Piezotronics and piezo-phototronics with third-generation semiconductors
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Piezotronic effect and piezotronics The piezoelectric nanogenerator was first proposed in 2006 using ZnO nanowires.1 The presence of a piezoelectric potential arising from piezoelectric polarization charges was proposed for understanding the observed electricity output by mechanical straining.2 At the same time, the piezoelectric-modulated potential barrier at the metal–ZnO interface was proposed to act as a “gate” voltage for explaining the observed transistorlike behavior of a metal–ZnO–metal structure3 and the straingated diode effect at a metal–ZnO interface.4 Following these early efforts, piezotronics was coined as the name for this new field in 2007.5–7
Fundamental effect As for a metal–semiconductor interface, if the semiconductor has a noncentrosymmetric structure (Figure 1a) and the doping in the semiconductor is moderate, the static polarization charges at the interface arising from piezoelectric effects are not completely screened. The height of the Schottky barrier can then be modulated by the applied strain (tensile or compressive), resulting in tuning of the barrier electronic-transport properties by the piezoelectric effect. The lowered barrier can enhance electron transport, while an increased barrier height can cut off the current, just like a diode. This is the piezotronic
effect, which involves using piezoelectric polarization charges as a “gating” voltage for tuning the electronic transport across an interface or junction (Figure 1b–e).8,9 The effect was further verified experimentally by the development of piezotronic strain-gated transistor and logic gates;10,11 the associated semiclassical theory was proposed recently.12
Basic device structure and applications Since the invention of piezotronics in 2007,5 rapid advances have been made in revealing the fundamental piezotronic process and implementing new device technologies. The basic structure of a typical piezotronic device includes two backto-back Schottky contacts and a semiconducting channel, or two ohmic contacts and a p–n junction. The strain-induced polarization in the piezoelectric semiconductor (e.g., ZnO nanowires) can act as the controlling signal in the piezotronic devices.8,13 The asymmetric change in the strain-dependent current–voltage (I–V) curves for piezotronic transistors is characteristic of the piezotronic effect. Piezotronic logic devices have been developed by integrating strain-gated piezotronic transistors for performing electronic logic computations over the mechanical strain signals.10 A piezotronically gated resistive memory device has also been demonstrated for recording strain information.14 The feasibility
Zhong Lin Wang, Georgia Institute of Technology, USA; and Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, China; [email protected] Wenzhuo Wu, Purdue University, USA; [email protected] Christian Falconi, University of Rome Tor Vergata, Italy; Sungkyunkwan University, Republic of Korea; and Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Ch
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