Fabrication and photoresponse of ZnO nanowires/CuO coaxial heterojunction
© Wu et al.; licensee Springer. 2013
Received: 25 July 2013
Accepted: 11 September 2013
Published: 17 September 2013
The fabrication and properties of n-ZnO nanowires/p-CuO coaxial heterojunction (CH) with a photoresist (PR) blocking layer are reported. In our study, c-plane wurtzite ZnO nanowires were grown by aqueous chemical method, and monoclinic CuO (111) was then coated on the ZnO nanowires by electrochemical deposition to form CH. To improve the device performance, a PR layer was inserted between the ZnO buffer layer and the CuO film to serve as a blocking layer to block the leakage current. Structural investigations of the CH indicate that the sample has good crystalline quality. It was found that our refined structure possesses a better rectifying ratio and smaller reverse leakage current. As there is a large on/off ratio between light on and off and the major light response is centered at around 424 nm, the experimental results suggest that the PR-inserted ZnO/CuO CH can be used as a good narrow-band blue light detector.
Because of its wide band gap (3.37 eV) and large exciton binding energy (60 meV), zinc oxide (ZnO) is one of the most promising materials for optoelectronic device applications in the ultraviolet (UV) region[1–3]. ZnO thin films can be produced by several techniques, such as reactive evaporation, molecular beam epitaxy (MBE)[4–6], magnetron sputtering technique, pulsed laser deposition (PLD), sol–gel technique, chemical vapor deposition, electrochemical deposition, and spray pyrolysis. In recent years, ZnO-based heterojunctions have been extensively studied for application as UV photodetectors. These ZnO-based heterojunctions can be classified into two categories: thin film heterojunction (FH) and coaxial heterojunction (CH). ZnO/SiC, ZnO/NiO, and ZnO/GaN belong to the category of thin film heterojunction which had been shown to possess good photoresponse in the UV region. On the other hand, p-copper oxide (CuO)/n-ZnO nanowires (NWs), which belong to the category of coaxial heterojunction, were found to have large enhancement in photocurrent under UV illumination.
ZnO NW possesses many attractive advantages over ZnO thin film. The light trapping ability and great photosensitivity owing to the presence of an oxygen-related hole-trap state at the ZnO NW surface make ZnO NW-based heterojunction very attractive for use as a photodetector. Due to the good optical properties of ZnO NWs and the strong absorption of CuO in the visible region, ZnO NW/CuO heterojunction has drawn much interest these days. A wide variety of processes, including sputtering method, sol–gel technique, thermal oxidation, and modified hydrothermal method, have been developed to fabricate ZnO/CuO CH. These works demonstrated that good rectification ratio and good photoresponse can be obtained with ZnO/CuO coaxial heterojunctions. However, in coating a CuO layer on ZnO nanowires, it is unavoidable that part of the CuO will be in contact with the ZnO buffer layer, and as there are two parallel channels for current conduction (one from the ZnO buffer layer to the CuO layer, and the other from ZnO nanowires to the CuO layer), it is not possible to take full advantage of the benefits that are associated with using the ZnO nanowires in making the photodetector[14, 18, 19]. In this letter, we report fabrication and characterization of a ZnO nanowire/CuO heterojunction photodetector with a photoresist blocking layer. In our study, the ZnO NWs were grown by hydrothermal method, and the sample was then spin-coated with a photoresist layer before the growth of the CuO layer. Structural investigations of the coaxial heterojunction indicate that the sample has good crystalline quality. It was found that our refined structure possesses a better rectifying ratio and a smaller reverse leakage current which are 110 and 12.6 μA, respectively. With the increase of reverse bias from 1 to 3 V, the responsivity increases from 0.4 to 3.5 A W−1 under a 424-nm light illumination.
ZnO NW arrays were grown on an indium tin oxide (ITO)-coated glass substrate by aqueous chemical method as reported in. The reaction solution was 0.05 M Zn(NO3)2 · 6H2O mixed with 0.05 M C6H12N4. The growth temperature and time are 90°C and 2 h, respectively. After the growth, the sample was baked at 100°C for complete dryness. In order to provide electrical blocking between the ZnO buffer layer and the CuO film, a layer of photoresist (DSAM) was spin-coated on ZnO NW arrays as a blocking layer. To remove the PR on top of the ZnO NWs, acetone was dropped onto the sample while it is spinning in a spin coater. With this method, the upper part of the nanowires is not covered by the PR but the bottom part of the nanowires and the ZnO buffer layer are still coated with PR, thus ensuring that the CuO layer which will be grown later will not be in contact with the ZnO buffer layer. Copper was then coated on ZnO NWs by ECD and was then annealed at 400°C for 2 h with the oxygen flow offset at 20 sccm. Finally, a 100-nm silver layer was deposited onto the CuO layer by thermal evaporation to serve as an ohmic contact for electrical measurements. The morphology of ZnO/CuO was examined using a HITACHI S-2400 scanning electron miscroscope (SEM; Chiyoda-ku, Japan). The crystal structure was examined using a transmission electron microscope (TEM; Philips Tecnai G2 F20 FEG-TEM) located at the Department of Physics, National Taiwan University, and by X-ray diffraction (PANalytical X’Pert PRO, Almelo, The Netherlands). Optical transmission spectra were measured using a JASCO V-570 UV/VIS/NIR spectrophotometer (Easton, MD, USA). Xenon arc lamp (LHX150 08002, Glasgow, UK) and iHR-320 monochromator (HORIBA Scientific, Albany, NY, USA ) were used in the photoresponse measurement, and the current–voltage (I-V) curves were measured using Keithley 236 and 4200-SCS (Cleveland, OH, USA).
Results and discussion
In summary, PR-inserted ZnO nanowires/CuO coaxial heterojunctions were fabricated by a low-cost and simple method. Structural studies demonstrated that the nanostructure has good crystalline quality. Optical and electrical characteristics were studied by transmission spectrum, current–voltage curve, and photoresponse measurements, and it is found that adding a PR blocking layer can effectively reduce the reverse bias leakage current and enhance the rectifying ratio. For our sample, the turn-on voltage is 1.7 V, the rectifying ratio between 3 and −3 V is 110, and the responsivity is 3.5 A W−1 at a reverse bias of 3 V in the visible region. As there is a large on/off ratio between light on and off and the light response is centered at around 424 nm, the experimental results suggest that the PR-inserted ZnO/CuO CH can be used as a good narrow-band blue light detector.
This work was funded by the National Science Council of Taiwan, Republic of China (grant number NSC 100-2112-M-002-017-MY3).
- Huang H, Fang G, Mo X, Yuan L, Zhou H, Wang M, Xiao H, Zhao X: Zero-biased near-ultraviolet and visible photodetector based on ZnO nanorods/ n -Si heterojunction. Appl Phys Lett 2009, 94: 063512. 10.1063/1.3082096View ArticleGoogle Scholar
- Alivov YI, Özgür Ü, Dogan S, Johnstone D, Avrutin V, Onojima N, Liu C, Xie J, Fan Q, Morkoç H: Photoresponse of n- ZnO/ p -SiC heterojunction diodes grown by plasma-assisted molecular-beam epitaxy. Appl Phys Lett 2005, 86: 241108. 10.1063/1.1949730View ArticleGoogle Scholar
- Chen W-J, Wu J-K, Lin J-C, Lo S-T, Lin H-D, Hang D-R, Shih MF, Liang C-T, Chang YH: Room-temperature violet luminescence and ultraviolet photodetection of Sb-doped ZnO/Al-doped ZnO homojunction array. Nanoscale Res Lett 2013, 8: 313. 10.1186/1556-276X-8-313View ArticleGoogle Scholar
- Wang H-C, Liao C-H, Chueh Y-L, Lai C-C, Chou P-C, Ting S-Y: Crystallinity improvement of ZnO thin film by hierarchical thermal annealing. Opt Mater Express 2013, 3: 295. 10.1364/OME.3.000295View ArticleGoogle Scholar
- Wang H-C, Liao C-H, Chueh Y-L, Lai C-C, Chen L-H, Tsiang RC-C: Synthesis and characterization of ZnO/ZnMgO multiple quantum wells by molecular beam epitaxy. Opt Mater Express 2013, 3: 237. 10.1364/OME.3.000237View ArticleGoogle Scholar
- Ting S-Y, Chen P-J, Wang H-C, Liao C-H, Chang W-M, Hsieh Y-P, Yang CC: Crystallinity improvement of ZnO thin film on different buffer layers grown by MBE. J Nanomater 2012, 2012: 929278.View ArticleGoogle Scholar
- Hoon JW, Chan KY, Ng ZN, Tou TY: Transparent ultraviolet sensors based on magnetron sputtered ZnO thin films. Adv Mater Res 2013, 686: 79.View ArticleGoogle Scholar
- Gluba MA, Nickel NH, Hinrichs K, Rappich J: Improved passivation of the ZnO/Si interface by pulsed laser deposition. J Appl Phys 2013, 113: 043502. 10.1063/1.4788675View ArticleGoogle Scholar
- Ting C-C, Li C-H, Kuo C-Y, Hsu C-C, Wang H-C, Yang M-H: Compact and vertically-aligned ZnO nanorod thin films by the low-temperature solution method. Thin Solid Films 2010, 518: 4156. 10.1016/j.tsf.2009.11.082View ArticleGoogle Scholar
- Benramache S, Benhaoua B, Khechai N, Chabane F: Elaboration and characterisation of ZnO thin films. Materiaux Tech 2012, 100: 573. 10.1051/mattech/2012052View ArticleGoogle Scholar
- Maldonado A, Guillén-Santiago A, Olvera ML, Castanedo-Pérez RG, Torres-Delgado G: The role of the fluorine concentration and substrate temperature on the electrical, optical, morphological and structural properties of chemically sprayed ZnO:F thin films. Mater Lett 2005, 59: 1146. 10.1016/j.matlet.2004.12.006View ArticleGoogle Scholar
- Ohta H, Hirano M, Nakahara K, Maruta H, Tanabe T, Kamiya M, Kamiya T, Hosono H: Fabrication and photoresponse of a pn -heterojunction diode composed of transparent oxide semiconductors, p -NiO and n -ZnO. Appl Phys Lett 2003, 83: 1029. 10.1063/1.1598624View ArticleGoogle Scholar
- Zhu H, Shan CX, Yao B, Li BH, Zhang JY, Zhao DX, Shen DZ, Fan XW: High spectrum selectivity ultraviolet photodetector fabricated from an n-ZnO/p-GaN heterojunction. J Phys Chem C 2008, 112: 20546. 10.1021/jp808870zView ArticleGoogle Scholar
- Hsueh HT, Chang SJ, Weng WY, Hsu CL, Hsueh TJ, Hung FY, Wu SL, Dai BT: Fabrication and characterization of coaxial p-copper oxide/n-ZnO nanowire photodiodes. IEEE Trans Nanotechnol 2012, 11: 127.View ArticleGoogle Scholar
- Soci C, Zhang A, Xiang B, Dayeh SA, Aplin DPR, Park J, Bao XY, Lo YH, Wang D: ZnO nanowire UV photodetectors with high internal gain. Nano Lett 2010, 7: 1003.View ArticleGoogle Scholar
- Jung S, Jeon S, Yong K: Fabrication and characterization of flower-like CuO–ZnO heterostructure nanowire arrays by photochemical deposition. Nanotechnology 2010, 22: 015606.View ArticleGoogle Scholar
- Wang P, Zhao X, Li B: ZnO-coated CuO nanowire arrays: fabrications, optoelectronic properties, and photovoltaic applications. Opt Express 2011, 19: 11271. 10.1364/OE.19.011271View ArticleGoogle Scholar
- Liao K, Shimpi P, Gao PX: Thermal oxidation of Cu nanofilm on three-dimensional ZnO nanorod arrays. J Mater Chem 2011, 21: 9564. 10.1039/c1jm10762cView ArticleGoogle Scholar
- Wang JX, Sun XW, Yang Y, Kyaw KK, Huang XY, Yin JZ, Wei J, Demir HV: Free-standing ZnO-CuO composite nanowire array films and their gas sensing properties. Nanotechnology 2011, 22: 325704. 10.1088/0957-4484/22/32/325704View ArticleGoogle Scholar
- Vayssieres L: Growth of arrayed nanorods and nanowires of ZnO from aqueous solutions. Adv Mater 2003, 15: 464. 10.1002/adma.200390108View ArticleGoogle Scholar
- Leung YH, He ZB, Luo LB, Tsang CHA, Wong NB, Zhang WJ, Lee ST: ZnO nanowires array p-n homojunction and its application as a visible-blind ultraviolet photodetector. Appl Phys Lett 2010, 96: 053102. 10.1063/1.3299269View ArticleGoogle Scholar
- Yang S, Prendergast D, Neaton JB: Strain-induced band gap modification in coherent core/shell nanostructures. Nano Lett 2010, 10: 3156. 10.1021/nl101999pView ArticleGoogle Scholar
- Wang SB, Hsiao CH, Chang SJ, Lam KT, Wen KH, Hung SC, Young SJ, Huang BR: A CuO nanowire infrared photodetector. Sensors Actuators A 2011, 171: 207. 10.1016/j.sna.2011.09.011View ArticleGoogle Scholar
- Lin S-K, Wu KT, Huang CP, Liang C-T, Chang YH, Chen YF, Chang PH, Chen NC, Chang C-A, Peng HC, Shih CF, Liu KS, Lin TY: Electron transport in In-rich In x Ga1−xN films. J Appl Phys 2005, 97: 046101. 10.1063/1.1847694View ArticleGoogle Scholar
- Chen JH, Lin JY, Tsai JK, Park H, Kim G-H, Youn D, Cho HI, Lee EJ, Lee JH, Liang C-T, Chen YF: Experimental evidence for Drude-Boltzmann-like transport in a two-dimensional electron gas in an AlGaN/GaN heterostructure. J Korean Phys Soc 2006, 48: 1539.Google Scholar
- Hsieh Y-P, Chen H-Y, Lin M-Z, Shiu S-C, Hoffmann M, Chern M-Y, Jia X, Yang Y-J, Chang H-J, Huang H-M, Tseng S-C, Chen L-C, Chen K-H, Lin C-F, Liang C-T, Chen YF: Electroluminescence from ZnO/Si-nanotips light-emitting diodes. Nano Lett 1839, 2009: 9.Google Scholar
- Zhai T, Fang X, Liao M, Xu X, Zeng H, Yoshio B, Golberg D: A comprehensive review of one-dimensional metal-oxide nanostructure photodetectors. Sensors 2009, 9: 6504. 10.3390/s90806504View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.