Open Access
Add to BibSonomyAbstract
We report ultraviolet electroluminescence from a hetero p-n junction between a single ZnO microsphere and p-GaN thin film. ZnO microspheres, which have high crystalline quality, have been synthesized by ablating a ZnO sintered target. It was found that synthesized ZnO microspheres had a high-optical property and exhibit the laser action in the whispering gallery mode under pulsed optical pumping. A hetero p-n junction was formed between the single ZnO microsphere/ p-GaN thin film, and a good rectifying property with a turn-on voltage of approximately 6 V was observed in I-V characteristic across the junction. Ultraviolet and visible electroluminescence were observed under forward bias.
© 2014 Optical Society of America
1. Introduction
ZnO is a very important II-VI semiconductor material with direct band-gap energy of approximately 3.37 eV and large exciton binding energy of approximately 60 meV, which is larger than the thermal energy at room temperature (26 meV). It is attractive material for highly efficient emission in near ultraviolet (UV) spectral region [1]. In addition, a variety of ZnO nano/microstructures can be synthesized, for example, wires [2], rods [3], walls [4], spheres [5], sheet [6] and so on. Since those ZnO nano/microstructures have high crystallinity and light-confinement effect, they are extremely attractive materials as the building blocks for a laser-diode (LD).
So far the lasing using ZnO nano/micro crystals has been reported by many research groups. The lasing mechanism using ZnO nano/micro crystals are categorized into a random laser and a micro-cavity laser. In a random laser, an optical cavity accidentally formed among the many ZnO crystals via multiple scattering [7, 8]. In a micro-cavity laser, on the other hand, a laser cavity formed within a single ZnO crystal. Usually the micro-cavity lasers have lower threshold energy for lasing than that of the random laser due to higher confinement ability in the micro-cavity. Furthermore, it is reported that there are two types of micro-cavity lasers, and those are a Fabry-Perot type laser and a whispering gallery mode (WGM) laser. The Fabry-Perot type lasing are reported using a ZnO nanowires where a laser cavity is formed between two end surfaces of the nanowire with a help of the waveguide effect of the nanowire [9, 10]. On the other hand, WGM laser often observed from relatively thick hexagonal ZnO nanorods, where the WGM cavity is formed along the hexagonal side wall with a help of total internal refraction at the walls [11–13].
The WGM laser in a single ZnO crystal is very attractive for the optoelectronic devices such as the bio-sensors in an ultraviolet spectral region because of its high cavity quality factor with a help of the total internal reflection. Recently we have reported the WGM lasing in a single spherical ZnO crystal under the optical pumping [14]. From a practical point of view, the electrical excitation is favorable. Although, many study groups have been trying to fabricate p-type ZnO crystals for realization of ZnO homo-junction LEDs, any stable and reliable methods have not been established at present. A number of studies had been reported on the ZnO nanocrystal based on hetero p-n junction by using other p-type materials such as GaN, Si, organic semiconductor, and so on [15–17].
In this report, the electroluminescence (EL) from hetero p-n junction between a single n-ZnO microsphere and p-GaN thin film has been investigated for the first time and compared with the photoluminescence from single ZnO sphere. Based on the results, the practical approach toward the realization of efficient WGM mode operation in single ZnO sphere under electrical pumping is discussed.
2. Fabrication of ZnO microspheres
ZnO microspheres were fabricated by ablating a ZnO sintered target with a purity of 99% in the air by using the fundamental of a Nd:YAG laser (Spectra-Physics, Quanta-Ray) (λ = 1064 nm) at a fluence of approximately 25 J/cm2 and a repetition rate of 10 Hz. ZnO liquid droplets produced from the surface of the ZnO sintered target were formed into spherical shapes due to the surface tension, and recrystallized during the dropping process since they were rapidly cooled down. The ZnO droplets were collected on a proper substrate which was located at approximately 5 mm away from the focal point on the target. More than 100 hundreds ZnO spheres in diameters of 0.1-10 µm were obtained on the substrate after three minutes ablation. Figure 1 shows the image of a ZnO microsphere on an ITO substrate observed by a scanning electron microscopy (SEM) (Keyence, VE-7800S). The diameter of the ZnO microsphere is found to be approximately 20 µm. It is turned out that ZnO microspheres with smooth surface can be fabricated by using this method.
Figure 2(a) shows the CCD image of the single ZnO microsphere on a glass substrate. It was investigated by a micro-Raman (Horiba, LabRAM ARAMIS) analysis, as shown in Fig. 2(b). It shows that there were the Raman-shift peaks only from ZnO wrutz structure [18, 19]. It was confirmed that those spheres fabricated by simple laser ablation method have a ZnO crystal structure from the micro-Raman result.
Fig. 2 (a) CCD image of the single ZnO microsphere on a glass substrate (b) Micro-Raman result from the single ZnO microsphere under excitation at 532 nm.
Room-temperature photoluminescence (RT-PL) spectrum of a ZnO microsphere was observed by a microscopic-spectroscopy system as shown in Fig. 3(a). A He-Cd laser (KIMMON KOHA, IK3301 R-G) (λ = 355 nm) irradiated the ZnO microsphere through the glass substrate. The luminescence from the crystal was observed by a monochrome CCD (Nikon, DS-QilMc) camera attached on the microscope. The image was also focused onto the entrance of an optical fiber with a half mirror, and a part of the image was transferred into the fiber and observed by a spectrometer (Lambda Vision, LVM-200-KS). Figure 3(b) shows PL spectrum from the single ZnO microsphere and the CCD image under excitation by the He-Cd laser. A strong UV emission peaked at approximately 395 nm was attributed to the near-band-edge (NBE) emission of the ZnO [20, 21].
Fig. 3 (a) Schematic of microscopic-spectroscopy system (b) RT-PL spectrum of the single ZnO microsphere and CCD image under excitation by the He-Cd laser (λ = 325 nm).
Next, a lasing characteristic of the single ZnO microsphere was also investigated using the microscopic-spectroscopy system. The single ZnO microsphere on a glass substrate was excited by the third harmonic of a Q-switched Nd:YAG laser beam (NEW WAVE, Polaris II, Δt = 5 ns, λ = 355 nm). Figure 4(a) shows the CCD image of the single ZnO microsphere under excitation, and its lasing spectra measured by a high resolution spectrometer (Lambda Vision, SA100SHPCB/TTK) is shown in Fig. 4 (b). The modal structure is found from the emission spectra and its mode spacing was approximately 0.43 nm. The measured diameter of a single ZnO microsphere shown in Fig. 4(a) was 25.2 ± 0.2 μm. Assuming the WGM operation, the total number of the internal reflection within the sphere was estimated to be 40 ± 7 times [14] for a diameter of 25.2 ± 0.2 μm. Figure 4(c) shows the peak intensities at different wavelength as a function of the excitation intensities. The threshold characteristic for the lasing was clearly observed and the threshold power was 380 kW/cm2, which is comparable to that of ZnO microwires [11]. Multiple strong peaks were overlapped due to the small mode spacing, which is comparable to the line width of each mode. The FWHM of the lasing peak was roughly estimated to be 0.5 nm, corresponding to the previous report [14], and the quality factor of 790 was achieved due to high light-confinement effect.
Fig. 4 (a) CCD image of the single ZnO microsphere excited by the third harmonic of a Q-switched Nd:YAG laser (λ = 355 nm), (b) emission spectra at different pump powers, and (c) emission intensities as a function of excitation power density.
3. Electroluminescence of a ZnO Microsphere
I-V characteristic of the single ZnO microsphere/p-type GaN thin film was investigated, as shown in Fig. 5(a). A Mg-doped GaN thin film (e PAK, 11OK1449-2) was used as a p-type semiconductor, and its thickness was approximately 2 μm. The electrodes attaching on the single ZnO microsphere and the p-GaN were a tungsten probe and an Au thin film with a thickness of 20 μm deposited by vapor deposition, respectively. We have confirmed that Ohmic contacts were formed on both electrodes [21,22]. Figure 5(b) is I-V characteristic of the single ZnO microsphere/p-type GaN thin film hetero-junction and shows good rectifying characteristics. It shows that the turn-on voltage of this hetero-junction was approximately 6 V. The current was increasing under forward bias and blocked under reverse bias.
Fig. 5 (a) Schematic of the I-V characteristic measurement (b) I-V characteristic of pn junction of the single ZnO microsphere/GaN thin film.
Using the experimental equipment shown in Fig. 3(a), the EL spectra of the single ZnO microsphere/ p-GaN thin film was measured while the applied voltage was varied from 10 V to 30 V in increments of 5 V. Figure 6(a) shows the single ZnO microsphere on p-GaN thin film under forward bias of 25 V. Although the half of single ZnO microsphere was covered with the tungsten probe, EL spectra from the area emitting the single ZnO microsphere was observed. Figure 6(b) shows the EL spectra observed by the spectrometer. In Fig. 6(b), the strong UV-emission peak of approximately 403 nm and the broad emission of visible region peaked at 622 nm was observed. Those emission peaks were clearly increased with the increase of applied voltage up from 10 V to 30 V. By comparing Fig. 3(b) with Fig. 6(b), it can be seen that UV emission peaks of EL spectra located very close to PL spectrum. It has been also reported that the NBE emission from n-ZnO/p-GaN hetero-junction appears around 400 nm [23, 24]. Therefore, the emission of UV region is considered to be the NBE emission from the ZnO. On the other hand, the broad emission centered to approximately 622 nm was due to the oxygen vacancy of ZnO defects [25]. The ripples are observed in the EL spectra. The wavelength separation between adjacent peaks is approximately 33.4 nm around 600 nm. Theoretical mode spacing based on the WGM cavity was estimated to be 2 nm, which is very different from the experimental result. We presume that the ripples are due to the interference effect within the p-GaN thin film. Because, the separation is corresponded to the mode spacing from the thin film interference theory, which is 32.4 nm.
Fig. 6 (a) CCD image of the single ZnO microsphere under forward bias of 25 V (b), and EL spectra from hetero p-n junction between the single ZnO microsphere and the p-GaN thin film.
In the present experiment, we have not observed the resonance mode of the WGM in the EL spectra in Fig. 6(b). The main reason is that the refractive index of the p-GaN is slightly higher than that of n-ZnO. Therefore, the light generated in n-ZnO sphere by the recombination is escaping from a contact point of p-n junction into p-GaN, resulting in the thin film interference ripple observed in Fig. 6(b). Another special issue related to the spherical shape is that the p-n junction is formed as a point contact. In this case, the emission region is confined around the small volume of the point contact. The point contact also limits the current which can flow through the contact. In the present experiment, the ZnO sphere was broken when the current reaches approximately 1.5 mA. In order to solve this problem, it is necessary to form p-n junction over a wider area of the ZnO sphere surface. The easiest way to make large area p-n junction over ZnO sphere surface is just to dip ZnO sphere into a p-type organic conductor.
4. Conclusions
The electroluminescence from hetero p-n junction between a single n-ZnO microsphere and a p-GaN thin film has been investigated for the first time and compared with the photoluminescence from single ZnO sphere. ZnO microsphere is very attractive as a building block for the WGM laser. ZnO microspheres were fabricated by ablating the ZnO sintered target with the fundamental of Nd:YAG laser in air. The laser oscillation around wavelength of 395 nm was observed by optical excitation with the third harmonic of the Q-switched Nd:YAG laser due to high light-confinement property of the single ZnO microsphere. Using such a high quality ZnO microsphere, hetero p-n junction was formed by simple mechanical contact between the single ZnO microsphere and the p-GaN thin film. This devise had rectifying I-V characteristic with a turn-on voltage of 6V and emitted UV light originated from the band edge transition of ZnO crystal.
Acknowledgments
This work was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS, No. 24656053 and 25286071).
References and links
1. D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen, and T. Goto, “Optically pumped lasing of ZnO at room temperature,” Appl. Phys. Lett. 70(17), 2230–2232 (1997). [CrossRef]
2. L. E. Greene, M. Law, D. H. Tan, M. Montano, J. Goldberger, G. Somorjai, and P. Yang, “General route to vertical ZnO Nanowire arrays using textured ZnO seeds,” Nano Lett. 5(7), 1231–1236 (2005). [CrossRef] [PubMed]
3. G. C. Yi, C. Wang, and W. I. Park, “ZnO nanorods: synthesis, characterization and applications,” Semicond. Sci. Technol. 20(4), S22–S34 (2005). [CrossRef]
4. J. Y. Lao, J. Y. Huang, D. Z. Wang, Z. F. Ren, D. Steeves, B. Kimball, and W. Porter, “ZnO nanowalls,” Appl. Phys., A Mater. Sci. Process. 78(4), 539–542 (2004). [CrossRef]
5. S. Xu, Z. H. Li, Q. Wang, L. J. Cao, T. M. He, and G. T. Zou, “A novel one-step method to synthesize nano/micron-sized ZnO sphere,” J. Alloy. Comp. 465(1-2), 56–60 (2008). [CrossRef]
6. J. Q. Hu, Y. Bando, J. H. Zhan, Y. B. Li, and T. Sekiguchi, “Two-dimensional micrometer-sized single-crystalline ZnO thin nanosheets,” Appl. Phys. Lett. 83(21), 4414–4416 (2003). [CrossRef]
7. S. F. Yu, C. Yuen, S. P. Lau, W. I. Park, and G. C. Yi, “Random laser action in ZnO nanorod arrays embedded in ZnO epilayers,” Appl. Phys. Lett. 84(17), 3241–3243 (2004). [CrossRef]
8. H. Cao, Y. G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang, and R. P. H. Chan, “Random laser action in semiconductor powder,” Phys. Rev. Lett. 82(11), 2278–2281 (1999). [CrossRef]
9. R. Q. Guo, J. Nishimura, M. Matsumoto, D. Nakamura, and T. Okada, “Catalyst-free synthesis of vertically-aligned ZnO nanowires by nanoparticle-assisted pulsed laser deposition,” Appl. Phys., A Mater. Sci. Process. 93(4), 843–847 (2008). [CrossRef]
10. L. K. van Vugt, S. Rühle, and D. Vanmaekelbergh, “Phase-correlated nondirectional laser emission from the end facets of a ZnO nanowire,” Nano Lett. 6(12), 2707–2711 (2006). [CrossRef] [PubMed]
11. C. Czekalla, T. Nobis, A. Rahm, B. Cao, J. Zúñiga-Pérez, C. Sturm, R. Schmidt-Grund, M. Lorenz, and M. Grundmann, “Whispering gallery modes in zinc oxide micro- and nanowires,” Phys. Status Solidi B 247(6), 1282–1293 (2010). [CrossRef]
12. X. Zhang, X. Zhang, J. Xu, X. Shan, J. Xu, D. Yu, M. Lorenz, and M. Grundmann, “Whispering gallery modes in single triangular ZnO nanorods,” Opt. Lett. 34(16), 2533–2535 (2009). [CrossRef] [PubMed]
13. L. Sun, Z. Chen, Q. Ren, K. Yu, W. Zhou, L. Bai, Z. Q. Zhu, and X. Shen, “Polarized photoluminescence study of whispering gallery mode polaritons in ZnO microcavity,” Phys. Status Solidi C 6(1), 133–136 (2009). [CrossRef]
14. K. Okazaki, T. Shimogaki, K. Fusazaki, M. Higashihata, D. Nakamura, N. Koshizaki, and T. Okada, “Ultraviolet whispering-gallery-mode lasing in ZnO micro/nano sphere crystal,” Appl. Phys. Lett. 101(21), 211105 (2012). [CrossRef]
15. N. H. Alvi, S. M. Usman Ali, S. Hussain, O. Nur, and M. Willander, “Fabrication and comparative optical characterization of n-ZnO nanostructures (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN white-light-emitting diodes,” Scr. Mater. 64(8), 697–700 (2011). [CrossRef]
16. R. Ghosh and D. Basak, “Electrical and ultraviolet photoresponse properties of quasialigned ZnO nanowires/ p-Si heterojunction,” Appl. Phys. Lett. 90(24), 243106 (2007). [CrossRef]
17. A. El-Shaer, A. Dev, J. P. Richters, S. R. Waldvogel, J. Waltermann, W. Schade, and T. Voss, “Hybrid LEDs based on ZnO-nanowire arrays,” Phys. Status Solidi B 247(6), 1564–1567 (2010). [CrossRef]
18. M. Rajalakshmi, A. K. Arora, B. S. Bendre, and S. Mahamuni, “Optical phonon confinement in zinc oxide nanoparticles,” J. Appl. Phys. 87(5), 2445–2448 (2000). [CrossRef]
19. A. Khan, “Raman spectroscopic study of the ZnO nanostructures,” J. Pak. Mater. Soc. 4, 5–9 (2010).
20. R. Guo, M. Matsumoto, T. Matsumoto, M. Higashihata, D. Nakamura, and T. Okada, “Aligned growth of ZnO nanowires by NAPLD and their optical characterizations,” Appl. Surf. Sci. 255(24), 9671–9675 (2009). [CrossRef]
21. R. Guo, J. Nishimura, M. Matsumoto, M. Higashihata, D. Nakamura, and T. Okada, “Electroluminescence from ZnO nanowire-based p-GaN/n-ZnO heterojunction light-emitting diodes,” Appl. Phys. B 94(1), 33–38 (2009). [CrossRef]
22. T. Shimogaki, K. Okazaki, D. Nakamura, M. Higashihata, T. Asano, and T. Okada, “Effect of laser annealing on photoluminescence properties of Phosphorus implanted ZnO nanorods,” Opt. Express 20(14), 15247–15252 (2012). [CrossRef] [PubMed]
23. 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]
24. 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]
25. S. Kishwar, K. Ul Hasan, N. H. Alvi, P. Klason, O. Nur, and M. Willander, “A comparative study of the electrodeposition and the aqueous chemical growth techniques for the utilization of ZnO nanorods on p-GaN for white light emitting diodes,” Superlattices Microstruct. 49(1), 32–42 (2011). [CrossRef]
