IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 12, NO. 2, MARCH2013 263 ZnO Branched Nanowires and the p-CuO/n-ZnO Heterojunction Nanostructured Photodetector Sheng-Bo Wang, Chih-Hung Hsiao, Shoou-Jinn Chang, Senior Member, IEEE, Z. Y. Jiao, Sheng-JoueYoung, Shang-Chao Hung, and Bohr-Ran Huang Abstract—The authors report the growth of ZnO branched nanowires on the CuO nanowires and the fabrication of p-CuO/ n-ZnO heterojunction nanostructured photodetector (PD). It was found that the hydrothermally grown ZnO branched nanowires were reasonably uniform with an average length of 200 nm and an average diameter of 50 nm. Under forward bias, it was found that turn on voltage of the fabricated PD reduced from ∼0.7 to ∼0.2 V under ultraviolet (UV) illumination. It was also found that UV-to-visible rejection ratio of the fabricated device was larger than 100. Index Terms—Branched nanowires, CuO, heterojunction, photodetector (PD), ZnO. I. INTRODUCTION I T is known that sun emits not only visible and infrared light but also emits ultraviolet (UV) radiation. Although ozone layer in the atmosphere can absorb most UV-C (100–280 nm) and UV-B (280–315 nm) radiation, UV-A (315–400 nm) light that reaches the earth can still cause diseases such as sunburn and even skin cancer. To protect human body from these diseases, we could use wide bandgap semiconductor-based photodetectors (PDs) to detect UV radiation. UV PDs can also be used in various applications such as space communications and flame detection. Manuscript received May 6, 2012; accepted January 25, 2013. Date of pub- lication January 30, 2013; date of current version March 6, 2013. This work was supported in part by the Center for Micro/Nano Science and Technology, in part by the Advanced Optoelectronic Technology Center, National Cheng Kung University under projects from the Ministry of Education, Taiwan. This work was also supported in part by the Center for Condensed Matter Sciences, National Taiwan University. The review of this paper was arranged by Associate Editor G. Ramanath. S.-B. Wang and C.-H. Hsiao are with the Institute of Microelec- tronics, National Cheng Kung University, Tainan 70101, Taiwan (e-mail: sbwang777@gmail.com; neel1203@gmail.com). S.-J. Chang is with the Department of Electrical Engineering, Center for Micro/Nano Science and Technology, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan (e-mail: changsj@mail.ncku.edu.tw). Z. Y. Jiao is with the College of Science, China University of Petroleum (East China) Qingdao, Shandong 266580, China S.-J. Young is with the Department of Electronic Engineering National For- mosa University, Huwei, Yunlin632, Taiwan (e-mail: youngsj@nfu.edu.tw). S.-C. Hung is with the Department of Information Technology and Com- munication Shih Chien University, Neimen, Kaohsiung 845, Taiwan (e-mail: schung99@gmail.com). B.-R. Huang is with the Graduate Institute of Electro-Optical Engineering and the Department of Electronic Engineering, National Taiwan University of Sci- ence and Technology, Taipei 106, Taiwan (e-mail: huangbr@mail.ntust.edu.tw). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNANO.2013.2243916 Recently, 1-D nanowires have attracted much attention for potential electronic and photonic device applications. Among the materials for 1-D nanowire devices, ZnO has attracted much attention. ZnO is natural n-type semiconductor with wide direct bandgap energy of 3.37 eV and a large exciton binding energy (60 meV) at room temperature [1], [2]. It has been shown that nanowires are particularly useful for PD applications. Compared with thin film PDs, 1-D nanowire PDs could provide a high photoconductive gain due to the surface-enhanced electron-hole separation efficiency [3], [4]. Recently, Ji et al. reported the fabrication of ZnO nanowire UV PDs [5]. It was found that ZnO nanowire PDs can provide a higher responsivity and a larger UV- to-visible rejection ratio, as compared to the conventional 2-D thin film ZnO PDs. Although ZnO nanowire photoconductive PDs have already been demonstrated, only few report on the ZnO nanowire p-n homojunction devices could be found in the literature due to the fact that stable p-type ZnO is difficult to achieve [6]. CuO is an interesting metal oxide which is also poten- tially useful for photonic device applications. CuO is a natural p-type semiconductor material with small bandgap energy of only 1.2 eV at room temperature. Similar to ZnO, CuO is also abundant in nature, low cost, environment friendly and easy to synthesize. Previously, growth of p-CuO/n-ZnO heterojunc- tion nanowires has been demonstrated [7], [8]. A p-CuO/n-ZnO nanowire solar cell has also been realized [9]. However, prop- erties of the p-CuO/n-ZnO heterojunction nanowire-based PD have never been reported. Very recently, we reported the growth of CuO nanowires on an oxidized Cu wire. The fabrication of CuO nanowire-based infrared PD and humidity sensor has also been demonstrated [10], [11]. In this study, we report hydrother- mal growth of ZnO branched nanowires on the CuO nanowires and the fabrication of the p-CuO/n-ZnO heterojunction nanos- tructured PD. Detailed growth method of the nanostructure, device fabrication process, and electro-optical properties of the fabricated PD will also be discussed. II. EXPERIMENTS The CuO nanowires were prepared by simply placing a Cu wire (1-cm-long 99.999% pure with a diameter of 0.1 mm) in a conventional quartz tube horizontal furnace under atmo- spheric pressure at 500 ◦ C in air for 2 h. Detailed growth procedures can be found elsewhere [10], [11]. ZnO branched nanowires were subsequently grown on CuO nanowires by hy- drothermal method [12]. To grow the ZnO branched nanowires, a ZnO seed layer was first deposited onto the surface of CuO nanowires by radio frequency magnetron sputtering at room 1536-125X/$31.00 © 2013 IEEE