Piezotronic materials and large-scale piezotronics array devices
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dividual piezotronic devices The concept of piezotronics was first introduced more than a decade ago.1–5 The field was established based on the use of semiconductor crystals that lack inversion symmetry. ZnO is a good example of such a material. It has a wurtzite crystal structure that is noncentrosymmetric, where Zn2+ cations and O2– anions are tetrahedrally coordinated within the lattice. Figure 1a shows an array of ZnO nanowires (NWs) and a piezoelectric potential induced by a mechanical stimulus. By applying stress along the c-axis of the NW, the centers of positive and negative charges can be efficiently displaced relative to each other to form dipole moments, exhibiting a strong piezoelectric potential.6–8 This piezoelectric potential can act as a virtual gate to control carrier transport. In addition to ZnO, a number of II–VI and III–V wurtzitestructured piezoelectric semiconductor materials such as GaN,9,10 InN,11 CdS,12 and CdSe13 NWs have also demonstrated potential in the field of piezotronics. In the III–V family, GaN is a more extensively studied third-generation semiconductor with wide applications in solid-state lighting and displays, data storage, and power devices. Discussions regarding distinctions between piezoresistive and piezotronic effects and their contributions have been a matter of debate for some time. However, we have evidence regarding the validity of a piezotronic concept. For example, in a ∼1-µm-long c-plane GaN NW, µN-range normal compressive forces induced piezoelectric
charges to change the Schottky barrier height (SBH) and thus modulate the current injected by the tip of conductive atomic force microscope.14 However, a similar modulation could not be observed in a nonpolar m-plane (1100 ) GaN NW under a normal compressive force.14 Such a comparison reveals that the piezotronic effect, rather than the piezoresistive effect, dominates the carrier-transport modulations. Liu et al. systematically investigated the effect of carrier concentration on piezoelectric nanogenerators (PENGs) that used GaN NWs.15 With increasing carrier concentration from 1.10 × 1017 cm–3 (undoped) to 5.63 × 1018 cm–3 (doped), the output current of PENGs gradually reached a maximum value of 50 nA, which was attributed to the reduction in the interior resistance. With a further increase in the carrier concentration to 1.53 × 1019 cm–3, the carrier screening effect on the piezoelectric potential became dominant and resulted in a significant reduction of the output current.15 Piezotronics provides a direct interface between micro-/ nanoelectronic devices and mechanical stimuli. Wang et al. fabricated strain-gated transistors (SGTs) based on GaN nanobelts (Figure 1b).9 In this configuration, the source-drain current decreases as the gate strain increases for which a threshold gate strain >0.4% and a large ON-OFF ratio of nearly 100 were observed. Thus far, piezotronic logic operations such as NOT, AND, OR, NAND, NOR, and XOR have been achieved using these GaN SGTs.9
Weiguo Hu, Beijing Institute of Nanoenergy and N
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