High-Performance Piezotronic Devices

After fourteen years of development,piezotronics has made great progress and shows huge application potential in areas such as human-computer interfacing,smart MEMS,nanorobotics and sensor network systems.Piezotronic device has received extensive attentions and research interests,especially for high-performance piezotronic devices.With the successful development of the flexible piezotronic chips and the piezo-phototronic light-emitting diode,piezotronic device has gradually entered the preliminary stage of implementing multifunctional systems by assembling and integrating single nanodevices.However,up to now,there still exist many problems,such as the buckling effect of nanowire and the inverse regulation of bipolar surfaces,to be solved to achieve high-performance piezotronic devices,especially for the sensitivity and integratability.With respect to the problem of existing devices in terms of structure,material and interface,this dissertation focuses on the research and development of high-performance piezotronic devices by improving device structure,material property and modulation interface.Here,piezotronic transistor based on Zn O nanoplatelet,2D out-of-plane piezotronic transistor and piezotronic tunneling transistor are designed and developed both with merits of high sensitivity.These works provide possible solutions to enhance the reliability and spatial resolution of the piezotronic device,and initially explore their compatibility with existing planar micro/nano processing technology.This will give some guiding significance for the design and fabrication of high-performance piezotronic devices,together with the theoretical construction and practical application of piezotronics.The detailed works carried out in this dissertation is as follows.1.To meet the designs of the piezotronic devices with high performance in this dissertation,Zn O nanoplatelets and two-dimensional Zn O nanosheet were successfully synthesized by low temperature hydrothermal method.And,the crystal structure,morphology and piezoelectric property of these Zn O nanomaterials were characterized by various means such as X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and atomic force microscopy（AFM）.The piezoelectric coefficient d₃₃ of the Zn O nanoplatelet with a thickness of about several hundred nanometers（>300 nm）was found to be 18.1～20.8 pm/V.While,the 2D Zn O nanosheet possessed a thickness of about 2～3 nm and an out-of-plane effective piezoelectric coefficient d_eff of about 21.5 pm/V.The sheet-like morphology and excellent out-of-plane piezoelectric properties of these materials provide a solid material foundation for the manufacture of high-performance piezotronic devices.2.Aiming at the problems of buckling effect and reverse regulation of bipolar facets in previous piezotronic devices,this dissertation designed and developed the vertical piezotronic transistor and the double channel piezotronic transistor based on Zn O nanoplatelet.Firstly,the negative influence of the buckling effect on piezotronic effect was effectively avoided by replacing vertical nanowire by nanoplatelet.Thus,a vertical piezotronic transistor based on Zn O nanoplatelet was successfully developed with both merits of high gauge factor（～1.5×10⁷）,high pressure sensitivity（60.97～78.23 me V/MPa）and fast response（<5 ms）.Also,based on its self-assembled array,a high spatial resolution of about 12700 dpi tactile imaging demonstration was achieved.Secondly,basing on above device,we further designed a new kind of double channel piezotronic transistor with a higher pressure sensitivity of about 84.2～104.4 me V/MPa and high reliability through fabricating both electrodes on only one single polar surface of Zn O nanoplatelet.This part of work greatly improves the performance of piezotronic device from the aspects of material selection and device structure redesign,and provides a practical and useful reference for the design and fabrication of future piezotronic devices.3.To achieve high-performance piezotronic devices,a preliminary exploration and research on the two-dimensional（2D）out-of-plane piezotronic effect was carried out at the atomic scale both in theory and experiment.Firstly,through theoretical calculations,a totally new channel width gating mechanism of piezotronic effect was discovered,which only exists in two-dimension.The gauge factor of such 2D out-of-plane piezotronics transistor is predicted to be as high as 2′10⁸.Then,based on the synthesized 2D Zn O nanosheet,a 2D out-of-plane piezotronic transistor was designed and fabricated,by which theoretically predicted modulation mechanism was experimentally verified.Benefiting from its compatibility with existing planar micro/nano processing technology,a logic unit based on2D piezotronic transistor with high performance was successfully achieved.By reducing the device scale,this part of work provides a new possible strategy for achieving high-performance piezotronic device,and expands the application of piezotronic effect of two-dimensional materials,which is of importance to the theoretical construction of 2D piezotronics.4.Considering the insurmountable shortcomings of the Schottky interface in traditional piezotronic devices,combined with the giant switching and fast response of tunneling junction,a high-performance piezotronic tunneling transistor with a metal-insulator-piezoelectric semiconductor heterostructure was rationally designed and developed through simultaneously modulation of the height and width of the tunnel barrier.For the first time,the effective control of the piezotronic effect on the quantum tunneling effect had been achieved.By characterizing the structure and performance of the tunnelling device,it was found to exhibit a great potential in tactile imaging,which presented high critical voltage sensitivity（～5.59 m V/MPa）,high gauge factor（～2.6×10⁸）,giant switching（～1.34′10⁵）and fast response（～4.38 ms）.And,based on the experimental data,combined with the existing piezotronic theory,a preliminary theoretical exploration of the piezotronic tunnel junction was then carried out.This part of work shows the effectiveness of piezotronic effect on tunneling junction.Its high performance suggests the great potential in adaptive interaction between quantum tunneling and mechanical stimuli,providing a new idea and strategy for realizing high-performance piezotronic device.