| Since piezotronics was proposed in 2006,it has been widely used in strain sensing,photoelectric detection and flexible electronic devices.The concept of piezotronics is based on the piezoelectric effect,which refers to the electromechanical coupling between the piezoelectric properties of a material and the mechanical stimulation.Piezoelectric semiconductor materials with piezoelectric properties have semiconductor properties at the same time,and their internal crystal structures generally have non-central symmetry,such as wurtzite ZnO and GaN.Piezotronic devices work based on piezotronic effect.Piezotronic effect is an interface effect which regulates the electric transport characteristics of devices by inducing the generation of piezoelectric polarized charges through mechanical stimulation.The polarity of piezoelectric polarized charge induced by strain in piezotronics can be judged based on the c-axis direction of piezoelectric semiconductor.When tensile strain occurs in the semiconductor,positive piezoelectric polarized charge will be generated in the c-axis direction,and negative piezoelectric polarized charge will be generated in the inverse c-axis direction.In piezotronics,device performance is studied based on energy band theory and piezoelectric theory to analyze the interface regulation effect induced by mechanical stimulus.Researchers fabricated the schottky junction contact interface with metal-semiconductor contact,the p-n junction contact interface with semiconductorsemiconductor contact,and the MOS junction contact interface with metal-oxidesemiconductor contact using piezoelectric semiconductor materials,and regulated the barrier at the junction interface by mechanical stimulus-induced strain.The positive piezoelectric polarized charge induced by strain can increase the energy band at the interface and increase the barrier height,while the negative piezoelectric polarized charge can lower the energy band and decrease the barrier height.Based on the working principle of piezotronic devices and the effect of strain on interface regulation,we know that piezotronics has the ability to build seamless interaction between electronic devices and human and environmental adaptive,which has important application value in the fields of Internet of Things,artificial intelligence and biomedical engineering.In this paper,the tunneling junction with Ag/HfO2/n-ZnO contact was prepared by depositing atomically thick HfO2 insulating layer on a single ZnO nanowire,and the regulation of the carrier tunneling behavior was successfully realized by piezotronic effect.The strain-regulated tunneling-junction device is called piezotronic tunneling strain sensor,or PTSS for short.PTSS uses strain-induced piezoelectric polarized charges to control the height and width of the tunneling barrier in parallel,thus coordinating the electrical transport process of the device.The specific regulation mechanism of strain on PTSS performance can be detailed as follows:In tunneling junction,the resistance of insulating layer and that of Schottky junction are in series,and there is a partial pressure relationship between them.The height of the Schottky barrier and the width of the depletion zone will be changed when the piezoelectric polarized charge is induced by mechanical stimulation of the piezoelectric semiconductor,which will change the voltage drop in the insulating layer of the tunneling junction.According to quantum mechanics,the probability of tunneling by carriers is related to the size of the electric field:as the electric field in the insulating layer increases,the probability of tunneling by carriers increases.Moreover,the greater the electric field change in the dielectric layer regulated by piezoelectric polarized charge,the better the strain sensing performance of the device.Therefore,the current transport characteristics of PTSS can be regulated by the strain of semiconductor materials.Based on the interface regulation effect of piezotronics on tunneling junction barrier,PTSS is studied from three aspects:1.Through the characterization of the I-V characteristics of PTSS,and the analysis of its switching current ratio,the change of Schottky barrier height,strain sensitivity factor(GF),the effect of insulating layer thickness,insulating medium material and temperature on the performance of PTSS strain sensor was studied.The results show that PTSS with 1.8 nm insulation layer has the best strain sensing performance.This is because when the thickness of the insulating layer is too thin or too thick,the regulation of piezoelectric polarization charge on the electric field in the insulating layer will be reduced.Moreover,when HfO2,ZrO2,Al2O3 and SiO2 with different dielectric constants and band gaps are used to prepare the tunneling junction devices,the strain sensing performance of tested PTSS shows no significant difference.In addition,from the characterization of the influencing factors of temperature,it is concluded that electron-hole pairs are induced by overheating excitation,which enhances the shielding effect of free carriers on piezoelectric polarized charge and inhibits the strain sensing performance of PTSS.2.Through experiments and theoretical analysis,the parallel regulation mechanism of mechanical stimuli on barrier height and width of tunnel junction is confirmed.In contrast to sensors that regulate only the height of the schottky barrier for metal-semiconductor contact,the piezotronic tunneling strain sensor shows a high switching current ratio of 478.4 and a high GF value of 4.8×105.The GF value is 17.8 times higher than that of the conventional Schottky strain sensor and the existing ZnO nanowires or nanoribbons sensor.This work has promoted the understanding of the basic mechanism of strain-regulated piezotronic tunneling junction,and realized the device-level piezotronic tunneling junction with high sensitivity strain sensor,which has great application potential in advanced micro/nano electromechanical devices and systems.3.The effect of pulse voltage stimulation on the strain sensing performance of Ag/HfO2/n-ZnO PTSS was studied.Firstly,different pulse voltages are applied to PTSS using Keilthly 2636 digital source meter to study the current response and recovery characteristics of the tunnel node.It is proved that both the amplitude of pulse voltage and the stimulation time will affect the performance of the device.Through the analysis of experimental data,we know that driven by electric field force,the concentration of positively charged ionic oxygen vacancies near the ZnO interface towards the tunnel interface leads to the reduction of barrier height.Therefore,PTSS exhibits high current characteristics and weakened strain sensing performance when the amplitude and stimulation time of pulse voltage are increased.In addition,we also verified experimentally that the change of pulse voltage polarity does not change its inhibition effect on PTSS performance. |
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