Bending effects of ZnO nanorod metal–semiconductor–metal photodetectors on flexible polyimide substrate

Tse-Pu Chen 1 Sheng-Joue Young 0 Shoou-Jinn Chang 1 Chih-Hung Hsiao 1 Yu-Jung Hsu 2 0 Department of Electronic Engineering, National Formosa University , Yunlin, 632, Taiwan 1 Institute of Microelectronics and Department of Electrical Engineering, Center for Micro/Nano Science and Technology, Advanced Optoelectronic Technology Center, National Cheng Kung University , Tainan, 701, Taiwan 2 Institute of Electro-Optical Science and Engineering, National Cheng Kung University , Tainan, 701, Taiwan The authors report the fabrication and I-V characteristics of ZnO nanorod metal-semiconductor-metal photodetectors on flexible polyimide substrate. From field-emission scanning electron microscopy and X-ray diffraction spectrum, ZnO nanorods had a (0002) crystal orientation and a wurtzite hexagonal structure. During the I-V and response measurement, the flexible substrates were measured with (i.e., the radius of curvatures was 0.2 cm) and without bending. From I-V results, the dark current decreased, and the UV-to-visible rejection ratio increased slightly in bending situation. The decreasing tendency of the dark current under bending condition may be attributed to the increase of the Schottky barrier height. - Background Zinc oxide (ZnO), a nanostructured material that has been widely investigated, has a wide energy band gap of 3.37 eV at room temperature, high optical gain of 300/ cm which is higher than that of GaN (100/cm) [1], and large exciton binding energy of 60 meV [2] which is higher than that of ZnSe (22 meV) and GaN (25 meV). The large exciton binding energy provides high-luminescence efficiency of light emission at or above room temperature. ZnO has slightly higher saturation velocity of 3.2 107 cm/s [3] than GaN, InGaN, and AlGaN [4,5], but the room temperature electron Hall mobility (205 cm2/V/s/) [6] is lower than that of GaN. ZnO has high mechanical and thermal stabilities, and radiation hardness for devices used in nuclear and space applications. ZnO also has lower growth temperature and material cost than III-nitride materials. According to the researches, GaN and its alloys with AlN and InN cover the spectral range from red to vacuum UV (1.9 to 6.2 eV). Therefore, III-nitride materials have attracted a great deal of attention since the commercialization of light-emitting diodes. Not only GaN but also ZnO can tune the value of band gap by forming the ternary alloy of ZnMgO and ZnCdO with MgO and CdO [7], respectively. Over the past decade, ZnO-based and III-nitride-based light-emitting diodes (LEDs) and laser diodes (LDs) have attracted much interest for display, illumination, and mobile phone backlights. Recently, Zhang et al. [8] studied on gain properties of high Al-content AlGaN-delta-GaN quantum wells (QWs) for mid- and deep-UV lasers. Zhao et al. [9] investigated the QW structures with large overlap design to enhance the internal quantum efficiency for InGaN QWbased LEDs. Shukla [10] studied a p-n junction LED employing ZnO/MgZnO QW active layer on a c-plane sapphire by the pulsed-laser deposition technique. Ahn et al. [11] showed p-n heterojunction LEDs that were formed from a p-Si thin film/nanostructured n-ZnO by a dielectrophoresis method. Except for the LEDs and LDs, ZnO makes it as a promising functional material for the electronic device manufacture such as field emission, photodetectors (PD), solar cells, waveguides, and chemical or biosensors. To date, many groups have reported encouraging results for ZnO-based photodetectors. It is important that ZnObased PDs be used in various military and commercial applications, like missile launching and flame detections, optical communications, and ozone layer monitoring. Indeed, ZnO-based PDs have various types, such as p-n junction PDs, p-i-n PDs, Schottky barrier PDs, and metal semiconductormetal (MSM) PDs [12-15]. Among the above structures, the MSM structure is a practical application due to its easy fabrication, low dark current and device noise values, high-response speed, and compatibility with integrated circuit technology. Recently, Yen et al. [16] obtained high photocurrent generation with ZnO/Si heterostructure MSM PDs by an avalanche multiplication in the ZnO layer. GaN-based MSM PDs also can detect the UV region, but a significant number of threading dislocations exit in GaN epilayers due to the large mismatches in lattice constant and thermal expansion coefficient between GaN and substrate, like sapphire and Si. Recently, Li et al. [17] found that dislocations had strong influence on the dark current and responsivity of the PDs. Consequently, ZnO-based materials have more advantages for PDs than GaN-based materials. Recently, various one-dimensional (1-D) ZnO semiconducting nanostructures have been synthesized like nanorods, nanotubes, and nanobelts [18-20]. Most importantly, ZnO is well known as a piezoelectric material. Because of semiconductor properties and the coupling of piezoelectric of ZnO, many groups extensive (...truncated)