Piezoelectric Gated Diode of a Single ZnO Nanowire

One-dimensional (1D) semiconducting nanostructures, such as nanowires (NWs) and nanobelts (NBs), are fundamental building blocks for constructing nanoscale electronic devices because of their small size and the enhanced charge carrier mobility owing to 1D confinement. Considerable efforts have been devoted to energy-band engineering by doping for controlling their electrical properties and assembling NWs into increasingly complex structures. The rectifier, 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 energy barrier at the junction. ZnO exhibits the most diverse and abundant configurations of nanostructures known so far, such as NWs, NBs, nanosprings, nanorings, nanobows, and nanohelices. Numerous studies based on ZnO nanostructures have demonstrated novel applications due to their semiconducting and piezoelectric properties. 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. This leads to the conclusion that the generation of p-type material is one of the last major obstacles hindering the development of ZnObased electronic and optoelectronic devices. There are many possible strategies for doping ZnO in order to make p–n junctions for advancing the technological uses of ZnO-based electronic and optoelectronic devices. As an alternative approach for achieving p-type ZnO, we have been exploring the potential of coupling the piezoelectric 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 mechanical bending of a ZnO NW. As a result, the potential energy barrier induced by piezoelectricity (wPZ) across the bent NW governs the electrical transport through the NW. To quantify wPZ, the current–voltage (I–V) characteristics received at different levels of deformation were included in theoretical calculations. The magnitude of the piezoelectric barrier 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) image 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 measurement (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 nanowire was precoated with a Ti/Au (30 nm:30 nm) film by electron-beam evaporation to obtain Ohmic contact between Ti and ZnO. Creating Ohmic contact is a key step for the measurements. Focusing the NW and the W nanotip in the C O M M U N IC A IO N