Piezotronic and Piezophototronic Effects

Owing to the polarization of ions in a crystal that has noncentral symmetry, a piezoelectric potential (piezopotential) is created in the material by applying a stress. The creation of piezopotential together with the presence of Schottky contacts are the fundamental physics responsible for a few important nanotechnologies. The nanogenerator is based on the piezopotential-driven transient flow of electrons in the external load. On the basis of nanomaterials in the wurtzite semiconductors, such as ZnO and GaN, electronics fabricated by using a piezopotential as a gate voltage are called piezotronics, with applications in strain/force/pressure-triggered/controlled electronic devices, sensors, and logic gates. The piezophototronic effect is a result of three-way coupling among piezoelectricity, photonic excitation, and semiconductor transport, which allows tuning and controlling of electro-optical processes by a strain-induced piezopotential. P iezoelectricity is a well-known effect that involves the production of an electrical potential in a substance as the pressure on it changes. This effect has beenwidely used for fabricating electromechanical sensors, actuators, and energy converters. The most well-known material that has a piezoelectric effect is Pb(Zr,Ti)O3 (PZT). However, PZT is an electric insulator, and it is less useful for building electronic devices. Piezoelectric materials that are used for fabricating electronic and optoelectronic devices are required to be semiconductors, such as ZnO, GaN, InN, and ZnS. One of the most common electronic devices is a singlechannel field effect transistor (FET)basedona semiconductor nanowire, in which a source and drain are located at the two ends of the device and a gate voltage is applied to the channel and the substrate. By applying an external gate voltage, Vds, at the source and drain, the charge carrier transport process in the semiconductor device is tuned/gated by the gate voltage Vg.We first suggested that the gate voltage can be replaced by the piezopotential in a piezoelectric nanowire, such as ZnO, so that the charge carrier transport process in FETcan be tuned/ gated by applying a stress to the device. This type of device is called a piezotronic device as triggered or driven by a mechanical deformation action. Alternatively, for a device with Schottky contacts at either or both the source or drain, by introducing a laser excitation at the source/drain, a coupling has been demonstrated among piezoelectricity, photoexcitation, and semiconductor characteristics, leading to the piezophototronic effect. This Perspective is to introduce the principle and potential applications of the devices fabricated based on piezotronic and piezophototronic effects. Piezopotential.We now use ZnO to elaborate on the piezopotential. For a crystal that lacks a center symmetry, the Zn2þ cations and O anions are tetrahedrally coordinated. Under a strain-free condition, the center of the positive charges and the center of negative charges overlap, with zero dipole moment. If a stress is applied at an apex of the tetrahedron, the center of the cations and the center of the anions are relatively displaced, resulting in a dipolemoment. Aconstructive addition of the dipolemoments created by all of the units in the crystal results in amacroscopic potential drop along the strain direction in the crystal. This is thepiezoelectric potential (piezopotential). The piezopotential is created by the nonmobile, nonannihilative ionic charges, and the piezopotential remains in the crystal as long as the stress remains, although its magnitude depends on the density of doping. The principle of the nanogenerator is a transient flow of electrons in an external load as driven by the