Piezotronics/piezo-phototronics: Devices and applications
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Piezoelectric effect has been widely used in electromechanical sensing, actuation and energy harvesting, which produces polarization charges under mechanical deformation in materials lacking inversion symmetry or with polarization domains. Conventional piezoelectric materials such as PZT and PVDF are electrically insulating and hence not feasible for constructing functional electronics or optoelectronics. The effect of mechanically-induced polarization on electronic and optoelectronic processes of charge carriers in piezoelectric materials has therefore been long overlooked. Semiconductor materials such as ZnO, GaN and CdS with wurtzite or zinc blende structures also possess piezoelectric properties but are not as extensively utilized in piezoelectric sensors and actuators as PZT due to their relatively small piezoelectric coefficients. The coupling of piezoelectric polarization with semiconductor properties in these materials has resulted in both novel fundamental phenomenon and unprecedented device applications, leading to the increasing research interests in the emerging field of piezotronics and piezo-phototronics. The basic of piezotronics and piezo-phototronics lies in the fact that strain-induced polarization charges at interface can effectively modulate the local band structure and hence the charge carrier transport across local junctions/contacts by exerting substantial influence on the concentration/distribution of free carriers and interfacial electronic charged states in the device. Fundamental physics about the piezotronics and piezo-phototronics are systematically illustrated at first in this dissertation. Functional electronic/optoelectronic devices based on piezoelectric semiconductor materials are presented to demonstrate the practical applications of the piezotronic and piezo-phototronic effects, including nanowire/microwire transistors, nanowire logic circuits, bio/chemical sensors and photo detectors. By successfully applying the piezotronic and piezo-phototronic effects in a wide range of electronics/optoelectronics, we have shown the universality of these two effects to be utilized in various practical applications as effective approaches to modify the physical properties of charge carriers in piezoelectric semiconductors.