146.9% in the 1st quantum well and 80.7% in the 5th quantum well region, as shown in Fig. 4(b). Figure 4(c) shows the conduction band energy diagram with and without p-type Mg0.15Zn0.85O EBL. The p-type Mg0.15Zn0.85O EBL produces an effective potential energy barrier of 102.9 meV for the electrons in the conduction band (marked with a circle in Fig. 4(c)). This potential energy barrier effectively blocks the overflow of electrons to the p-type MgZnO region and increases the electron concentration in the MQWs. Figure 4(d) shows the valence band diagram of ZnO LEDs with and without p-type Mg0.15Zn0.85O EBL at 20 mA. The p-type Mg0.15Zn0.85O EBL shows a potential notch and spike in the valence band (circles in Fig. 4(d)) at the interfaces of the 5th quantum barrier/EBL/p-type Mg0.1Zn0.9O layer due to the polarized-electric field in the layers [14, 27, 28]. The holes can be accumulated in the notch, leading to a high hole concentration in the MQWs of the ZnO LEDs with the p-type Mg0.15Zn0.85O EBL, as shown in Fig. 4(b) [14]. Although the hole concentration in the spike region is decreased by potential barrier, the hole concentration is rapidly recovered in the EBL region. This result shows that the holes can easily tunnel into EBL region through the spike [29]. Therefore, the improved EL property and optical output power of ZnO MQWs LEDs with a p-type Mg0.15Zn0.85O EBL are attributed to the increased electron and hole concentrations in the ZnO quantum well layer. 4. Summary We have investigated the effect of a p-type MgZnO EBL on the properties of ZnO MQWs LEDs. The UV emission intensity is enhanced and the deep-level emission is suppressed by the p-type Mg0.15Zn0.85O EBL. The total output power and integrated UV emission intensity of ZnO MQWs LEDs are improved by 21.5% and 111.2% at 60 mA with the p-type Mg0.15Zn0.85O EBL, respectively. The energy band structures and distribution of carrier concentrations show that p-type Mg0.15Zn0.85O EBL efficiently blocks electron overflow and increases the electron and hole concentrations in the MQWs, increasing the UV output power of ZnO MQWs LEDs. Acknowledgments This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant No. R15-2008-006-02001-0) and the Inter-ER Cooperation Project from the Ministry of Trade, Industry & Energy and Korea Institute for Advancement of Technology (KIAT) (Grant No. R0000499).
#200227 - $15.00 USD Received 28 Oct 2013; revised 30 Nov 2013; accepted 30 Nov 2013; published 13 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031560 | OPTICS EXPRESS 31566
