- Email: [email protected]
Surface plasmon enhanced ultraviolet light-emitting devices
Journal of Luminescence 134 (2013) 754–757
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Surface plasmon enhanced ultraviolet light-emitting devices Qian Qiao a,b, Chong-Xin Shan a,n, Jian Zheng a,b, Bing-Hui Li a, Zhen-Zhong Zhang a, De-Zhen Shen a a State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, No. 3888 Dongnanhu Road, Changchun 130033, People’s Republic of China b Graduate University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2012 Received in revised form 12 June 2012 Accepted 29 June 2012 Available online 7 July 2012
In this paper, n-ZnO/i-ZnO/MgO/p-GaN structured light-emitting devices have been designed and constructed, and Ag nanoparticles whose surface plasmon resonance absorption spectrum overlaps well with the electroluminescence (EL) of the structure were employed to improve the emission characteristics of the devices. A noticeable enhancement in the EL intensity has been obtained, and the enhancement can be attributed to the resonant coupling between the electron–hole pairs in the structure and the surface plasmons of the Ag nanoparticles. & 2012 Elsevier B.V. All rights reserved.
Keywords: Surface plasmons Ag nanoparticles Ultraviolet Light-emitting devices
1. Introduction Ultraviolet (UV) light-emitting devices (LEDs) hold the promise for a variety of applications, including solid state lighting, photolithography, high density data storage, biomedical analysis, etc[1]. Zinc oxide (ZnO), with a direct band gap of 3.37 eV and large exciton binding energy of 60 meV, is a promising candidate for the above-mentioned applications [2,3]. Recently, ZnO based homojunction and heterojunction structured UV LEDs have been reported [4–8]. Nevertheless, the performance of the LEDs is still far below expectation. Surface plasmons (SPs) are collective oscillations of electrons at interfaces between two materials where the real part of the dielectric function changes sign across the interface, and they can be coupled with excited electrons and holes to increase the emission efficiency of a luminescent material [9–13]. Recently, the output power of InGaN/GaN quantum well and Si quantum dots based LEDs has been considerably increased via the SP coupling [14–17]. However, the emissions of the InGaN and Si based LEDs are located in visible region, while only two reports on SPs enhanced electroluminescence (EL) of UV LEDs can be found to date [18,19], in which Ag NPs LSP enhanced ZnO-based heterojunction has been demonstrated. In this paper, Ag NPs were sandwiched by two MgO dielectric layers to reduce the energy diminishment of SPs, and the sandwiched Ag NPs were incorporated into n-ZnO/i-ZnO/MgO/p-GaN structured LEDs. Obvious UV emission at around 400 nm has been
n
Corresponding author. Tel./fax: þ 86 43186176298. E-mail address: [email protected] (C.-X. Shan).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.06.052
observed from the structures. It is found that the emission of the LEDs can be enhanced noticeably by the Ag NPs, and the enhancement has been attributed to the coupling of the Ag NPs confined surface plasmon modes with the injected carriers at the active layer of the structures.
2. Experimental To study SP enhanced ZnO based UV emissions, n-ZnO/i-ZnO/ MgO/p-GaN structures have been constructed, and Ag NPs sandwiched by two thin MgO layers were placed between the n-ZnO and i-ZnO layers. The ZnO and MgO films were deposited onto commercially available p-GaN/sapphire (0 0 0 1) templates in a VG V80H plasma-assisted molecular-beam epitaxy (MBE) system. Firstly, a 30 nm MgO layer, a 50 nm i-ZnO layer, and a 10 nm MgO layer were deposited onto the p-GaN templates in sequence at 650 1C. For the incorporation of Ag NPs into the structure, the sample was removed from the MBE chamber, and colloidal Ag NPs were spin-coated onto the structure. The colloidal Ag NPs were prepared by chemical reduction of silver nitrate in aqueous solution employing sodium borohydride in the presence of sodium citrate as a stabilizer [20]. Then, the sample was reloaded into the MBE chamber; another 10 nm MgO layer and a 220 nm n-ZnO layer were grown in sequence onto the sample at 450 1C. Finally, bilayer Ni/Au and Ti/Au electrodes were deposited onto the p-GaN and n-ZnO layers by vacuum evaporation. For comparison, another device was also fabricated under the same conditions except that no Ag NPs were incorporated.
Q. Qiao et al. / Journal of Luminescence 134 (2013) 754–757
755
The morphology of the Ag NPs was characterized by scanning electron microscopy (SEM) using a Hitachi S4800 microscope. The optical absorption spectra of the samples were recorded in a Shimadzu UV-3101PC spectrometer. The electrical characteristics of the devices were measured by a Lakeshore 7707 Hall measurement system. EL measurements were carried out in a Hitachi F4500 spectrometer, and a continuous-current power source was used to excite the devices. Note that all the measurements were performed at room temperature.
3. Results and discussions Fig. 1 shows a schematic diagram of the Ag NPs incorporated n-ZnO/i-ZnO/MgO/p-GaN structure. The thickness of the p-GaN layer is around 2 mm, and the hole concentration and Hall mobility of this layer are about 5.0 1017 cm 3 and 22.0 cm2 V 1 s 1, respectively. The p-GaN layer acts as the holeinjection layer in the structure. The n-ZnO layer with an electron concentration of 5.0 1018 cm 3 and a mobility of 16.0 cm2 V 1 s 1 is employed as an electron source. The i-ZnO layer with an electron concentration of 3.0 1017 cm 3 and a Hall mobility of 28.0 cm2 V 1 s 1 is employed as the active layer of the device. The carrier transportation process in n-ZnO/i-ZnO/ MgO/p-GaN structure has been elucidated elsewhere [19,21–23]. Ag NPs were employed as the source of SPs for the structure. One notable character of surface plasmon lies in the fact that the electron oscillations are mainly focused at the near surface of the metals, and their intensity diminishes greatly with distance from the metals. To obtain a strong resonant coupling between the surface plasmon of Ag NPs and the carriers in the structure, the Ag NPs layer was inserted between the n-ZnO and i-ZnO layer. In addition, a major disadvantage of metal plasmon is the large ohmic loss, which is one of the major hurdles that hinder the application of surface plasmons. Recently, some groups have demonstrated that the ohmic losses can be significantly reduced by inserting a dielectric layer with lower refractive index between the metal and semiconductor [24–27]. Herein, the MgO dielectric layer beside the NPs is used to reduce the ohmic loss of the Ag NPs[28]. Fig. 2(a) shows a SEM image of the Ag NPs spin-coated onto a MgO/ZnO surface. Ag NPs with a diameter of 10–20 nm have been dispersed onto the film randomly, as indicated by the white spots in the image. In order to determine the extinction spectrum of the Ag NPs, the absorption spectra of the MgO/ZnO/MgO/sapphire film with and without the Ag NPs are shown in Fig. 2(b). It is found that the ZnO film exhibits strong absorption in the spectrum region shorter than 380 nm, and is almost transparent in the range from 390 nm to 800 nm. Compared with the
Fig. 1. Schematic diagram of SPs enhanced n-ZnO/i-ZnO/MgO/p-GaN structured LEDs.
Fig. 2. (a) SEM images of the Ag NPs deposited onto the MgO/ZnO/MgO/sapphire film and (b) the absorption spectra of the MgO/ZnO/MgO/sapphire films with and without of Ag NPs, and the inset shows the SP resonance absorption spectrum of the Ag NPs obtained by subtracting the absorption spectrum of the structure with Ag NPs to that of the structure without Ag NPs.
ZnO film, the absorption of the ZnO film with Ag NPs shows a similar absorption edge, but exhibits an extra band at around 400 nm. The differential absorption spectrum of the Ag NPs obtained by subtracting the spectrum of the sample without Ag NPs from that of the sample with Ag NPs is shown in the inset of Fig. 2(b). The spectrum exhibits a peak at around 400 nm, which can be attributed to the absorption by the excitation of dipole plasmon modes in the Ag NPs. To investigate the optical properties of the SPs enhanced n-ZnO/i-ZnO/MgO/p-GaN LEDs, the EL spectra of the structure without and with Ag NPs have been measured, as shown in Fig. 3(a), and a typical photograph of the devices under the injection of continuous current is shown in Fig. 3(b). Obvious emission is visible from the image. From Fig. 3(a) one can find that the EL spectrum of the device without Ag NPs exhibits an emission peak at around 400 nm, which has been frequently observed in ZnO-based LEDs, and it can be attributed to the near-band-edge emission of ZnO. The carrier transportation process and emission mechanism of the n-ZnO/i-ZnO/MgO/ p-GaN LEDs have been detailed before [22,23]. Note that the EL spectrum overlaps well with the SP resonance absorption spectrum of the Ag NPs shown in the inset of Fig. 2(b), which promises that the emission of the n-ZnO/i-ZnO/MgO/p-GaN structure may be enhanced by the SPs of the Ag NPs. Actually, the EL intensity of the structure with Ag NPs is obviously larger than that of the structure without Ag NPs, but the lineshape of the spectrum changes little, as shown in Fig. 3(a). It is accepted that the emission of a LED may also be varied by altering the carrier transportation in the structure. To clarify the origin of the enhancement, the current–voltage (I–V) characteristics of the devices without and with Ag NPs were measured, as shown in Fig. 4. One can see that these two curves have very similar shapes, indicating that the carrier transportation in the nZnO/i-ZnO/MgO/p-GaN structure has not been altered much by the Ag NPs; thus the possibility that the enhancement is caused by the change in carrier transportation can be excluded. It is thus
756
Q. Qiao et al. / Journal of Luminescence 134 (2013) 754–757
Fig. 3. (a) EL spectra of the n-ZnO/i-ZnO/MgO/p-GaN structured devices with and without Ag NPs, and the inset shows the EL enhancement ratio of the devices as a function of wavelength. (b) Digital photograph of the device under the injection of continuous current.
the electron–hole pairs being captured by nanoradiative centers can be decreased [12,18,29,30]. As a result, the emission of the devices can be enhanced significantly. In summary, by incorporating Ag NPs into n-ZnO/i-ZnO/MgO/ p-GaN structures, the emission of the UV LEDs has been enhanced noticeably, and the enhancement can be attributed to the coupling between the electron–hole pairs in i-ZnO layer and the SP modes of the Ag NPs. The results reported in this paper may promise a route to enhanced performance of UV LEDs using SPs of metal NPs.
Acknowledgments This work is financially supported by the National Basic Research Program of China (2011CB302005), the National Natural Science Foundation of China (21101146, 11104265, and 61177040), and the Science and Technology Developing Project of Jilin Province (20111801). Fig. 4. I–V characteristics of the n-ZnO/i-ZnO/MgO/p-GaN structures with and without Ag NPs.
References
speculated that the enhancement may come from the coupling of the Ag NPs surface plasmon modes with the injected carriers at the active layer of the structures. To confirm this speculation, the EL intensity ratio obtained by dividing the emission intensity of the structure with Ag NPs with that of the structure without Ag NPs as a function of wavelength is shown in the inset of Fig. 3. The intensity ratio shows a very similar shape to that of the SP resonance absorption spectrum of the Ag NPs shown in the inset of Fig. 2(b), which confirms that the enhancement is caused by the SPs of the Ag NPs. The mechanism for the enhancement can be elucidated as follows [19]: electron–hole pairs in the active layer of the structure, which are injected by the external circuit, can couple with the electron oscillation of the Ag NPs. The SP modes can then be extracted as light from the surface of MgO and n-ZnO layer. Because the SP coupling rate is much faster than the recombination rate of the electron–hole pairs, the possibility of
[1] A. Sandhu, Nat. Photon 1 (2007) 38. [2] U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.J. Cho, H. Morkoc, J. Appl. Phys. 98 (2005) 041301. [3] U. Ozgur, D. Hofstetter, H. Morkoc- , Proc. IEEE 98 (2010) 1255. [4] Z.P. Wei, Y.M. Lu, D.Z. Shen, Z.Z. Zhang, B. Yao, B.H. Li, J.Y. Zhang, D.X. Zhao, X.W. Fan, Z.K. Tang, Appl. Phys. Lett. 90 (2007) 042113. [5] J. Kong, S. Chu, M. Olmedo, L. Li, Z. Yang, J. Liu, Appl. Phys. Lett. 93 (2008) 132113. [6] Y. Yang, X.W. Sun, B.K. Tay, G.F. You, S.T. Tan, K.L. Teo, Appl. Phys. Lett. 93 (2008) 253107. [7] O. Lupan, T. Pauporte´, B. Viana, Adv. Mater. 22 (2010) 3298. [8] J. Sun, Q. Feng, J. Bian, D. Yu, M. Li, C. Li, H. Liang, J. Zhao, H. Qiu, G. Du, J. Lumin. 131 (2011) 825. [9] S.A. Maier, Plasmonics: Fundamentals and Applications, Springer, New York, 2007. [10] M.L. Brongersma, P.G. Kik, Surface Plasmon Nanophotonics, Springer Dordrecht, The Netherlands, 2007. [11] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107 (2002) 668. [12] K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, A. Scherer, Nat. Mater. 3 (2004) 601. [13] C.W. Lai, J. An, H.C. Ong, Appl. Phys. Lett. 86 (2005) 251105.
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[14] M.K. Kwon, J.Y. Kim, B.H. Kim, I.K. Park, C.Y. Cho, C.C. Byeon, S.J. Park, Adv. Mater. 20 (2008) 1253. [15] C.Y. Cho, M.K. Kwon, S.J. Lee, S.H. Han, J.W. Kang, S.E. Kang, D.Y. Lee, S.J. Park, Nanotechnology 21 (2010) 205201. [16] C.Y. Cho, S.J. Lee, J.H. Song, S.H. Hong, S.M. Lee, Y.H. Cho, S.J. Park, Appl. Phys. Lett. 98 (2011) 051106. [17] B.H. Kim, C.H. Cho, J.S. Mun, M.K. Kwon, T.Y. Park, J.S. Kim, C.C. Byeon, J. Lee, S.J. Park, Adv. Mater. 20 (2008) 3100. [18] S.G. Zhang, X.W. Zhang, Z.G. Yin, J.X. Wang, J.J. Dong, H.L. Gao, F.T. Si, S.S. Sun, Y. Tao, Appl. Phys. Lett. 99 (2011) 181116. [19] Q. Qiao, C.X. Shan, J. Zheng, B.H. Li, Z.Z. Zhang, L.G. Zhang, D.Z. Shen, J. Mater. Chem. 22 (2012) 9481. [20] L. Mulfinger, S.D. Solomon, M. Bahadory, A.V. Jeyarajasingam, S.A. Rutkowsky, C. Boritz, J. Chem. Educ. 84 (2007) 322. [21] S.J. Jiao, Y.M. Lu, D.Z. Shen, Z.Z. Zhang, B.H. Li, J.Y. Zhang, B. Yao, Y.C. Liu, X.W. Fan, Phys. Status Solidi C 3 (2006) 972.
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Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Surface plasmon enhanced ultraviolet light-emitting devices Qian Qiao a,b, Chong-Xin Shan a,n, Jian Zheng a,b, Bing-Hui Li a, Zhen-Zhong Zhang a, De-Zhen Shen a a State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, No. 3888 Dongnanhu Road, Changchun 130033, People’s Republic of China b Graduate University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2012 Received in revised form 12 June 2012 Accepted 29 June 2012 Available online 7 July 2012
In this paper, n-ZnO/i-ZnO/MgO/p-GaN structured light-emitting devices have been designed and constructed, and Ag nanoparticles whose surface plasmon resonance absorption spectrum overlaps well with the electroluminescence (EL) of the structure were employed to improve the emission characteristics of the devices. A noticeable enhancement in the EL intensity has been obtained, and the enhancement can be attributed to the resonant coupling between the electron–hole pairs in the structure and the surface plasmons of the Ag nanoparticles. & 2012 Elsevier B.V. All rights reserved.
Keywords: Surface plasmons Ag nanoparticles Ultraviolet Light-emitting devices
1. Introduction Ultraviolet (UV) light-emitting devices (LEDs) hold the promise for a variety of applications, including solid state lighting, photolithography, high density data storage, biomedical analysis, etc[1]. Zinc oxide (ZnO), with a direct band gap of 3.37 eV and large exciton binding energy of 60 meV, is a promising candidate for the above-mentioned applications [2,3]. Recently, ZnO based homojunction and heterojunction structured UV LEDs have been reported [4–8]. Nevertheless, the performance of the LEDs is still far below expectation. Surface plasmons (SPs) are collective oscillations of electrons at interfaces between two materials where the real part of the dielectric function changes sign across the interface, and they can be coupled with excited electrons and holes to increase the emission efficiency of a luminescent material [9–13]. Recently, the output power of InGaN/GaN quantum well and Si quantum dots based LEDs has been considerably increased via the SP coupling [14–17]. However, the emissions of the InGaN and Si based LEDs are located in visible region, while only two reports on SPs enhanced electroluminescence (EL) of UV LEDs can be found to date [18,19], in which Ag NPs LSP enhanced ZnO-based heterojunction has been demonstrated. In this paper, Ag NPs were sandwiched by two MgO dielectric layers to reduce the energy diminishment of SPs, and the sandwiched Ag NPs were incorporated into n-ZnO/i-ZnO/MgO/p-GaN structured LEDs. Obvious UV emission at around 400 nm has been
n
Corresponding author. Tel./fax: þ 86 43186176298. E-mail address: [email protected] (C.-X. Shan).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.06.052
observed from the structures. It is found that the emission of the LEDs can be enhanced noticeably by the Ag NPs, and the enhancement has been attributed to the coupling of the Ag NPs confined surface plasmon modes with the injected carriers at the active layer of the structures.
2. Experimental To study SP enhanced ZnO based UV emissions, n-ZnO/i-ZnO/ MgO/p-GaN structures have been constructed, and Ag NPs sandwiched by two thin MgO layers were placed between the n-ZnO and i-ZnO layers. The ZnO and MgO films were deposited onto commercially available p-GaN/sapphire (0 0 0 1) templates in a VG V80H plasma-assisted molecular-beam epitaxy (MBE) system. Firstly, a 30 nm MgO layer, a 50 nm i-ZnO layer, and a 10 nm MgO layer were deposited onto the p-GaN templates in sequence at 650 1C. For the incorporation of Ag NPs into the structure, the sample was removed from the MBE chamber, and colloidal Ag NPs were spin-coated onto the structure. The colloidal Ag NPs were prepared by chemical reduction of silver nitrate in aqueous solution employing sodium borohydride in the presence of sodium citrate as a stabilizer [20]. Then, the sample was reloaded into the MBE chamber; another 10 nm MgO layer and a 220 nm n-ZnO layer were grown in sequence onto the sample at 450 1C. Finally, bilayer Ni/Au and Ti/Au electrodes were deposited onto the p-GaN and n-ZnO layers by vacuum evaporation. For comparison, another device was also fabricated under the same conditions except that no Ag NPs were incorporated.
Q. Qiao et al. / Journal of Luminescence 134 (2013) 754–757
755
The morphology of the Ag NPs was characterized by scanning electron microscopy (SEM) using a Hitachi S4800 microscope. The optical absorption spectra of the samples were recorded in a Shimadzu UV-3101PC spectrometer. The electrical characteristics of the devices were measured by a Lakeshore 7707 Hall measurement system. EL measurements were carried out in a Hitachi F4500 spectrometer, and a continuous-current power source was used to excite the devices. Note that all the measurements were performed at room temperature.
3. Results and discussions Fig. 1 shows a schematic diagram of the Ag NPs incorporated n-ZnO/i-ZnO/MgO/p-GaN structure. The thickness of the p-GaN layer is around 2 mm, and the hole concentration and Hall mobility of this layer are about 5.0 1017 cm 3 and 22.0 cm2 V 1 s 1, respectively. The p-GaN layer acts as the holeinjection layer in the structure. The n-ZnO layer with an electron concentration of 5.0 1018 cm 3 and a mobility of 16.0 cm2 V 1 s 1 is employed as an electron source. The i-ZnO layer with an electron concentration of 3.0 1017 cm 3 and a Hall mobility of 28.0 cm2 V 1 s 1 is employed as the active layer of the device. The carrier transportation process in n-ZnO/i-ZnO/ MgO/p-GaN structure has been elucidated elsewhere [19,21–23]. Ag NPs were employed as the source of SPs for the structure. One notable character of surface plasmon lies in the fact that the electron oscillations are mainly focused at the near surface of the metals, and their intensity diminishes greatly with distance from the metals. To obtain a strong resonant coupling between the surface plasmon of Ag NPs and the carriers in the structure, the Ag NPs layer was inserted between the n-ZnO and i-ZnO layer. In addition, a major disadvantage of metal plasmon is the large ohmic loss, which is one of the major hurdles that hinder the application of surface plasmons. Recently, some groups have demonstrated that the ohmic losses can be significantly reduced by inserting a dielectric layer with lower refractive index between the metal and semiconductor [24–27]. Herein, the MgO dielectric layer beside the NPs is used to reduce the ohmic loss of the Ag NPs[28]. Fig. 2(a) shows a SEM image of the Ag NPs spin-coated onto a MgO/ZnO surface. Ag NPs with a diameter of 10–20 nm have been dispersed onto the film randomly, as indicated by the white spots in the image. In order to determine the extinction spectrum of the Ag NPs, the absorption spectra of the MgO/ZnO/MgO/sapphire film with and without the Ag NPs are shown in Fig. 2(b). It is found that the ZnO film exhibits strong absorption in the spectrum region shorter than 380 nm, and is almost transparent in the range from 390 nm to 800 nm. Compared with the
Fig. 1. Schematic diagram of SPs enhanced n-ZnO/i-ZnO/MgO/p-GaN structured LEDs.
Fig. 2. (a) SEM images of the Ag NPs deposited onto the MgO/ZnO/MgO/sapphire film and (b) the absorption spectra of the MgO/ZnO/MgO/sapphire films with and without of Ag NPs, and the inset shows the SP resonance absorption spectrum of the Ag NPs obtained by subtracting the absorption spectrum of the structure with Ag NPs to that of the structure without Ag NPs.
ZnO film, the absorption of the ZnO film with Ag NPs shows a similar absorption edge, but exhibits an extra band at around 400 nm. The differential absorption spectrum of the Ag NPs obtained by subtracting the spectrum of the sample without Ag NPs from that of the sample with Ag NPs is shown in the inset of Fig. 2(b). The spectrum exhibits a peak at around 400 nm, which can be attributed to the absorption by the excitation of dipole plasmon modes in the Ag NPs. To investigate the optical properties of the SPs enhanced n-ZnO/i-ZnO/MgO/p-GaN LEDs, the EL spectra of the structure without and with Ag NPs have been measured, as shown in Fig. 3(a), and a typical photograph of the devices under the injection of continuous current is shown in Fig. 3(b). Obvious emission is visible from the image. From Fig. 3(a) one can find that the EL spectrum of the device without Ag NPs exhibits an emission peak at around 400 nm, which has been frequently observed in ZnO-based LEDs, and it can be attributed to the near-band-edge emission of ZnO. The carrier transportation process and emission mechanism of the n-ZnO/i-ZnO/MgO/ p-GaN LEDs have been detailed before [22,23]. Note that the EL spectrum overlaps well with the SP resonance absorption spectrum of the Ag NPs shown in the inset of Fig. 2(b), which promises that the emission of the n-ZnO/i-ZnO/MgO/p-GaN structure may be enhanced by the SPs of the Ag NPs. Actually, the EL intensity of the structure with Ag NPs is obviously larger than that of the structure without Ag NPs, but the lineshape of the spectrum changes little, as shown in Fig. 3(a). It is accepted that the emission of a LED may also be varied by altering the carrier transportation in the structure. To clarify the origin of the enhancement, the current–voltage (I–V) characteristics of the devices without and with Ag NPs were measured, as shown in Fig. 4. One can see that these two curves have very similar shapes, indicating that the carrier transportation in the nZnO/i-ZnO/MgO/p-GaN structure has not been altered much by the Ag NPs; thus the possibility that the enhancement is caused by the change in carrier transportation can be excluded. It is thus
756
Q. Qiao et al. / Journal of Luminescence 134 (2013) 754–757
Fig. 3. (a) EL spectra of the n-ZnO/i-ZnO/MgO/p-GaN structured devices with and without Ag NPs, and the inset shows the EL enhancement ratio of the devices as a function of wavelength. (b) Digital photograph of the device under the injection of continuous current.
the electron–hole pairs being captured by nanoradiative centers can be decreased [12,18,29,30]. As a result, the emission of the devices can be enhanced significantly. In summary, by incorporating Ag NPs into n-ZnO/i-ZnO/MgO/ p-GaN structures, the emission of the UV LEDs has been enhanced noticeably, and the enhancement can be attributed to the coupling between the electron–hole pairs in i-ZnO layer and the SP modes of the Ag NPs. The results reported in this paper may promise a route to enhanced performance of UV LEDs using SPs of metal NPs.
Acknowledgments This work is financially supported by the National Basic Research Program of China (2011CB302005), the National Natural Science Foundation of China (21101146, 11104265, and 61177040), and the Science and Technology Developing Project of Jilin Province (20111801). Fig. 4. I–V characteristics of the n-ZnO/i-ZnO/MgO/p-GaN structures with and without Ag NPs.
References
speculated that the enhancement may come from the coupling of the Ag NPs surface plasmon modes with the injected carriers at the active layer of the structures. To confirm this speculation, the EL intensity ratio obtained by dividing the emission intensity of the structure with Ag NPs with that of the structure without Ag NPs as a function of wavelength is shown in the inset of Fig. 3. The intensity ratio shows a very similar shape to that of the SP resonance absorption spectrum of the Ag NPs shown in the inset of Fig. 2(b), which confirms that the enhancement is caused by the SPs of the Ag NPs. The mechanism for the enhancement can be elucidated as follows [19]: electron–hole pairs in the active layer of the structure, which are injected by the external circuit, can couple with the electron oscillation of the Ag NPs. The SP modes can then be extracted as light from the surface of MgO and n-ZnO layer. Because the SP coupling rate is much faster than the recombination rate of the electron–hole pairs, the possibility of
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Q. Qiao et al. / Journal of Luminescence 134 (2013) 754–757
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