Open Access
Add to BibSonomy
Abstract
A strategy to realize ZnO-based near-white-light electroluminescence (EL) was proposed by utilizing and regulating the intrinsic defect-related emissions of solution-processed ZnO nanocrystals (NCs). Prototype near-white light-emitting diodes (LEDs) based upon this strategy were demonstrated by using n-ZnO NCs/n-Si isotype heterojunctions. The emission color of the n-ZnO NCs/n-Si isotype heterojunction LEDs was tuned toward near white by using an Al-doped ZnO (AZO) spectral “scissor” which can tailor the green light more severely, rather than the blue or red light. Moreover, quantum size effect was clearly observed in both the photoluminescence (PL) and EL spectra via the redshift of the near-band-edge UV emission of the ZnO NCs. The strategy using AZO spectral “scissors” to regulate the VO-related green emission of ZnO may present a promising pathway to realize ZnO-based white-light LEDs.
© 2017 Optical Society of America
1. Introduction
Solid-state white lighting based on GaN and related compounds has been successfully developed to commercialization in past decades [1, 2]. Seeking new classes of inorganic semiconductor candidates for the next-generation white light-emitting diodes (LEDs) has attracted intensive interests worldwide. Among them, ZnO is very promising because of its wide direct bandgap (3.37 eV) and superior optoelectronic properties, which makes ZnO be widely used in solar cells, photodetectors, LEDs and bioimaging [3–6]. Among the ZnO materials, ZnO nanocrystals (NCs) have drawn considerable attentions due to their solution processing capability, narrow emission line-width, and ease for printable electronics [7–9]. Besides, ZnO NCs possess large exciton binding energy of 60 meV, much larger than that of the GaN with ~25 meV, which means that highly efficient exciton emission around 380 nm can be expectable from ZnO NCs at room temperature (RT) or higher temperatures [7, 10]. To date, numerous LEDs based on ZnO thin film, ZnO nanowires, and ZnO NCs have been extensively reported, either using homojunctions (p-ZnO [11–13]) or heterojunctions (p-GaN [14–17], p-Si [18–20], p-NiO [21–24], p-MgZnO [25], p-polymers [8, 26–31], n-Si [32–35], etc.). Among the reported works, in order to obtain white LEDs based on ZnO, most of the literatures were devoted to achieving the UV emission of ZnO around 380 nm as intense as possible, and intended to utilize the UV light to stimulate an appropriate phosphor to generate white light, which merely follows the similar route as that in the GaN industry. However, this strategy to achieve the ZnO-based white LEDs has been rebuffed till now due to the lack of stable and reliable p-ZnO with high hole concentration and mobility [36]. It is notable that except for the UV emission around 380 nm, ZnO itself, especially in the solution produced ZnO NCs, is also capable of emitting stronger red, green, and blue (RGB) light either in photoluminescence (PL) or electroluminescence (EL) due to the naturally abundant intrinsic point defects in ZnO. Unfortunately, however, in most of the reported works, these intrinsic RGB emissions are considered as big drawbacks that should be eliminated as completely as possible and thus few researchers attempt to utilize them. If one can utilize and regulate the intrinsic RGB emissions of the ZnO, it might be an alternative promising strategy to realize ZnO-based white LEDs.
Here in this work, following this strategy, we present a technique to utilize and regulate the defect-related RGB emissions of ZnO NCs, simply by introducing an Al-doped ZnO (AZO) spectral “scissor”. Prototype near-white LEDs are realized and demonstrated by using n-ZnO NCs/n-Si isotype heterojunctions. The emission color of the isotype heterojunction LEDs can be tuned toward near white by using the AZO spectral “scissor” which can tailor the green light more severely, rather than the blue or red light. Furthermore, quantum size effect with respect to ZnO NCs was also observed in both the PL and EL spectra. Relevant phenomena and EL mechanisms are revealed and discussed in this work.
2. Experimental section
The schematic structure of the AZO/n-ZnO NCs/n-Si isotype heterojunction LEDs is depicted in Fig. 1(a). N-Si slices cut from an n-type Si wafer (100, with resistivity of 10−3 Ω∙cm) were used as substrates for device fabrication. Prior to producing ZnO NC film, the n-Si slices were cleaned in an ultrasonic bath sequentially in acetone, ethanol and deionized water each for 10 minutes, and then dried by an argon gas flow. ZnO NCs were prepared via a low-cost solution method using Zn(CH3COO)2∙2H2O (99.0% purity) and NaOH (96.0% purity) and the experimental details can be found in our previous work [37]. Specifically, 5 mL NaOH solution (200 mM/L) was slowly dropwise added into 70 mL Zn(CH3COO)2 solution (9.8 mM/L) and the mixture was transferred into a 60 °C automatic oven to react for 8 hours to produce ZnO NCs. The resultant ZnO NCs were collected via centrifugation and ultrasonically redispersed in ethanol (~1.5 mg/mL) for use. The as-grown ZnO NCs solution was then spin-casted onto the cleaned n-Si substrates under 2000 rpm for 40 s to produce a thin ZnO NC film. This procedure was repeated for several times (optimized to be 9 times) and dried at 80 °C for 20 min to completely evaporate the solvent. This was followed by an annealing treatment at 700 °C for 30 min in air to yield a ~280 nm uniform ZnO NC film. After that, a ~80 nm thick AZO film was deposited on the ZnO NC film as a spectral scissor in a radio frequency magnetron sputtering system at 200 °C using an AZO ceramic target (2 wt% Al2O3). The sputtering power was optimized to be 80 W under an argon flow of 10 standard cubic centimeter per minute (sccm). Finally, a semitransparent Ag electrode with a diameter of 2 mm was sputtered on the AZO film as the cathode by direct current magnetron sputtering with an optimized power of 20 W for 2.5 min under an argon flow of 20 sccm, and In-Ga alloy was used as the anode on the backside of the n-Si substrates. Both contacts were proved to be good ohmic contacts. Three types of LEDs were investigated: LED 1 for LEDs with the structure of Ag/unannealed n-ZnO NCs/n-Si; LED 2 for LEDs with Ag/annealed n-ZnO NCs/n-Si; LED 3 for LEDs with Ag/AZO/annealed n-ZnO NCs/n-Si.
Fig. 1 (a) Schematic diagram of the AZO/n-ZnO NCs/n-Si isotype heterojunction LEDs. (b) XRD patterns of the ZnO NC film grown on n-Si substrate with and without annealing. (c)-(e) Surface SEM images of LED 1, LED 2 and LED 3, respectively. The insets are the corresponding zoomed-in SEM images with scale bars of 500 nm. (f) Cross-sectional SEM image of LED 3.
Scanning electron microscopy (SEM) images of the LEDs were measured by Hitachi SU8220. X-ray diffraction (XRD) was introduced to characterize the crystal structure of the ZnO NCs (Tongda TD-3500, Cu Kα radiation). PL spectra were obtained under a 325 nm He-Cd laser at RT and the PL emissions were collected by a Zolix monochromator coupled with a CCD detector (Andor iDus DU401A-BVF). The EL spectra were also collected at RT by the Zolix monochromator whose grating was set 1800 g/mm (blazed at 500 nm), and light emission signals were detected by a photo-multiplier tube (PMT) with a scanning step of 1 nm. Current-voltage (I-V) characteristics of the LEDs were measured with a Keithley 2400 sourcemeter. The transmittance spectra of the AZO film were measured using a UV-VIS spectrophotometer (Shimadzu UV-2700).
3. Results and discussion
Figure 1(b) shows the XRD patterns with and without annealing the as-grown ZnO NC film. All of the diffraction peaks correspond to wurtzite ZnO with preferred orientation along the (101) facets and no impurity peak is observed. Intensities for all the diffraction peaks are dramatically boosted after annealing, indicative of a highly improved crystallinity for the ZnO NC film. The average crystal size D significantly increases from 7.1 to 34.6 nm (by Scherrer equation), suggesting a recrystallization of the ZnO NCs by annealing at 700 °C in air. As compared with Fig. 1(c), the enlarged grain size observed from the surface SEM images in Fig. 1(d) can clearly characterize such recrystallization. Specifically in Figs. 1(c) and 1(d), the size of the closely packed grains that consist of the ZnO NC film is increased clearly from ~60 to ~90 nm after annealing, while pinholes throughout the whole film are greatly reduced to yield a much smoother surface. Depositing the AZO film can further modify and smoothen the surface of the ZnO NC film, as shown in Fig. 1(e). Pinholes are completely eliminated via the AZO modification and the grain size is greatly reduced to ~20 nm. The cross-sectional SEM image of LED 3 in Fig. 1(f) displays clear interfaces of the AZO/n-ZnO NCs/n-Si isotype heterojunction with ZnO NC film and AZO thicknesses to be ~280 nm and ~80 nm, respectively.
Figure 2(a) depicts the RT PL spectra for LED 1, LED 2, and LED 3. A distinct ZnO near-band-edge (NBE) emission peak at 375 nm is observed from the as-grown ZnO NCs, accompanying a much stronger deep level (DL)-related emission centered at 585 nm (eg. Oi [38]). After annealing, nevertheless, the NBE emission of ZnO is greatly boosted with the emission peak redshift to 380 nm. This redshift, consistent with the XRD and SEM results, is commonly seen in the PL of ZnO NCs [7] and must be attributed to the quantum size effect of ZnO NCs. Note that the DL emission from the annealed ZnO NCs is dramatically decreased, suggesting a greatly reduced DL defects and improved crystallinity for the ZnO NC film by annealing. With the AZO film, the ZnO NBE emission can be further boosted and the DL emission is also slightly increased, which might be ascribed to superposition of the AZO emission generated by the 325 nm laser [39].
Fig. 2 (a) RT PL spectra excited by a 325 nm He-Cd laser and (b) I-V characteristics of LED 1, LED 2 and LED 3. (c) Chromaticity coordinates (CIE 1931) of LED 1, LED 2 and LED 3 and (d) their corresponding EL spectra with a same ordinate under the forward injection current of 90 mA.
Figure 2(b) shows the I-V characteristics of LED 1, LED 2 and LED 3, all of which demonstrate nonlinear rectifying behaviors (unless stated elsewhere, forward voltage is denoted as positive voltage on the n-Si). For LED 1 with the as-grown ZnO NCs, a reverse breakdown is observed as the reverse bias is increased beyond ~3 V. For LED 2 with annealed ZnO NCs, this drawback is conquered and the device stability is improved due to the markedly reduced reverse leakage current. The reverse leakage current can further be suppressed with the AZO film (LED 3) and a moderate forward turn-on voltage is achieved, which should be attributed to the current spreading effect by the AZO surface modification as well as the elimination of the pin-holes shown in Fig. 1(e).
Figure 3 presents the RT EL spectra of LED 1, LED 2 and LED 3, from which it is seen that all the EL intensities of the LEDs are boosted with increasing the forward injection currents. For LED 1 with as-grown ZnO NCs, specifically, the EL spectra are composed of a broad dominant DL emission centered at ~560 nm as well as a weak ZnO NBE UV emission peaked at ~375 nm. For LED 2 and LED 3 with annealed ZnO NCs, nevertheless, the EL spectra are markedly changed with the broad dominant DL emission blueshift to ~520 nm but the UV ZnO NBE emission redshift to ~390 nm. Larger crystals that are regrown during the annealing treatment at 700 °C (LED 2 and LED 3) can emit light with longer wavelength, thus leading the ZnO NBE emission peak redshift to ~390 nm. Therefore, the interesting redshift of the ZnO NBE emission should be attributed to the quantum size effect of the ZnO NCs, which agrees well with the PL and XRD results obtained above. The blueshift of the DL emission is also very interesting and might be related to evolution from Oi to VO of the ZnO NCs during annealing [40]. However, Yang et al. believed that the blueshift of the DL emission was attributed to the evolution of the intrinsic defects from Oi to OZn as the annealing temperature increased, which could follow the reactions Oi + VZn→OZn [41]. Both effects might play a role in our case and be responsible for the blueshift phenomenon.
Considering the big difference of the EL spectra, we calculate the corresponding chromaticity coordinates at CIE 1931 as (0.39, 0.44), (0.32, 0.40), and (0.33, 0.38) for LED 1, LED 2 and LED 3, respectively. As plotted in Fig. 2(c), one can clearly observe that the EL emission color of LED 3 is tuned toward near white, suggesting a great performance improvement. Since ZnO NCs itself possess abundant intrinsic defects that can emit RGB emissions simultaneously under appropriate injection currents (see the broad DL emission in the EL spectra in Fig. 3), near-white or even ideal white emission can be quite expectable by the superpositon of these RGB emissions if one can regulate and match the proportion of the individual R, G, and B emissions carefully. In order to further explicate the tunability toward near-white light, the EL spectra of LED 1, LED 2, and LED 3 were plotted with a same ordinate in Fig. 2(d). Four spectral regions can be obtained from the EL spectra: UV, blue, green and red. For all of the LEDs, they emit nearly the same intensity of the red light. For LED 1, however, the UV light is stronger than the other two LEDs. Since the UV light rarely influences the emission color (or chromaticity ordinates), it must be the much weaker blue and green light emissions than that of LED 2 and LED 3 make the emission color yellowish for LED 1. For LED 2 and LED 3, nevertheless, the intensity of the blue light is almost the same but the green light of LED 2 is much stronger than LED 3, which leads the emission color of LED 2 toward green. In other words, rich of the green light is unfavorable to achieve white-light emission for this isotype heterojunction LED. Furthermore, it can be seen that insufficient blue light can further hinder the emission color of LED 3 from tuning toward ideal white with CIE coordinates at (0.33, 0.33). As a result, we conclude that in order to obtain white-light LEDs with n-ZnO NC/n-Si isotype heterojunctions, we should find routes to improve the blue light (e.g. introducing Zni [42] or rare earth [32]) but suppress the green light (eg. eliminating VO [38]).
In order to further understand the roots of the EL emissions, peak deconvolutions with Gaussian functions are applied to the EL spectra of LED 1, LED 2 and LED 3, respectively, as shown in Figs. 4(a)-4(c). Five distinct fit peaks corresponding to five different EL mechanisms are obtained by the peak deconvolutions. The UV “Peak 1” (~392 nm) is consistent with the PL results and commonly considered as the NBE emission of ZnO NCs [37, 43]. The blue “Peak 2” (~424 nm) is probably attributed to the transition from the shallow donors (Zni) to the valence band [44]. The green “Peak 3” (~500 nm) should be originated from transition from deep donor level to valence band due to the VO levels [45, 46]. The orange “Peak 4” (~590 nm) should result from the Oi-related defect levels [38]. The red “Peak 5” (~688 nm for LED 2 and ~715 nm for LED 3) was also observed by other researchers and considered to be attributed to the combination of emissions related to Oi and VO defects of ZnO [47]. Peak intensities of these five fit peaks are also concluded from the peak deconvolution results and depicted in Fig. 4(d), from which one can see that the significant decrease of “Peak 3” in LED 3 (as compared with LED 2) should be the primary reason for the tunability of LED 3 toward near-white light after depositing the AZO film. Note again that the only difference between LED 2 and LED 3 is the introduction of the AZO film on the ZnO NC film, and thus the significant decrease of the green “Peak 3” should not be attributed to the impact of the VO. Instead, the AZO film should play an important role for the decrease of the green “Peak 3”.
Fig. 4 (a)-(c) Peak deconvolutions of the EL spectra with Gaussian functions for LED 1, LED 2 and LED 3, respectively. (d) Fitting peak intensity from peak 1 to 5, as concluded from the peak-deconvolution results.
To further better understand the effect of the AZO film, we measured the transmission spectrum of the AZO film deposited on a glass substrate (the same growth condition as that used in LED 3), as plotted in Fig. 5(a). According to Fig. 5(a), it can be clearly seen that the transmittance of the green light (~500 nm) is much smaller than that of the blue and red light. In other words, the AZO film can block the green light more severely when the EL has to escape from the AZO surface, whereas the blue and red light are less affected. Xia et al. found that grain size and thickness of the AZO films can greatly influence the transmittance [48]. Therefore, AZO layer with proper grain size and thickness in our work may result in the fact that the green light is scattered more severely by the AZO layer, rather than the blue or red light. Besides, the Al2O3 content [49] and surface roughness of the AZO layer might also play a role in affecting the transmittance, thus influencing the extraction of the EL emission light. The AZO layer works like a spectral “scissor” that regulates the green light emission and indeed it is this “scissor” effect makes the green light tailored for LED 3 and thus leads the emission color tuning toward near white. Since the grain size and thickness can directly affect its transmission spectrum [48], the AZO film may become a very promising candidate to regulate the luminous spectra of LEDs. Figure 5(b) shows the energy band diagram of the LED 3 and illustrates the spectral “scissor” effect of the AZO film, from which one can see that the EL emission generated at the n-ZnO NCs/SiOx interface are tailored by the AZO film, thus shrinking the green light emission and tuning the emission color toward near white.
Fig. 5 (a) Transmittance spectra of the AZO film deposited on Glass substrates. (b) The energy band diagram of LED 3.
4. Conclusion
In summary, we have observed quantum size effect from both the PL of the ZnO NCs and the EL of the n-ZnO NCs/n-Si isotype heterojunction LEDs. We have also obtained near-white emission from such isotype heterojunction LEDs with the help of an AZO spectral “scissor”. It is found that as compared with the blue or red light, the green light can be tailored more severely when the EL generated at the n-ZnO NCs/SiOx interface escapes from the AZO film, thus tuning the emission color toward near white. A route to obtain an ideal white light emission with CIE coordinates at (0.33, 0.33) is proposed by suppressing the green light but improving the blue light. The strategy presented in this work may give a promising pathway to realize ZnO-based white-light LEDs.
Appendix Supplementary materials
Detailed process to get the plot of Fig. 4(d) from the fitting of the EL spectra
Prior to get the intensity of UV, blue, green, orange, and red emissions in LED 1, LED 2 and LED 3 from the fitting of the EL spectra, we had to measure the EL spectra for the three types of LEDs under different forward currents, as shown in Fig. 3. Then peak deconvolutions with Gaussian functions were applied to the EL spectra of LED 1, LED 2 and LED 3 @90 mA, respectively. Five distinct fit peaks (Peak 1 to Peak 5 that represents UV to red emissions) can be obtained by the peak deconvolutions for each type of LED. We measured the peak intensity for each fitting peak and recorded it. Finally, the fitting peak intensity vs each fitting peak was plotted, as shown in Fig. 4(d).
Funding
National Natural Science Foundation of China (11504060, 11405034); the Scientific Research Project for Higher Education of Guangxi Zhuang Autonomous Region (KY2015ZD006); and the Doctoral Scientific Research Foundation of Guangxi University (XBZ160084).
References and links
1. N. Shuji, M. Takashi, and S. Masayuki, “High-power GaN P-N junction blue-light-emitting diodes,” Jpn. J. Appl. Phys. 30(2), L1998–L2001 (1991). [CrossRef]
2. A. Isamu and A. Hiroshi, “Breakthroughs in improving crystal quality of GaN and Invention of the p-n junction blue-light-emitting diode,” J. Appl. Phys. 45(12), 9001–9010 (2006). [CrossRef]
3. Z. Zang, A. Nakamura, and J. Temmyo, “Single cuprous oxide films synthesized by radical oxidation at low temperature for PV application,” Opt. Express 21(9), 11448–11456 (2013). [CrossRef] [PubMed]
4. Z. Zang, X. Zeng, J. Du, M. Wang, and X. Tang, “Femtosecond laser direct writing of microholes on roughened ZnO for output power enhancement of InGaN light-emitting diodes,” Opt. Lett. 41(15), 3463–3466 (2016). [CrossRef] [PubMed]
5. Z. G. Zang and X. S. Tang, “Enhanced fluorescence imaging performance of hydrophobic colloidal ZnO nanoparticles by a facile method,” J. Alloys Compd. 619, 98–101 (2015). [CrossRef]
6. J. Gomez and O. Tigli, “Zinc oxide nanostructures: from growth to application,” J. Mater. Sci. 48(2), 612–624 (2013). [CrossRef]
7. T. Toyama, H. Takeuchi, D. Yamaguchi, H. Kawasaki, K. Itatani, and H. Okamoto, “Solution-processed ZnO nanocrystals in thin-film light-emitting diodes for printed electronics,” J. Appl. Phys. 108(8), 084302 (2010). [CrossRef]
8. D. I. Son, B. W. Kwon, D. H. Park, W. S. Seo, Y. Yi, B. Angadi, C. L. Lee, and W. K. Choi, “Emissive ZnO-graphene quantum dots for white-light-emitting diodes,” Nat. Nanotechnol. 7(7), 465–471 (2012). [CrossRef] [PubMed]
9. X. Xu, C. Xu, X. Wang, Y. Lin, J. Dai, and J. Hu, “Control mechanism behind broad fluorescence from violet to orange in ZnO quantum dots,” CrystEngComm 15(5), 977–981 (2013). [CrossRef]
10. D. C. Look, “Recent advances in ZnO materials and devices,” Mater. Sci. Eng. B 80(1–3), 383–387 (2001). [CrossRef]
11. H. Sun, Q. Zhang, J. Zhang, T. Deng, and J. Wu, “Electroluminescence from ZnO nanowires with a p-ZnO film/n-ZnO nanowire homojunction,” Appl. Phys. B 90(3), 543–546 (2008). [CrossRef]
12. X. W. Sun, B. Ling, J. L. Zhao, S. T. Tan, Y. Yang, Y. Q. Shen, Z. L. Dong, and X. C. Li, “Ultraviolet emission from a ZnO rod homojunction light-emitting diode,” Appl. Phys. Lett. 95(13), 133124 (2009). [CrossRef]
13. S. Chu, G. Wang, W. Zhou, Y. Lin, L. Chernyak, J. Zhao, J. Kong, L. Li, J. Ren, and J. Liu, “Electrically pumped waveguide lasing from ZnO nanowires,” Nat. Nanotechnol. 6(8), 506–510 (2011). [CrossRef] [PubMed]
14. O. Lupan, T. Pauporté, and B. Viana, “Low-voltage UV-electroluminescence from ZnO-nanowire Array/p-GaN light-emitting diodes,” Adv. Mater. 22(30), 3298–3302 (2010). [CrossRef] [PubMed]
15. S. Xu, C. Xu, Y. Liu, Y. Hu, R. Yang, Q. Yang, J. H. Ryou, H. J. Kim, Z. Lochner, S. Choi, R. Dupuis, and Z. L. Wang, “Ordered nanowire array blue/near-UV light emitting diodes,” Adv. Mater. 22(42), 4749–4753 (2010). [CrossRef] [PubMed]
16. J. Dai, C. X. Xu, and X. W. Sun, “ZnO-microrod/p-GaN heterostructured whispering-gallery-mode microlaser diodes,” Adv. Mater. 23(35), 4115–4119 (2011). [CrossRef] [PubMed]
17. C. Pan, L. Dong, G. Zhu, S. Niu, R. Yu, Q. Yang, Y. Liu, and Z. L. Wang, “High-resolution electroluminescent imaging of pressure distribution using a piezoelectric,” Nat. Photonics 7(9), 752–758 (2013). [CrossRef]
18. K. Kim, T. Moon, J. Kim, and S. Kim, “Electrically driven lasing in light-emitting devices composed of n-ZnO and p-Si nanowires,” Nanotechnology 22(24), 245203 (2011). [CrossRef] [PubMed]
19. H. Huang, G. Fang, X. Mo, H. Long, H. Wang, S. Li, Y. Li, Y. Zhang, C. 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]
20. E. Nannen, T. Kümmell, A. Ebbers, and G. Bacher, “p-Si/n-ZnO Nanocrystal Heterojunction Light Emitting Device,” Appl. Phys. Express 5(3), 035001 (2012). [CrossRef]
21. Y. Y. Xi, Y. F. Hsu, A. B. Djurišić, A. M. C. Ng, W. K. Chan, H. L. Tam, and K. W. Cheah, “NiO/ZnO light emitting diodes by solution-based growth,” Appl. Phys. Lett. 92(11), 113505 (2008). [CrossRef]
22. J. Y. Wang, C. Y. Lee, Y. T. Chen, C. T. Chen, Y. L. Chen, C. F. Lin, and Y. F. Chen, “Double side electroluminescence from p-NiO/n-ZnO nanowire heterojunctions,” Appl. Phys. Lett. 95(13), 131117 (2009). [CrossRef]
23. R. Deng, B. Yao, Y. F. Li, Y. Xu, J. C. Li, B. H. Li, Z. Z. Zhang, L. G. Zhang, H. F. Zhao, and D. Z. Shen, “Ultraviolet electroluminescence from n-ZnO/p-NiO heterojunction light-emitting diode,” J. Lumin. 134(0), 240–243 (2013). [CrossRef]
24. B. O. Jung, Y. H. Kwon, D. J. Seo, D. S. Lee, and H. K. Cho, “Ultraviolet light emitting diode based on p-NiO/n-ZnO nanowire heterojunction,” J. Cryst. Growth 370(5), 314–318 (2013). [CrossRef]
25. 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]
26. X. W. Sun, J. Z. Huang, J. X. Wang, and Z. Xu, “A ZnO Nanorod Inorganic/Organic Heterostructure Light-Emitting Diode Emitting at 342 nm,” Nano Lett. 8(4), 1219–1223 (2008). [CrossRef] [PubMed]
27. Q. Zhang, J. Qi, X. Li, F. Yi, Z. Wang, and Y. Zhang, “Electrically pumped lasing from single ZnO micro/nanowire and poly(3,4-ethylenedioxythiophene):poly (styrenexulfonate) hybrid heterostructures,” Appl. Phys. Lett. 101(4), 043119 (2012). [CrossRef]
28. Q. Yang, Y. Liu, C. Pan, J. Chen, X. Wen, and Z. L. Wang, “Largely enhanced efficiency in ZnO nanowire/p-polymer hybridized inorganic/organic ultraviolet light-emitting diode by piezo-phototronic effect,” Nano Lett. 13(2), 607–613 (2013). [CrossRef] [PubMed]
29. D. I. Son, C. H. You, W. T. Kim, and T. W. Kim, “White light-emitting diodes fabricated utilizing hybrid polymer-colloidal ZnO quantum dots,” Nanotechnology 20(36), 365206 (2009). [CrossRef] [PubMed]
30. T. Toyama, H. Kawasaki, K. Itatani, and H. Okamoto, “Top-emission ultraviolet-light-emitting diodes containing solution-processed ZnO nanocrystals,” Appl. Phys. Express 4(6), 065005 (2011). [CrossRef]
31. J. Oliva, L. P. Mayen, E. D. I. Rosa, L. A. Diaz-Torres, A. T. Castro, and P. Salas, “Tunable white light from photo- and electroluminescence of ZnO nanoparticles,” J. Phys. D Appl. Phys. 47(1), 015104 (2014). [CrossRef]
32. Y. Yang, Y. Li, C. Wang, C. Zhu, C. Lv, X. Ma, and D. Yang, “Rare-earth doped ZnO films: a material platform to realize multicolor and near-infrared electroluminescence,” Adv. Opt. Mater. 2(3), 240–244 (2014). [CrossRef]
33. X. W. Sun, J. L. Zhao, S. T. Tan, L. H. Tan, C. H. Tung, G. Q. Lo, D. L. Kwong, Y. W. Zhang, X. M. Li, and K. L. Teo, “Epitaxially grown n-ZnO/MgO/TiN/n+-Si (111) heterostructured light-emitting diode,” Appl. Phys. Lett. 92(11), 111113 (2008). [CrossRef]
34. S. T. Tan, X. W. Sun, J. L. Zhao, S. Iwan, Z. H. Cen, T. P. Chen, J. D. Ye, G. Q. Lo, D. L. Kwong, and K. L. Teo, “Ultraviolet and visible electroluminescence from n-ZnO/SiOx/(n, p)-Si heterostructured light-emitting diodes,” Appl. Phys. Lett. 93(1), 013506 (2008). [CrossRef]
35. P. Chen, X. Ma, and D. Yang, “Fairly pure ultraviolet electroluminescence from ZnO-based light-emitting devices,” Appl. Phys. Lett. 89(11), 111112 (2006). [CrossRef]
36. Y. W. Zhang, X. M. Li, W. D. Yu, X. D. Gao, Y. F. Gu, C. Yang, J. L. Zhao, X. W. Sun, S. T. Tan, J. F. Kong, and W. Z. Shen, “Epitaxial growth and luminescence properties of ZnO-based heterojunction light-emitting diode on Si(1 1 1) substrate by pulsed-laser deposition,” J. Phys. D Appl. Phys. 41(20), 205105 (2008). [CrossRef]
37. X. Mo, H. Long, H. Wang, S. Li, Z. Chen, J. Wan, Y. Feng, Y. Liu, Y. Ouyang, and G. Fang, “Enhanced ultraviolet electroluminescence and spectral narrowing from ZnO quantum dots/GaN heterojunction diodes by using high-k HfO2 electron blocking layer,” Appl. Phys. Lett. 105(6), 063505 (2014). [CrossRef]
38. X. L. Wu, G. G. Siu, C. L. Fu, and H. C. Ong, “Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films,” Appl. Phys. Lett. 78(16), 2285–2287 (2001). [CrossRef]
39. K. Prabakar, C. Kim, and C. Lee, “UV, violet and blue-green luminescence from RF sputter deposited ZnO:Al thin films,” Cryst. Res. Technol. 40(12), 1150–1154 (2005). [CrossRef]
40. G. Rani and P. D. Sahare, “Structural and spectroscopic characterizations of ZnO quantum dots annealed at different temperatures,” J. Mater. Sci. Technol. 29(11), 1035–1039 (2013). [CrossRef]
41. Y. L. Yang, H. W. Yan, Z. P. Fu, B. F. Yang, L. S. Xia, Y. D. Xu, J. Zuo, and F. Q. Li, “Photoluminescence and Raman studies of electrochemically as-grown and annealed ZnO films,” Solid State Commun. 138(10–11), 521–525 (2006). [CrossRef]
42. H. Zeng, S. Yang, X. Xu, and W. Cai, “Dramatic excitation dependence of strong and stable blue luminescence of ZnO hollow nanoparticles,” Appl. Phys. Lett. 95(19), 191904 (2009). [CrossRef]
43. H. Huang, G. Fang, Y. Li, S. Li, X. Mo, H. Long, H. Wang, D. L. Carroll, and X. Zhao, “Improved and color tunable electroluminescence from n-ZnO/HfO2/p-GaN heterojunction light emitting diodes,” Appl. Phys. Lett. 100(23), 233502 (2012). [CrossRef]
44. Z. Chen, B. Li, X. Mo, K. Zhou, S. Li, Z. Song, H. Lei, J. Wen, Z. Zhu, and G. Fang, “Improved light emission from n-ZnO/p-Si heterojunction with HfO2 as an electron blocking layer,” J. Lumin. 184(0), 211–216 (2017). [CrossRef]
45. H. S. Kang, J. S. Kang, J. W. Kim, and S. Y. Lee, “Annealing effect on the property of ultraviolet and green emissions of ZnO thin films,” J. Appl. Phys. 95(3), 1246–1250 (2004). [CrossRef]
46. A. F. Kohan, G. Ceder, D. Morgan, and C. G. Van de Walle, “First-principles study of native point defects in ZnO,” Phys. Rev. B 61(22), 15019–15027 (2000). [CrossRef]
47. 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]
48. Y. Xia, P. Wang, G. He, M. Zhang, S. Shi, Y. Liu, and Z. Sun, “Microstructure, opotoelectrical and pre-strain dependent electrical properties of AZO films on flexible glass substrates for flexible electronics,” Surf. Coat. Tech. 320(25), 34–38 (2017). [CrossRef]
49. K. H. Kim, K. C. Park, and D. Y. Ma, “Structural, electrical and optical properties of aluminum doped zinc oxide films prepared by radio frequency magnetron sputtering,” J. Appl. Phys. 81(12), 7764–7772 (1997). [CrossRef]
