DOI: 10.1002/adma.200601908 Piezoelectric Gated Diode of a Single ZnO Nanowire** By Jr H. He, Cheng L. Hsin, Jin Liu, Lih J . Chen,* and Zhong L. Wang* One-dimensional (1D) semiconducting nanostructures, [1] such as nanowires (NWs) and nanobelts (NBs), are funda- mental building blocks for constructing nanoscale electronic devices [2] because of their small size and the enhanced charge carrier mobility owing to 1D confinement. Considerable ef- forts have been devoted to energy-band engineering by dop- ing for controlling their electrical properties and assembling NWs into increasingly complex structures. [2a,2b,2d,3] The recti- fier, a fundamental device for electronics, normally consists of a p–n junction diode. The role of the dopants is the formation of a p–n junction and creation of an electrostatic potential en- ergy barrier at the junction. ZnO exhibits the most diverse and abundant configurations of nanostructures known so far, such as NWs, NBs, nano- springs, nanorings, nanobows, and nanohelices. [4] Numerous studies based on ZnO nanostructures have demonstrated nov- el applications due to their semiconducting and piezoelectric properties. [1a,1e,5] For ZnO, n-type conductivity is relatively easy to realize via excess Zn, or with Al, Ga, or In doping, but p-type doping has only recently been achieved. [6] This leads to the conclusion that the generation of p-type material is one of the last major obstacles hindering the development of ZnO- based electronic and optoelectronic devices. There are many possible strategies for doping ZnO in order to make p–n junc- tions for advancing the technological uses of ZnO-based elec- tronic and optoelectronic devices. [6] As an alternative approach for achieving p-type ZnO, we have been exploring the potential of coupling the piezoelec- tric effect with the semiconducting property of ZnO to achieve a few unique applications. In this Communication, we show how an n-type ZnO NW can be used to produce a p–n junction that serves as a diode. Our design is based on the me- chanical bending of a ZnO NW. As a result, the potential en- ergy barrier induced by piezoelectricity (w PZ ) across the bent NW governs the electrical transport through the NW. To quantify w PZ , the current–voltage (I–V) characteristics re- ceived at different levels of deformation were included in the- oretical calculations. The magnitude of the piezoelectric bar- rier dominates the rectifying effect. The rectifying ratio could be as high as 8.7:1 by simply bending a NW. The operation current ratio of a straight to a bent ZnO NW could be as high as 9.3:1 at reverse bias. This also shows that the NW can serve as a random access memory (RAM) unit. Figure 1a shows a typical scanning electron microscopy (SEM) image of a well-aligned ZnO NW array. Figure 1b shows a typical transmission electron microscopy (TEM) im- age of a single ZnO NW. The corresponding selected-area electron diffraction pattern (Fig. 1c) confirms that the phase of the NWs is hexagonal wurtzite-structured ZnO. Figure 1d is a high-resolution TEM (HRTEM) image from the outlined region indicated in Figure 1b. Figure 1c and d shows that the ZnO NWs are single-crystalline and free of dislocations. The growth direction of the ZnO NW was determined to be [0001]. In situ I–V measurements and the manipulation of ZnO NWs were carried out in a multiprobe nanoelectronics mea- surement (MPNEM) system. The two-terminal method was applied for electrical transport measurements at high vacuum to minimize influences from the environment. The tungsten nanotip used for measuring the electrical transport of a nano- wire was precoated with a Ti/Au (30 nm:30 nm) film by elec- tron-beam evaporation to obtain Ohmic contact between Ti and ZnO. Creating Ohmic contact is a key step for the mea- surements. Focusing the NW and the W nanotip in the COMMUNICATION Adv. Mater. 2007, 19, 781–784 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 781 – [*] Prof. Z. L. Wang, Dr. J. H. He, J. Liu School of Materials Science and Engineering Georgia Institute of Technology Atlanta, GA 30332-0245 (USA) E-mail: zhong.wang@mse.gatech.edu Prof. L. J. Chen, Dr. J. H. He, C. L. Hsin Department of Materials Science and Engineering National Tsing Hua University Hsinchu 300 (Taiwan) E-mail: ljchen@mx.nthu.edu.tw [**] This research was supported by the NSF, DARPA, NASA, NIH, and NSC. Figure 1. a) An SEM image of a well-aligned ZnO NW array; b) TEM im- age of a single ZnO nanorod; c) Corresponding selected-area electron diffraction pattern, and d) high-resolution TEM image of the ZnO nano- wire at the boxed region marked in (b).