Theory of piezotronics and piezo-phototronics
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Introduction Piezotronics and piezo-phototronics are emerging fields that couple piezoelectricity, semiconducting properties, and photon excitation for achieving high-performance devices,1–3 such as nanogenerators,4–6 piezotronic field-effect transistors,7 strain sensors,8 light-emitting diodes (LEDs),9 solar cells,10 straingated vertical nanowire arrays,11 piezo-phototronic strain sensor arrays,12 two-dimensional (2D) piezotronic transistors,13 and strain-gated logic devices.14 The built-in piezoelectric potential effectively controls carrier-transport characteristics in piezoelectric semiconductors,15–18 such as ZnO, GaN, InN, CdS, and monolayer MoS2. The first piezotronic strain sensor fabricated using a ZnO nanowire showed high strain sensitivity in which the ratio of resistance change to strain (gauge factor) is higher than carbon nanotube devices (∼1000) and reached 1250.8 The strain-induced piezoelectric potential modulates carrier-transport properties, generation, and recombination at the junction or interface of a semiconductor. There are two novel features of piezotronic and piezo-phototronic devices. First, piezoelectric materials convert mechanical stimuli to an electric signal; second, strain-induced piezoelectric charges directly control carriers in the depletion layer of a p–n junction or metal–semiconductor contact, which amplifies the piezoelectric signal. Therefore, piezotronic
transistors simultaneously have energy/signal conversion and amplification functions, which is the high sensitivity mechanism. In particular, piezotronic and piezo-phototronic devices exhibit novel sensitivity enhancement for strain-sensing applications8,19 and energy-conversion improvement for solar cells.10,20 Based on semiconductor physics and the piezoelectric theory, the fundamental piezotronic theory has been established, and piezotronic p–n junction and metal– semiconductor contact models have been constructed.15,18 Piezotronic and piezo-phototronic effects provide a fundamental understanding of the results of previous theoretical and experimental studies, for example, the piezopotential modulates the Schottky barrier heights at metal–semiconductor contacts,21 changes the electric-field distribution in CdS-based photovoltaics,22 and separates photon-induced electron–hole pairs in core–shell structures.23 Coupling properties can be used to convert the mechanical or optical signal to ON or OFF output signals. Piezotronic logic devices based on strain-gated transistors have been fabricated for NAND, NOR, and XOR logic units,4 and for piezo-phototronic binary computation using a piezoelectric semiconductor with ZnO and CdS with wurtzite structure.8 Piezotronic transistors can be strain sensors, signal comparators and amplifiers, used for designed novel piezotronic analog-
Yan Zhang, University of Electronic Science and Technology of China, China; [email protected] Yongsheng Leng, George Washington University, USA; [email protected] Morten Willatzen, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sci
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