- Email: [email protected]
Fabricating ZnO single microwire light-emitting diode with transparent conductive ITO film
Journal of Luminescence 149 (2014) 313–316
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Fabricating ZnO single microwire light-emitting diode with transparent conductive ITO film Yingtian Xu a, Jun Dai b, Zhifeng Shi a, Beihong Long c, Bin Wu a, Xupu Caia, Xianwei Chu a, Guotong Du a, Baolin Zhang a, Jingzhi Yin a,n a State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People's Republic of China b State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, People's Republic of China c College of Materials Science and Engineering, Jinlin University, 2699 Qianjin Street, Changchun 130012, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 July 2013 Received in revised form 18 January 2014 Accepted 19 January 2014 Available online 25 January 2014
In this paper, n-ZnO single microwire/p þ -Si heterojunction LEDs are fabricated using the transparent conductive ITO film as an electrode. A distinct UV emission resulting from free exciton recombination in a ZnO single microwire is observed in the electroluminescence. Size difference of ZnO single microwire shows significant influence on emission efficiency. The EL spectra of n-ZnO single microwire/p-Si heterostructure exhibited relatively stronger UV emission which was compared with the EL spectra of n-ZnO single nanowire/p-Si heterostructure and n-ZnO film/p-Si heterostructure, respectively. & 2014 Elsevier B.V. All rights reserved.
Keywords: ZnO single microwire Electroluminescence Transparent conductive film
1. Introduction Because of the excellent properties such as passive waveguides, resonance cavities and gain media, a one-dimensional semiconductor microwire has received considerable attention in constructing optoelectronic devices [1–5]. Semiconductor microwire structures are recently emerging as prospective candidates for even further miniaturization [6–8]. ZnO is a semiconductor material with large band-gap (Eg ¼ 3.37 eV at 300 K) and high exciton binding energy (60 meV). It is, therefore, recognized as a high efficiency emitter for ultraviolet light-emitting devices (UV-LEDs) [9–11]. In particular, the high exciton binding energy can cause a strong Coulomb interaction between electrons and holes. Electroluminescence from n-ZnO single microwire/p-GaN and n-ZnO single nanowire/p-Si heterojunctions has been reported recently [12,13]. However, ZnO single microwire based p-Si optoelectronic devices have been less reported so far. Because ZnO-based LEDs could be compatible with the silicon micro-electronic technology, the research on ZnO/Si heterostructure is very significant. In this paper, we report electroluminescence from n-ZnO single microwire/p þ -Si heterojunction. The transparent conductive ITO film is deposited as a cathode by magnetron sputtering and this kind of electrode has better light transmissivity than a metal electrode.
n
Corresponding author. Tel.: þ 86 431 85168359; fax: þ 86 431851 68270. E-mail address: [email protected] (J. Yin).
0022-2313/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2014.01.056
The current–voltage (I–V) curves exhibited a typical rectification characteristic. Both ultraviolet (UV) and visible emissions could be detected in the electroluminescence (EL) measurement. Furthermore, the result had been compared with the EL spectra of n-ZnO film/p-Si heterostructure and n-ZnO single nanowire/p-Si heterostructure, respectively, and an improved UV emission was realized. 2. Experiments The ZnO microwires were synthesized with a vapor phase transport method. One single ZnO microwire can form a hexagonal resonant cavity and faces act as reflecting mirrors [14]. Fig. 1 illustrates a scanning electron microscope (SEM) image of ZnO microwires. The procedure for fabricating n-ZnO single microwire/ p þ -Si heterojunction is shown in Fig. 2(a). The ZnO microwires were first randomly dispersed on a p-Si substrate. A single ZnO microwire was adsorbed by a fiber moistened with alcohol and then transferred to a heavily doped p-Si substrate. Thereafter, the sample was annealed at 500 1C in nitrogen ambient for 1 h to evaporate the air and moisture which helps to form a good contact between the ZnO microwire and the substrate. A thin film of poly (methyl methacrylate) (PMMA) was spin-coated on the ZnO microwire to prevent the shorting of connection between the top and the bottom electrodes in the two-terminal devices. Following this, reactive ion etching (RIE) was used to remove the top surface of the ZnO single microwire for the electrode. Finally,
314
Y. Xu et al. / Journal of Luminescence 149 (2014) 313–316
Fig. 1. SEM image of ZnO microwires. Fig. 3. Current–voltage (I–V) curves of sample A and sample B.
Fig. 4. Electroluminescence spectra of sample A and sample B under forward bias current of 15 mA.
3. Results and discussion
Fig. 2. (a) The procedure for fabricating n-ZnO single microwire/p þ -Si heterojunction. (b) OM image of n-ZnO single microwire/p þ -Si heterojunction.
transparent conductive ITO film and Al thin-film were deposited on the ZnO single microwire and p þ -Si substrate by magnetron sputtering and electron beam evaporation as electrodes, respectively. The sample was then annealed at 300 1C in nitrogen ambient for 3 min to form the ohmic contact. An optical microscope (OM) image of the as-fabricated n-ZnO single microwire/ p þ -Si heterojunction is shown in Fig. 2(b).
The current–voltage (I–V) curves of samples with different diameters and lengths of the n-ZnO single microwire (sample A: length 192 mm, diameter 21 mm; sample B: length 90 mm, diameter 18 mm) are illustrated in Fig. 3, which shows non-ideal p–n junction I–V characteristics. This is because the ZnO single microwire is transferred to p-Si by a fiber and p–n junctions formed under the condition of annealing. In contrast to standard epitaxy grown p–n junctions, covalent chemical bonds between ZnO microwire and Si are not formed and van der Waals forces are generated in the interface region by a mechanical contact [7]. The apparent variation between the two samples could be due to the size differences of ZnO single microwires, as well as the different contact areas [15,16]. Fig. 4 distinctly shows the electroluminescence spectra of sample A and sample B. The EL spectra of these two samples are fed with the same forward bias current of 15 mA. Due to the different diameters, lengths and different contact areas of sample A
Y. Xu et al. / Journal of Luminescence 149 (2014) 313–316
Fig. 5. (a) Electroluminescence spectra for sample A under various forward bias currents. (b) The intensity ratio of UV/DLE versus injection current.
and sample B, the prominent differences of light emission are observed. The emitting efficiency of sample A is much higher than that of sample B, wherever light emission is in the range of ultraviolet to visible. The result also exhibits relatively strong UV emission and weak deep-level emission (DLE) which is compared with the EL spectra of n-ZnO single nanowire/p-Si heterostructure LEDs reported by Bao et al. [17]. This can be explained that the emission intensity strongly depends on the out-coupling efficiency, which is influenced by nonuniformity along the microwire length and the size of ZnO wire. The room-temperature electroluminescence spectra for sample A under a continuous current injection are presented in Fig. 5(a). The EL spectra show that there is no UV emission peak when the forward bias current is 5 mA. However, with increasing forward bias current at 15 mA and 20 mA, a UV emission peak appears at around 400 nm. When the forward bias current reaches 25 mA, an intense UV emission peak has been detected. However, the peak appeared to be blue-shifted. The UV emission peak shift is related to polarization effects [18]. Attributing the hexagonal wurtzite structure of ZnO single microwire and lattice array lacks reversal symmetry; spontaneous polarization is generated in the material. However, spontaneous polarization will reduce because the device
315
used a nonpolar face of ZnO contacting with p-Si. In addition, piezoelectric polarization will be generated in the process of annealing. The device was annealed at 500 1C in nitrogen ambient for 1 h to form a good contact between ZnO single microwire and Si. In this annealing process, mismatch of lattice constant between ZnO single microwire and Si will generate a strain in the interface. The strain between ZnO single microwire and Si will lead to the phenomenon of piezoelectric polarization. A combined effect of spontaneous polarization and piezoelectric polarization causes an intense built-in electric field in the p–n junction. Because the relaxation time of carrier in conduction band is much shorter than carrier lifetime, a large number of free carriers are gathered in the p–n junction under the influence of this built-in electric field, which is generated by the external current injection. This result leads to ground state rising. According to the well-known Burstein–Moss effect, also called the bandgap widening effect, the UV emission peak will show blue-shift for short wavelength [19]. Furthermore, the EL spectra present a weak UV emission peak accompanying a dominated DLE located around 500 nm. In accordance with foregone research works, the UV emission has been attributed to excitonic recombination, and the DEL originates from defects and surface states. Fig. 5(b) shows the intensity ratio of UV/ DLE as a function of injection current. The ratio of UV/DLE increases approximately linearly with increasing forward bias currents, indicating a high UV light extraction efficiency for the n-ZnO single microwire/p þ -Si heterojunction LEDs. For the reason that the surface of p þ -Si substrate has been oxidized by oxygen molecules, a thin SiO2 film was formed between the p–n junctions [20]. Therefore, this device structure can be identified as n-ZnO single microwire / p þ -Si heterojunction with a SiO2 barrier layer. The EL spectra of sample A exhibit relatively strong UV emission compared with the EL spectra of nZnO film/p-Si heterostructures experimented by You et al. [21]. This phenomenon could explain that the ZnO single microwire has better crystallinity than that of ZnO film and the SiO2 barrier layer could prevent the electrons transport. The inclusion of SiO2 barrier layer will restrict the movement of electrons from n-ZnO to p-Si. This is because upon applying forward bias to n-ZnO single microwire/SiO2/p-Si heterojunction, the energy band of p-Si would move more downwards than that of n-ZnO, near the ZnO/ SiO2 and SiO2/Si interfaces. EV (Si) would move downwards to EV (ZnO) gradually. As a result, the electrons will not be allowed to flow actively between n-ZnO and p-Si, due to high conduction band offsets. However, the holes at the SiO2/p-Si interface can move to n-ZnO over threshold voltage, because the valence band offsets are relatively low. Therefore, it is beneficial to realize electron–hole recombination in ZnO area effectively.
4. Conclusion In summary, we have demonstrated a method of using the transparent conductive ITO film to achieve reliable electrical injection into the ZnO single microwire, and fabrication of n-ZnO single microwire/p þ -Si heterojunction LEDs. A distinct UV emission around 380–400 nm from the ZnO single microwire was obtained. In addition, the EL spectra of two samples, which are constructed by the ZnO single microwire with different sizes, are compared with ZnO single nanowire LEDs. Size difference of ZnO single wire has great influence on emitting efficiency. Furthermore, the EL spectra of ZnO single microwire exhibited relatively strong UV emission which was compared with that of n-ZnO film/ p-Si heterostructures. These results propose a potential application of ZnO microwire in micro-optoelectronics.
316
Y. Xu et al. / Journal of Luminescence 149 (2014) 313–316
Acknowledgments We thank Professor Chunxiang Xu of Southeast University. He provided the ZnO microwires. This work was supported by Natural Basic Research Program of China (973 Program) No. 2011CB302005 and by Natural Science Foundation of China Contract nos. 61076046 and 6174023. References [1] H.X. Dong, Y. Liu, J. Lu, Z.H. Chen, J. Wang, L. Zhang, J. Mater. Chem. C 1 (2013) 202. [2] Q. Zhang, J.J. Qi, X. Li, F. Yi, Z.Z. Wang, Y. Zhang, Appl. Phys. Lett. 101 (2012) 043119. [3] M. Ding, D.X. Zhao, B. YaoS.L. EZ. Guo, L.G. Zhang, D.Z. Shen, Opt. Express 20 (2012) 13657. [4] Z.F. Shi, L. Zhao, X.C. Xia, W. Zhao, H. Wang, J. Wang, X. Dong, B.L. Zhang, G.T. Du, J. Lumin. 131 (2011) 1645. [5] G.Y. Chai, L. Chow, O. Lupan, E. Rusu, G.I. Stratan, H. Heinrich, V.V. Ursaki, I.M. Tiginyanu, Solid State Sci. 13 (2011) 1205. [6] D. Vanmaekelbergh, B. Vugt, Nanoscale 3 (2011) 2783. [7] M.A. Zimmler, J.M. Bao, I. Shalish, W. Yi, J. Yoon, V. Narayanamurti, F. Capasso, Nanotechnology 18 (2007) 235205.
[8] C. Czekalla, J. Lenzner, A. Rahm, T. Nobis, M. Grundmann, Superlattice Microstruct. 41 (2007) 347. [9] Z.F. Shi, X.C. Xia, W. Yin, S.K. Zhang, H. Wang, J. Wang, L. Zhao, X. Dong, B.L. Zhang, G.T. Du, Appl. Phys. Lett. 100 (2012) 101112. [10] G.T. Du, W. Zhao, G.G. Wu, Z.F. Shi, X.C. Xia, Y. Liu, H.W. Liang, X. Dong, Y. Ma, B.L. Zhang, Appl. Phys. Lett. 101 (2012) 053503. [11] S. Ruhle, L.K. van Vugt, H.Y. Li, N.A. Keizer, L. Kuipers, D. Vanmaekelbergh, Nano Lett. 8 (2008) 119. [12] J. Dai, C.X. Xu, X.W. Sun, Adv. Mater. 23 (2011) 4115. [13] W.Q. Yang, H.B. Huo, L. Dai, R.M. Ma, S.F. Liu, G.Z. Ran, B. Shen, C.L. Lin, G.G. Qin, Nanotechnology 17 (2006) 4868. [14] G.P. Zhu, C.X. Xu, J. Zhu, C.G. Lv, Y.P. Cui, Appl. Phys. Lett. 94 (2009) 051106. [15] J.D. Ye, S.L. Gu, S.M. Zhu, W. Liu, S.M. Liu, R. Zhang, Y. Shi, Y.D. Zheng, Appl. Phys. Lett. 88 (2006) 182112. [16] 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, K.L. Teo, Appl. Phys. Lett. 93 (2008) 013506. [17] J.M. Bao, M.A. Zimmler, F. Capasso, X.W. Wang, Z.F. Ren, Nano Lett. 6 (2006) 1719. [18] Y.F. Hu, Y. Zhang, Y.L. Chang, R.L. Snyder, Z.L. Wang, ACS Nano 4 (2010) 4220. [19] M.T. Chen, M.P. Lu, Y.J. Wu, J.H. Song, C.Y. Lee, M.Y. Lu, Y.C. Chang, L.J. Chou, Z.L. Wang, L.J. Chen, Nano lett. 10 (2010) 4387. [20] J. Ahn, M.A. Mastro, P.B. Klein, J.K. Hite, B. Feigelson, C.R. Eddy, J. Kim, Opt. Express 19 (2011) 21692. [21] J.B. You, X.W. Zhang, S.G. Zhang, H.R. Tan, J. Ying, J. Appl. Phys. 107 (2010) 083701.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Fabricating ZnO single microwire light-emitting diode with transparent conductive ITO film Yingtian Xu a, Jun Dai b, Zhifeng Shi a, Beihong Long c, Bin Wu a, Xupu Caia, Xianwei Chu a, Guotong Du a, Baolin Zhang a, Jingzhi Yin a,n a State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People's Republic of China b State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, People's Republic of China c College of Materials Science and Engineering, Jinlin University, 2699 Qianjin Street, Changchun 130012, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 July 2013 Received in revised form 18 January 2014 Accepted 19 January 2014 Available online 25 January 2014
In this paper, n-ZnO single microwire/p þ -Si heterojunction LEDs are fabricated using the transparent conductive ITO film as an electrode. A distinct UV emission resulting from free exciton recombination in a ZnO single microwire is observed in the electroluminescence. Size difference of ZnO single microwire shows significant influence on emission efficiency. The EL spectra of n-ZnO single microwire/p-Si heterostructure exhibited relatively stronger UV emission which was compared with the EL spectra of n-ZnO single nanowire/p-Si heterostructure and n-ZnO film/p-Si heterostructure, respectively. & 2014 Elsevier B.V. All rights reserved.
Keywords: ZnO single microwire Electroluminescence Transparent conductive film
1. Introduction Because of the excellent properties such as passive waveguides, resonance cavities and gain media, a one-dimensional semiconductor microwire has received considerable attention in constructing optoelectronic devices [1–5]. Semiconductor microwire structures are recently emerging as prospective candidates for even further miniaturization [6–8]. ZnO is a semiconductor material with large band-gap (Eg ¼ 3.37 eV at 300 K) and high exciton binding energy (60 meV). It is, therefore, recognized as a high efficiency emitter for ultraviolet light-emitting devices (UV-LEDs) [9–11]. In particular, the high exciton binding energy can cause a strong Coulomb interaction between electrons and holes. Electroluminescence from n-ZnO single microwire/p-GaN and n-ZnO single nanowire/p-Si heterojunctions has been reported recently [12,13]. However, ZnO single microwire based p-Si optoelectronic devices have been less reported so far. Because ZnO-based LEDs could be compatible with the silicon micro-electronic technology, the research on ZnO/Si heterostructure is very significant. In this paper, we report electroluminescence from n-ZnO single microwire/p þ -Si heterojunction. The transparent conductive ITO film is deposited as a cathode by magnetron sputtering and this kind of electrode has better light transmissivity than a metal electrode.
n
Corresponding author. Tel.: þ 86 431 85168359; fax: þ 86 431851 68270. E-mail address: [email protected] (J. Yin).
0022-2313/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2014.01.056
The current–voltage (I–V) curves exhibited a typical rectification characteristic. Both ultraviolet (UV) and visible emissions could be detected in the electroluminescence (EL) measurement. Furthermore, the result had been compared with the EL spectra of n-ZnO film/p-Si heterostructure and n-ZnO single nanowire/p-Si heterostructure, respectively, and an improved UV emission was realized. 2. Experiments The ZnO microwires were synthesized with a vapor phase transport method. One single ZnO microwire can form a hexagonal resonant cavity and faces act as reflecting mirrors [14]. Fig. 1 illustrates a scanning electron microscope (SEM) image of ZnO microwires. The procedure for fabricating n-ZnO single microwire/ p þ -Si heterojunction is shown in Fig. 2(a). The ZnO microwires were first randomly dispersed on a p-Si substrate. A single ZnO microwire was adsorbed by a fiber moistened with alcohol and then transferred to a heavily doped p-Si substrate. Thereafter, the sample was annealed at 500 1C in nitrogen ambient for 1 h to evaporate the air and moisture which helps to form a good contact between the ZnO microwire and the substrate. A thin film of poly (methyl methacrylate) (PMMA) was spin-coated on the ZnO microwire to prevent the shorting of connection between the top and the bottom electrodes in the two-terminal devices. Following this, reactive ion etching (RIE) was used to remove the top surface of the ZnO single microwire for the electrode. Finally,
314
Y. Xu et al. / Journal of Luminescence 149 (2014) 313–316
Fig. 1. SEM image of ZnO microwires. Fig. 3. Current–voltage (I–V) curves of sample A and sample B.
Fig. 4. Electroluminescence spectra of sample A and sample B under forward bias current of 15 mA.
3. Results and discussion
Fig. 2. (a) The procedure for fabricating n-ZnO single microwire/p þ -Si heterojunction. (b) OM image of n-ZnO single microwire/p þ -Si heterojunction.
transparent conductive ITO film and Al thin-film were deposited on the ZnO single microwire and p þ -Si substrate by magnetron sputtering and electron beam evaporation as electrodes, respectively. The sample was then annealed at 300 1C in nitrogen ambient for 3 min to form the ohmic contact. An optical microscope (OM) image of the as-fabricated n-ZnO single microwire/ p þ -Si heterojunction is shown in Fig. 2(b).
The current–voltage (I–V) curves of samples with different diameters and lengths of the n-ZnO single microwire (sample A: length 192 mm, diameter 21 mm; sample B: length 90 mm, diameter 18 mm) are illustrated in Fig. 3, which shows non-ideal p–n junction I–V characteristics. This is because the ZnO single microwire is transferred to p-Si by a fiber and p–n junctions formed under the condition of annealing. In contrast to standard epitaxy grown p–n junctions, covalent chemical bonds between ZnO microwire and Si are not formed and van der Waals forces are generated in the interface region by a mechanical contact [7]. The apparent variation between the two samples could be due to the size differences of ZnO single microwires, as well as the different contact areas [15,16]. Fig. 4 distinctly shows the electroluminescence spectra of sample A and sample B. The EL spectra of these two samples are fed with the same forward bias current of 15 mA. Due to the different diameters, lengths and different contact areas of sample A
Y. Xu et al. / Journal of Luminescence 149 (2014) 313–316
Fig. 5. (a) Electroluminescence spectra for sample A under various forward bias currents. (b) The intensity ratio of UV/DLE versus injection current.
and sample B, the prominent differences of light emission are observed. The emitting efficiency of sample A is much higher than that of sample B, wherever light emission is in the range of ultraviolet to visible. The result also exhibits relatively strong UV emission and weak deep-level emission (DLE) which is compared with the EL spectra of n-ZnO single nanowire/p-Si heterostructure LEDs reported by Bao et al. [17]. This can be explained that the emission intensity strongly depends on the out-coupling efficiency, which is influenced by nonuniformity along the microwire length and the size of ZnO wire. The room-temperature electroluminescence spectra for sample A under a continuous current injection are presented in Fig. 5(a). The EL spectra show that there is no UV emission peak when the forward bias current is 5 mA. However, with increasing forward bias current at 15 mA and 20 mA, a UV emission peak appears at around 400 nm. When the forward bias current reaches 25 mA, an intense UV emission peak has been detected. However, the peak appeared to be blue-shifted. The UV emission peak shift is related to polarization effects [18]. Attributing the hexagonal wurtzite structure of ZnO single microwire and lattice array lacks reversal symmetry; spontaneous polarization is generated in the material. However, spontaneous polarization will reduce because the device
315
used a nonpolar face of ZnO contacting with p-Si. In addition, piezoelectric polarization will be generated in the process of annealing. The device was annealed at 500 1C in nitrogen ambient for 1 h to form a good contact between ZnO single microwire and Si. In this annealing process, mismatch of lattice constant between ZnO single microwire and Si will generate a strain in the interface. The strain between ZnO single microwire and Si will lead to the phenomenon of piezoelectric polarization. A combined effect of spontaneous polarization and piezoelectric polarization causes an intense built-in electric field in the p–n junction. Because the relaxation time of carrier in conduction band is much shorter than carrier lifetime, a large number of free carriers are gathered in the p–n junction under the influence of this built-in electric field, which is generated by the external current injection. This result leads to ground state rising. According to the well-known Burstein–Moss effect, also called the bandgap widening effect, the UV emission peak will show blue-shift for short wavelength [19]. Furthermore, the EL spectra present a weak UV emission peak accompanying a dominated DLE located around 500 nm. In accordance with foregone research works, the UV emission has been attributed to excitonic recombination, and the DEL originates from defects and surface states. Fig. 5(b) shows the intensity ratio of UV/ DLE as a function of injection current. The ratio of UV/DLE increases approximately linearly with increasing forward bias currents, indicating a high UV light extraction efficiency for the n-ZnO single microwire/p þ -Si heterojunction LEDs. For the reason that the surface of p þ -Si substrate has been oxidized by oxygen molecules, a thin SiO2 film was formed between the p–n junctions [20]. Therefore, this device structure can be identified as n-ZnO single microwire / p þ -Si heterojunction with a SiO2 barrier layer. The EL spectra of sample A exhibit relatively strong UV emission compared with the EL spectra of nZnO film/p-Si heterostructures experimented by You et al. [21]. This phenomenon could explain that the ZnO single microwire has better crystallinity than that of ZnO film and the SiO2 barrier layer could prevent the electrons transport. The inclusion of SiO2 barrier layer will restrict the movement of electrons from n-ZnO to p-Si. This is because upon applying forward bias to n-ZnO single microwire/SiO2/p-Si heterojunction, the energy band of p-Si would move more downwards than that of n-ZnO, near the ZnO/ SiO2 and SiO2/Si interfaces. EV (Si) would move downwards to EV (ZnO) gradually. As a result, the electrons will not be allowed to flow actively between n-ZnO and p-Si, due to high conduction band offsets. However, the holes at the SiO2/p-Si interface can move to n-ZnO over threshold voltage, because the valence band offsets are relatively low. Therefore, it is beneficial to realize electron–hole recombination in ZnO area effectively.
4. Conclusion In summary, we have demonstrated a method of using the transparent conductive ITO film to achieve reliable electrical injection into the ZnO single microwire, and fabrication of n-ZnO single microwire/p þ -Si heterojunction LEDs. A distinct UV emission around 380–400 nm from the ZnO single microwire was obtained. In addition, the EL spectra of two samples, which are constructed by the ZnO single microwire with different sizes, are compared with ZnO single nanowire LEDs. Size difference of ZnO single wire has great influence on emitting efficiency. Furthermore, the EL spectra of ZnO single microwire exhibited relatively strong UV emission which was compared with that of n-ZnO film/ p-Si heterostructures. These results propose a potential application of ZnO microwire in micro-optoelectronics.
316
Y. Xu et al. / Journal of Luminescence 149 (2014) 313–316
Acknowledgments We thank Professor Chunxiang Xu of Southeast University. He provided the ZnO microwires. This work was supported by Natural Basic Research Program of China (973 Program) No. 2011CB302005 and by Natural Science Foundation of China Contract nos. 61076046 and 6174023. References [1] H.X. Dong, Y. Liu, J. Lu, Z.H. Chen, J. Wang, L. Zhang, J. Mater. Chem. C 1 (2013) 202. [2] Q. Zhang, J.J. Qi, X. Li, F. Yi, Z.Z. Wang, Y. Zhang, Appl. Phys. Lett. 101 (2012) 043119. [3] M. Ding, D.X. Zhao, B. YaoS.L. EZ. Guo, L.G. Zhang, D.Z. Shen, Opt. Express 20 (2012) 13657. [4] Z.F. Shi, L. Zhao, X.C. Xia, W. Zhao, H. Wang, J. Wang, X. Dong, B.L. Zhang, G.T. Du, J. Lumin. 131 (2011) 1645. [5] G.Y. Chai, L. Chow, O. Lupan, E. Rusu, G.I. Stratan, H. Heinrich, V.V. Ursaki, I.M. Tiginyanu, Solid State Sci. 13 (2011) 1205. [6] D. Vanmaekelbergh, B. Vugt, Nanoscale 3 (2011) 2783. [7] M.A. Zimmler, J.M. Bao, I. Shalish, W. Yi, J. Yoon, V. Narayanamurti, F. Capasso, Nanotechnology 18 (2007) 235205.
[8] C. Czekalla, J. Lenzner, A. Rahm, T. Nobis, M. Grundmann, Superlattice Microstruct. 41 (2007) 347. [9] Z.F. Shi, X.C. Xia, W. Yin, S.K. Zhang, H. Wang, J. Wang, L. Zhao, X. Dong, B.L. Zhang, G.T. Du, Appl. Phys. Lett. 100 (2012) 101112. [10] G.T. Du, W. Zhao, G.G. Wu, Z.F. Shi, X.C. Xia, Y. Liu, H.W. Liang, X. Dong, Y. Ma, B.L. Zhang, Appl. Phys. Lett. 101 (2012) 053503. [11] S. Ruhle, L.K. van Vugt, H.Y. Li, N.A. Keizer, L. Kuipers, D. Vanmaekelbergh, Nano Lett. 8 (2008) 119. [12] J. Dai, C.X. Xu, X.W. Sun, Adv. Mater. 23 (2011) 4115. [13] W.Q. Yang, H.B. Huo, L. Dai, R.M. Ma, S.F. Liu, G.Z. Ran, B. Shen, C.L. Lin, G.G. Qin, Nanotechnology 17 (2006) 4868. [14] G.P. Zhu, C.X. Xu, J. Zhu, C.G. Lv, Y.P. Cui, Appl. Phys. Lett. 94 (2009) 051106. [15] J.D. Ye, S.L. Gu, S.M. Zhu, W. Liu, S.M. Liu, R. Zhang, Y. Shi, Y.D. Zheng, Appl. Phys. Lett. 88 (2006) 182112. [16] 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, K.L. Teo, Appl. Phys. Lett. 93 (2008) 013506. [17] J.M. Bao, M.A. Zimmler, F. Capasso, X.W. Wang, Z.F. Ren, Nano Lett. 6 (2006) 1719. [18] Y.F. Hu, Y. Zhang, Y.L. Chang, R.L. Snyder, Z.L. Wang, ACS Nano 4 (2010) 4220. [19] M.T. Chen, M.P. Lu, Y.J. Wu, J.H. Song, C.Y. Lee, M.Y. Lu, Y.C. Chang, L.J. Chou, Z.L. Wang, L.J. Chen, Nano lett. 10 (2010) 4387. [20] J. Ahn, M.A. Mastro, P.B. Klein, J.K. Hite, B. Feigelson, C.R. Eddy, J. Kim, Opt. Express 19 (2011) 21692. [21] J.B. You, X.W. Zhang, S.G. Zhang, H.R. Tan, J. Ying, J. Appl. Phys. 107 (2010) 083701.