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ZnO/GaN-based light-emitting diodes (LEDs) with improved asymmetric double heterostructure of Ta2O5/ZnO/HfO2 have been fabricated. Electroluminescence (EL) performance has been enhanced by the HfO2 electron blocking layer and further improved by continuing inserting the Ta2O5 hole blocking layer. The origins of the emission have been identified, which indicated that the Ta2O5/ZnO/HfO2 asymmetric structure could more effectively confine carriers in the active i-ZnO layer and meanwhile suppresses of radiation from GaN. This device exhibits superior stability in long-time running. It’s hoped that the asymmetric double heterostructure may be helpful for the development of the future ZnO-based LEDs.
© 2014 Optical Society of America
1. Introduction
GaN and ZnO are both direct wide-bandgap semiconductors and have been extensively considered as the most suitable materials for fabricating short-wavelength light sources [1–4]. GaN-based light-emitting diodes (LEDs) have been commercialized. However, the development of ZnO-based LEDs has been blocked by lacking of stable and reproducible p-type ZnO, though ZnO exhibits larger exciton binding energy and lower cost than GaN [1]. For realizing the electroluminescence (EL) emission from ZnO, numerous efforts have been made on heterostructures with n-type ZnO and other p-type materials, such as p-GaN [5–11], p-AlGaN [12], p-NiO [13], p-Si [14–17], p-SrCu2O2 [18], p-CuGaS2 [19], poly (3,4-ethylene-dioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) [20]. Among these structures, n-ZnO/p-GaN heterojunction has attracted much attention for their similar physical properties. However, for the similar bandgap of ZnO (3.37 eV) and GaN (3.4 eV), it is usually difficult to identify whether the near-band-edge (NBE) emission comes from ZnO or GaN. Furthermore, because of lower hole mobility and carrier concentration in p-GaN, the EL usually emits mainly from the p-GaN instead of n-ZnO. To obtain the EL emission from n-ZnO, electron blocking layers (EBLs), such as MgO [6, 8, 21], SiO2 [5], AlN [11], HfO2 [7], Ga2O3 [8], ZnS [22], were introduced between GaN and ZnO. Furthermore, the ZnO-based symmetry double heterostructures (for example, MgZnO/ZnO/MgZnO [23–25] and SiO2/ZnO/SiO2 [26] structures) were reported to enhance the radiative recombination in the active ZnO layer for their better carrier confinement. Shi et al. reported an asymmetric double heterostructure of MgZnO/ZnO/SiO2/Si [27]. The SiO2 layer was used as the EBL, and a fashioned asymmetric waveguide mechanism leads to an improved light extraction efficiency. ZnO/GaN-based heterojunction LEDs with the asymmetric double heterostructure are seldom reported.
In this study, a new improved asymmetric double heterostructure of Ta2O5/ZnO/HfO2 has been designed and utilized in the ZnO/GaN-based heterojunction LEDs. The large conduction (p side) and valence (n side) band offsets at the interfaces of the active layer provide significant suppression of both electron and hole leakage from the active region [28, 29]. HfO2 was used as the EBL and Ta2O5 was introduced as the hole blocking layer (HBL) for its low valence band maximum energy level. Compared with conventional symmetric double heterostructure [23–26], Ta2O5/ZnO/HfO2 asymmetric structure is more effective to confine carriers in the active i-ZnO layer and meanwhile suppresses of radiation from GaN. The n-ZnO/Ta2O5/i-ZnO/HfO2/p-GaN heterostructure LED has been fabricated. The room temperature (RT) current-voltage (I-V), photoluminescence (PL) and EL characteristics were observed. The origins of the EL emission were identified by peak-deconvolution with Gaussian functions and related mechanisms about the enhancement of radiative recombination were discussed and in terms of energy band diagram.
2. Experiment
The device structure is illustrated in the inset of Fig. 1. The substrate is a commercially available p-type GaN wafer with a 2.8-μm-thick p-GaN layer on an undoped GaN/sapphire substrate. The resistivity and hole concentration of p-GaN layer was obtained as 0.27 Ω cm and 6.80 × 1017 cm−3, respectively, by a LakeShore 7704 Hall measurement system. A 30-nm HfO2 layer, a 100-nm i-ZnO layer and a 30-nm Ta2O5 layer were deposited on the p-GaN wafer sequentially at 300 °C by a radio frequency (RF) magnetron sputtering system with controlled Ar and O2 flow ratio. The samples were then annealed at 700 °C in air for 1 h. After that, a 400-nm Al-doped n-ZnO film with an electron concentration of 4.32 × 1019 cm−3 was deposited on Ta2O5/i-ZnO/HfO2/p-GaN samples as the electron-injection layer by RF magnetron sputtering at 300 °C with Ar flow only. Finally, disc-shaped Ag with a diameter of 1 mm was sputtered as the electrode on the n-ZnO layer, and In was used as another electrode on the p-GaN layer. Good Ohmic contacts were achieved for both electrodes as shown in the inset of Fig. 1. The device of n-ZnO/Ta2O5/i-ZnO/HfO2/p-GaN is labeled as LED 3. As a contrast, LED 1 with structure of n-ZnO/i-ZnO/p-GaN and LED 2 with structure of n-ZnO/i-ZnO/HfO2/p-GaN were also fabricated. The I-V characteristic of the devices measured by a Keithley 4200 electrometer in dark at RT is illustrated in Fig. 1. Rectifying diode-like behavior was demonstrated from the curves. LED 1 shows a highest current density at the same forward voltage, while LED 2 and LED 3 show lower current densities with high-k interfacial layers inserted.
Fig. 1 The I-V characteristic of the LEDs in dark at RT. The insets show the schematic diagram of LED 3 with structure of n-ZnO/Ta2O5/i-ZnO/HfO2/p-GaN and the I-V curves of the In/p-GaN and the Ag/n-ZnO contacts, respectively.
3. Results and discussion
The EL spectra of the LEDs under different injection currents are shown in the left column of Fig. 2. The EL emission is remarkably enhanced with increasing injection current for all LEDs. The right column of Fig. 2 shows the Gaussian fits of the EL spectra of LEDs under the injection current of 2.00 mA. The EL spectra of LED 1 and 2 both consist of 5 distinct bands centered at ~425 nm, ~520 nm, ~565 nm, ~630 nm and ~710 nm which are defined as Peak I, II, III, IV and V, respectively. EL spectrum of LED 3 consists of above peaks except Peak I.
Fig. 2 The left column shows the EL spectra of the LEDs under different injection currents. The right column shows the Gaussian fits of the EL spectra of LEDs under the same injection current of 2.00 mA.
In order to investigate the emission mechanism, RT PL of p-GaN and Ta2O5/i-ZnO/HfO2/p-GaN was taken by a 325 nm He–Cd laser and shown in Fig. 3.Peak-deconvolution with Gaussian function is also applied in the PL spectra. As shown in Fig. 3, the ultraviolet (UV) emission of p-GaN centered at ~375 nm is attributed to the NBE emission of GaN. The curve becomes collapsed when the wavelength gets less than 370 nm. RT transmission spectrum of p-GaN in the inset of Fig. 3 demonstrates that the steep curve around 370 nm is caused by the absorption edge of GaN. The violet-blue emission of ~425 nm should be originated from the transitions between the conduction-band shallow donors and Mg-related acceptors [5, 11], and the green emission of ~520 nm is probably attributed to the defects emission in GaN [30]. In the PL spectrum of Ta2O5/i-ZnO/HfO2/p-GaN, green emission of ~520 nm is enhanced compared with that in the p-GaN PL spectrum. Therefore, it is believed that this green emission of Ta2O5/i-ZnO/HfO2/p-GaN is not only originated from the defects emission in GaN but also from the radiative recombination of shallowly trapped electrons with deeply trapped holes at zinc vacancies (VZn) in ZnO [31]. The peak at ~565 nm might be attributed to transitions from the conduction band or shallow donors to oxygen interstitials (Oi) level in ZnO [32]. However, the peak of ~650 nm has not been exactly identified to a recombination process but it is within the emission range of 620 to 690 nm, which might also originate from the conduction to Oi level in ZnO [33]. Because of the similar band gap of ZnO and GaN, it is usually difficult to tell whether the NBE emission comes from ZnO or GaN. The defect emission discussed above help us have a better understanding of the effect of asymmetric double heterostructures in the EL emission enhancement. Comparing with the results in Fig. 2, one can deduce the EL emission mechanism investigation of LEDs: Peak I (~425 nm) comes from GaN; Peak II (~520 nm) comes from both GaN and ZnO; Peak III (~565 nm) and IV (~630 nm) are attributed to ZnO (The deviation of 650-nm PL emission and 630-nm EL emission may be caused by the difference in radiative recombination pathway and excited state density between PL and EL [34]); the remaining Peak V (~710 nm), which has not been detected in PL measurement because of the measurement limit of the equipment, might be origined of the radiative recombination of shallowly trapped electrons with deeply trapped holes at Oi and/or a donor-acceptor transition taking place between oxygen vacancies (VO) and VZn in ZnO [31].
Fig. 3 RT PL spectra of p-GaN and Ta2O5/i-ZnO/HfO2/p-GaN. Peak-deconvolution with Gaussian function is applied in the PL spectra. The inset shows RT transmission spectrum of p-GaN.
Figure 4(a) shows the plots of EL integrated intensity versus injection current of the devices. With the increase of injection current, the EL integrated intensities are enhanced for all LEDs. The EL integrated intensities of LED 1 and 2 are almost the same when the injection current is below 1 mA. When the current is further increased, the emission integrated intensity of LED 2 grows stronger than that of LED 1, which is attributed to the EBL of HfO2.
Fig. 4 (a) The plots of EL integrated intensity versus injection current of LEDs. (b) EL intensity of respective peaks versus various LEDs under the same injection current of 2.00 mA.
In recent years, many kinds of materials have been used as the EBL in n-ZnO/p-GaN LEDs. The conduction-band offsets (ΔEC) of ZnO/EBLs and valence-band offsets (ΔEV) of EBLs/GaN of these materials are listed in Table 1.As shown in Table 1, HfO2 is one of the most suitable materials for the EBL in n-ZnO/p-GaN LEDs because of its large ΔEC (2.29 eV) for ZnO/HfO2 interface and small ΔEV (0.30 eV) for HfO2/GaN interface, which means that electrons can be well blocked from ZnO to GaN and meanwhile holes from GaN to ZnO are rarely affected, thus effectively enhancing the possibility of radiative recombination in ZnO. The energy band structures for LED 1 and 2 are shown in Figs. 5(a) and 5(b), respectively, illustrating the effectiveness of the HfO2 EBL, and the improved EL performance of LED 2 further confirms this. Figure 4(b) shows the EL intensity of respective peaks versus various LEDs under the same injection current of 2.00 mA. The intensity of Peak I (GaN emission) is weakened and the intensity of Peak III, IV and V (ZnO emission) is enhanced by inserting the HfO2 EBL, which is also consistent with the analysis above. Based on the enhancement of LED 2, we further inserted a Ta2O5 HBL into LED 2, to be n-ZnO/Ta2O5/i-ZnO/HfO2/p-GaN device (LED 3). As shown in Fig. 4(a), LED 3 exhibited the maximum emission integrated intensity at the same injection current among the LEDs. The band gap, conduction band minimum (CBM) and valence band maximum (VBM) of Ta2O5 are 4.65 eV, 3.75 eV and 8.40 eV, respectively [35]. The ΔEC for n-ZnO/Ta2O5 interface and ΔEV for Ta2O5/i-ZnO interface are calculated to be 0.60 eV and 0.68 eV, respectively. The band diagram of LED 3 has been constructed and is shown in Fig. 5(c). Comparing with the conventional MgZnO/ZnO/MgZnO and SiO2/ZnO/SiO2 symmetric double heterostructures used in ZnO-based LEDs, Ta2O5, as a barrier layer contacting with n-type electron injection layer and ZnO active layer, has a much lower CBM which is very close to that of ZnO. As a result, the Ta2O5 HBL blocks the hole injection from i-ZnO into n-ZnO while relatively less affects electron injection from n-ZnO into i-ZnO. This is the main reason that the EL intensity of LED 3 is greater than that of LED 2. As shown in Fig. 4(b), Peak I (GaN emission) almost disappears in LED 3, and intensity of Peak IV and V (ZnO emission) is about 13 and 19 times as much as that of LED 2, respectively, indicating that the ZnO emission has been further improved. However, the intensity of Peak III (ZnO emission) becomes one quarter comparing with LED 2. The variation of this ZnO emission by inserting Ta2O5 HBL in LED 3 may be caused by the change of radiative recombination region in the active i-ZnO layer with a certain depth distribution of defects. According to our results, one can expect to find a more suitable HBL material with smaller ΔEC and larger ΔEV with ZnO to enhance the emission in future ZnO-based LEDs.
Table 1. ΔEC of ZnO/EBLs and ΔEV of EBLs/GaN of various materials
The emission intensity of the band at 633 nm of LED 3 was recorded every 120 second intermittently with a continuous injection current of 2.00 mA. The emission intensity as a function of the driving time is shown in Fig. 6(a). In the first 8 hours, the emission intensity decreased about 19%, i.e. 2.4% per hour. After the first 8 hours, the curve exhibits good stability with an intensity degradation of less than 6.4% over 16 hours, i.e. 0.40% per hour. The EL spectra of LED 3 before and after running 24 hours are shown in Fig. 6(b). After stopping the 24-hour stability test for 2 hours, the EL spectrum was also tested. According to the results, it can be inferred that in the first 8 hours, the LED starts to generate heat which causes a junction temperature increment and a decrease in the EL emission intensity. After about 8 hours, the heat production and dissipation gradually reach a dynamic balancing so that the junction temperature can keep almost the same and the EL intensity decreases very slowly. After stopping the stability test for 2 hours, the junction temperature is close to room temperature and the EL intensity almost returns to the value before the 24-hour running. The inset of Fig. 6(a) shows the result of long-time stability test recorded every 10 hour intermittently also with a continuous injection current of 2.00 mA. The LED lost ~30% of the EL intensity at 160 hours of operation. The ZnO-based LED lifetime in this work is longer than that demonstrated in previous reports [10, 25, 36, 37].
Fig. 6 (a) The emission intensity of the band at 633 nm of LED 3 as a function of the 24-hour driving time recorded every 120 second intermittently with a continuous injection current of 2.00 mA. The inset shows the result of long-time stability test recorded every 10 hour intermittently also with a continuous injection current of 2.00 mA. (b) The EL spectra of LED 3 before and after running 24 hours, and after stopping continuous work 2 hours.
4. Conclusions
In summary, LEDs of n-ZnO/i-ZnO/p-GaN, n-ZnO/i-ZnO/HfO2/p-GaN and n-ZnO/Ta2O5/i-ZnO/HfO2/p-GaN structures have been fabricated. With the help of HfO2 EBL, the EL intensity has been enhanced. Further inserting the Ta2O5 HBL, the EL performance can be further improved. These results are attributed to the asymmetric double heterostructure of Ta2O5/ZnO/HfO2 which improves carrier confinement and then the radiative recombination rate in the active i-ZnO side and in the meantime the counterpart from p-GaN is suppressed. The double heterostructure LED exhibits superior stability in long-time running. We believe this result should be helpful for the development of fabricating ZnO-based light emitting devices.
Acknowledgments
The work at Wuhan University was supported by the National Natural Science Foundation of China (61376013, J1210061), the 973 Program (2011CB933300) of China, the Research Program of Wuhan Science & Technology Bureau (2013010501010141), the Academic Award for Excellent Ph.D. Candidates funded by Ministry of Education of China (5052012202002), and the Fundamental Research Funds for the Central Universities. The work at National Taiwan University was supported by Grants NSC (98-2221-E-002-015-MY3) and by NTU Excellent Research Projects (10R80908, 102R890954).
References and links
1. 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]
2. S. Pimputkar, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Prospects for LED lighting,” Nat. Photonics 3(4), 180–182 (2009). [CrossRef]
3. Z. C. Feng, Handbook of Zinc Oxide and Related Materials (Taylor & Francis, 2012).
4. Z. C. Feng, III-Nitride Devices and Nanoengineering (Imperial College, 2008).
5. C. P. Chen, M. Y. Ke, C. C. Liu, Y. J. Chang, F. H. Yang, and J. J. Huang, “Observation of 394 nm electroluminescence from low-temperature sputtered n-ZnO∕SiO2 thin films on top of the p-GaN heterostructure,” Appl. Phys. Lett. 91(9), 091107 (2007). [CrossRef]
6. H. Zhu, C. X. Shan, B. Yao, B. H. Li, J. Y. Zhang, Z. Z. Zhang, D. X. Zhao, D. Z. Shen, X. W. Fan, Y. M. Lu, and Z. K. Tang, “Ultralow-threshold laser realized in zinc oxide,” Adv. Mater. 21(16), 1613–1617 (2009). [CrossRef]
7. H. H. Huang, G. J. Fang, Y. Li, S. Z. Li, X. M. Mo, H. Long, H. N. Wang, D. L. Carroll, and X. Z. Zhao, “Improved and color tunable electroluminescence from n-ZnO/HfO2/p-GaN heterojunction light emitting diodes,” Appl. Phys. Lett. 100(23), 233502 (2012). [CrossRef]
8. L. C. Zhang, Q. S. Li, L. Shang, Z. J. Zhang, R. Z. Huang, and F. Z. Zhao, “Electroluminescence from n-ZnO : Ga/p-GaN heterojunction light-emitting diodes with different interfacial layers,” J. Phys. D Appl. Phys. 45(48), 485103 (2012). [CrossRef]
9. S. Z. Li, W. W. Lin, G. J. Fang, F. Huang, H. H. Huang, H. Long, X. M. Mo, H. N. Wang, W. J. Guan, and X. Z. Zhao, “Ultraviolet/violet dual-color electroluminescence based on n-ZnO single crystal/p-GaN direct-contact light-emitting diode,” J. Lumin. 140, 110–113 (2013). [CrossRef]
10. Z. F. Shi, Y. T. Zhang, J. X. Zhang, H. Wang, B. Wu, X. P. Cai, X. J. Cui, X. Dong, H. W. Liang, B. L. Zhang, and G. T. Du, “High-performance ultraviolet-blue light-emitting diodes based on an n-ZnO nanowall networks/p-GaN heterojunction,” Appl. Phys. Lett. 103(2), 021109 (2013). [CrossRef]
11. J. B. You, X. W. Zhang, S. G. Zhang, J. X. Wang, Z. G. Yin, H. R. Tan, W. J. Zhang, P. K. Chu, B. Cui, A. M. Wowchak, A. M. Dabiran, and P. P. Chow, “Improved electroluminescence from n-ZnO/AlN/p-GaN heterojunction light-emitting diodes,” Appl. Phys. Lett. 96(20), 201102 (2010). [CrossRef]
12. Y. I. Alivov, E. V. Kalinina, A. E. Cherenkov, D. C. Look, B. M. Ataev, A. K. Omaev, M. V. Chukichev, and D. M. Bagnall, “Fabrication and characterization of n-ZnO/p-AlGaN heterojunction light-emitting diodes on 6H-SiC substrates,” Appl. Phys. Lett. 83(23), 4719–4721 (2003). [CrossRef]
13. H. Long, G. J. Fang, H. H. Huang, X. M. Mo, W. Xia, B. Z. Dong, X. Q. Meng, and X. Z. Zhao, “Ultraviolet electroluminescence from ZnO/NiO-based heterojunction light-emitting diodes,” Appl. Phys. Lett. 95(1), 013509 (2009). [CrossRef]
14. H. H. Huang, G. J. Fang, X. M. Mo, H. Long, H. N. Wang, S. Z. Li, Y. Li, Y. P. Zhang, C. X. Pan, and D. L. Carroll, “Improved and orange emission from an n-ZnO/p-Si heterojunction light emitting device with NiO as the intermediate layer,” Appl. Phys. Lett. 101(22), 223504 (2012). [CrossRef]
15. Y. Yang, Y. P. Li, L. L. Xiang, X. Y. Ma, and D. R. Yang, “Low-voltage driven ∼1.54 μm electroluminescence from erbium-doped ZnO/p+-Si heterostructured devices: Energy transfer from ZnO host to erbium ions,” Appl. Phys. Lett. 102(18), 181111 (2013). [CrossRef]
16. J. Ahn, H. Park, M. A. Mastro, J. K. Hite, C. R. Eddy Jr, and J. Kim, “Nanostructured n-ZnO / thin film p-silicon heterojunction light-emitting diodes,” Opt. Express 19(27), 26006–26010 (2011). [CrossRef] [PubMed]
17. Y. F. Chan, W. Su, C. X. Zhang, Z. L. Wu, Y. Tang, X. Q. Sun, and H. J. Xu, “Electroluminescence from ZnO-nanofilm/Si-micropillar heterostructure arrays,” Opt. Express 20(22), 24280–24287 (2012). [CrossRef] [PubMed]
18. H. Ohta, K.-i. Kawamura, M. Orita, M. Hirano, N. Sarukura, and H. Hosono, “Current injection emission from a transparent p–n junction composed of p-SrCu2O2/n-ZnO,” Appl. Phys. Lett. 77(4), 475–477 (2000). [CrossRef]
19. S. F. Chichibu, T. Ohmori, N. Shibata, T. Koyama, and T. Onuma, “Greenish-white electroluminescence from p-type CuGaS2 heterojunction diodes using n-type ZnO as an electron injector,” Appl. Phys. Lett. 85(19), 4403–4405 (2004). [CrossRef]
20. A. Nadarajah, R. C. Word, J. Meiss, and R. Könenkamp, “Flexible inorganic nanowire light-emitting diode,” Nano Lett. 8(2), 534–537 (2008). [CrossRef] [PubMed]
21. S. Z. Li, G. J. Fang, H. Long, X. M. Mo, H. H. Huang, B. Z. Dong, and X. Z. Zhao, “Enhancement of ultraviolet electroluminescence based on n-ZnO/n-GaN isotype heterojunction with low threshold voltage,” Appl. Phys. Lett. 96(20), 201111 (2010). [CrossRef]
22. L. C. Zhang, Q. S. Li, L. Shang, F. F. Wang, C. Qu, and F. Z. Zhao, “Improvement of UV electroluminescence of n-ZnO/p-GaN heterojunction LED by ZnS interlayer,” Opt. Express 21(14), 16578–16583 (2013). [CrossRef] [PubMed]
23. P. C. Wu, H. Y. Lee, and C. T. Lee, “Enhanced light emission of double heterostructured MgZnO/ZnO/MgZnO in ultraviolet blind light-emitting diodes deposited by vapor cooling condensation system,” Appl. Phys. Lett. 100(13), 131116 (2012). [CrossRef]
24. S. Chu, J. Z. Zhao, Z. Zuo, J. Y. Kong, L. Li, and J. L. Liu, “Enhanced output power using MgZnO/ZnO/MgZnO double heterostructure in ZnO homojunction light emitting diode,” J. Appl. Phys. 109(12), 123110 (2011). [CrossRef]
25. H. Long, S. Z. Li, X. M. Mo, H. N. Wang, H. H. Huang, Z. Chen, Y. P. Liu, and G. J. Fang, “Electroluminescence from ZnO-nanorod-based double heterostructured light-emitting diodes,” Appl. Phys. Lett. 103(12), 123504 (2013). [CrossRef]
26. M. Y. Ke, T. C. Lu, S. C. Yang, C. P. Chen, Y. W. Cheng, L. Y. Chen, C. Y. Chen, J. H. He, and J. J. Huang, “UV light emission from GZO/ZnO/GaN heterojunction diodes with carrier confinement layers,” Opt. Express 17(25), 22912–22917 (2009). [CrossRef] [PubMed]
27. Z. F. Shi, Y. T. Zhang, X. C. Xia, W. Zhao, H. Wang, L. Zhao, X. Dong, B. L. Zhang, and G. T. Du, “Electrically driven ultraviolet random lasing from an n-MgZnO/i-ZnO/SiO2/p-Si asymmetric double heterojunction,” Nanoscale 5(11), 5080–5085 (2013). [CrossRef] [PubMed]
28. S. V. Ivanov, V. A. Solov’ev, K. D. Moiseev, I. V. Sedova, Y. V. Terent’ev, A. A. Toropov, B. Y. Meltzer, M. P. Mikhailova, Y. P. Yakovlev, and P. S. Kop’ev, “Room-temperature midinfrared electroluminescence from asymmetric AlSbAs/InAs/CdMgSe heterostructures grown by molecular beam epitaxy,” Appl. Phys. Lett. 78(12), 1655 (2001). [CrossRef]
29. S. V. Ivanov, V. A. Kaygorodov, S. V. Sorokin, B. Y. Meltser, V. A. Solov’ev, Y. V. Terent’ev, O. G. Lyublinskaya, K. D. Moiseev, E. A. Grebenshchikova, M. P. Mikhailova, A. A. Toropov, Y. P. Yakovlev, P. S. Kop’ev, and Z. I. Alferov, “A 2.78-μm laser diode based on hybrid AlGaAsSb/InAs/CdMgSe double heterostructure grown by molecular-beam epitaxy,” Appl. Phys. Lett. 82(21), 3782 (2003). [CrossRef]
30. T. Ogino and M. Aoki, “Mechanism of yellow luminescence in GaN,” Jpn. J. Appl. Phys. 19(12), 2395–2405 (1980). [CrossRef]
31. M. S. Wang, Y. J. Zhou, Y. P. Zhang, J. K. Eui, H. H. Sung, and G. S. Seung, “Near-infrared photoluminescence from ZnO,” Appl. Phys. Lett. 100(10), 101906 (2012). [CrossRef]
32. C. H. Ahn, Y. Y. Kim, D. C. Kim, S. K. Mohanta, and H. K. Cho, “A comparative analysis of deep level emission in ZnO layers deposited by various methods,” J. Appl. Phys. 105(1), 013502 (2009). [CrossRef]
33. N. H. Alvi, K. Ul Hasan, O. Nur, and M. Willander, “The origin of the red emission in n-ZnO nanotubes/p-GaN white light emitting diodes,” Nanoscale Res. Lett. 6(1), 130 (2011). [CrossRef] [PubMed]
34. T. P. Yang, H. C. Zhu, J. M. Bian, J. C. Sun, X. Dong, B. L. Zhang, H. W. Liang, X. P. Li, Y. G. Cui, and G. T. Du, “Room temperature electroluminescence from the n-ZnO/p-GaN heterojunction device grown by MOCVD,” Mater. Res. Bull. 43(12), 3614–3620 (2008). [CrossRef]
35. S. Miyazaki, “Photoemission study of energy-band alignments and gap-state density distributions for high-k gate dielectrics,” J. Vac. Sci. Technol. B 19(6), 2212 (2001). [CrossRef]
36. J. S. Liu, C. X. Shan, H. Shen, B. H. Li, Z. Z. Zhang, L. Liu, L. G. Zhang, and D. Z. Shen, “ZnO light-emitting devices with a lifetime of 6.8 hours,” Appl. Phys. Lett. 101(1), 011106 (2012). [CrossRef]
37. X. Y. Liu, C. X. Shan, C. Jiao, S. P. Wang, H. F. Zhao, and D. Z. Shen, “Pure ultraviolet emission from ZnO nanowire-based p-n heterostructures,” Opt. Lett. 39(3), 422–425 (2014). [CrossRef] [PubMed]
