| With the rapid development of the 5G era and the Internet of Things technology,a variety of emerging technologies have sprung up including wearable electronic devices.Wearable technology has the great potential in detection of human activities and personal health,in which the flexible sensors are regarded as the core components.In this thesis,the piezoelectric properties of ZnO films on the flexible substrate are studied in order to achieve high-performance wearable piezoelectric sensors.Flexible electrode is the main component of the sensor,its conductivity directly affects the performance of the sensor.Besides,the thickness is one of the important factors affecting its service life in flexible devices.Therefore,ultra-thin AZO electrodes and ultra-thin ZnO piezoelectric devices with linear piezoelectric response were fabricated on flexible Hastelloy substrates by epitaxial growth technique.Based on the observation that strain can affect the piezoelectric output of the device,the effect of film thickness on the performance of the device was studied.The main research contents and results are as follows:1.Ultra thin,high conductivity,single crystal and mechanical durable AZO electrodes were prepared on flexible Hastelloy substrates by RF magnetron sputtering.The epitaxial strain of AZO electrode on MgO is produced along the axis of AZO electrode.The epitaxial strain can be adjusted by changing the thickness of the AZO electrode film.With the increase of epitaxial strain,the carrier concentration gradually increases in the range of 1021-1023 cm-3,forming a highly conductive AZO electrode.The resistivity of thick AZO electrode(532nm)is about?166 ??·-cm,which is equivalent to the mainstream value of AZO electrode on various substrates.The ultra-thin AZO electrode(?28nm)achieves an ultra-low resistivity of?22 ??·cm,which surpasses most AZO electrodes prepared on rigid glass and flexible materials.More importantly,it is still stable after 1000 bending tests at a bending radius of?1.5 cm.It meets the requirements of flexible display and solar cell.In addition,by using the flexible Hastelloy tapes as the substrates,the large-scale roll to roll production and conformal surface application can be realized.2.The piezoelectric devices of AZO/ZnO/AZO thin films with linear piezoelectric response were prepared on the flexible Hastelloy substrate with a MgO buffer layer.The model of d33 tuned by epitaxial strain was verified by experiments.ZnO film has good linear piezoelectric response characteristics,and it is expected to achieve high precision positioner and improve the performance of piezoelectric sensor in the future.The piezoelectric constant d33 of epitaxial samples is 12.2 ± 0.3 pm/v,which is about 85%higher than that of fiber texture samples.The texture extension?? and ?? of epitaxial samples are?2.12° and?5.84° respectively,and the fiber texture samples are?9.41° and ? respectively,which may account for the difference in piezoelectric responses.However,the modeling of piezoelectric constants as a function of texture spreads shows that the effects of ?? and ?? are negligible.For the general ZnO(002)films,the maximum contribution of the optimized texture to the improvement of d33 is 7.4%,and less than 1%in the experiments.On the contrary,the internal strain detected in the epitaxial samples likely account for the improvement of d33 compared with the strain-free fiber texture samples.3.ZnO thin film piezoelectric devices with linear piezoelectric response were grown on the flexible Hastelloy substrate with CeO2 buffer layer.By changing the thickness of the ZnO film,it is found that the thinner samples show higher piezoelectric output.The bottom and top electrodes of the Piezoelectric ZnO layer are made of AZO films.The ultra-thin Piezoelectric ZnO thin film sensor fabricated on the Hastelloy substrate solves the contradiction between low film thickness and high voltage response,and paves the way for the low-cost and the large-scale production of high-performance flexible ZnO sensor. |
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