The Resistive Switching Of The As-fabricated PZT Film And ZnO Nanowire Array

Recently,the requirement of novel non-volatile random access memory with high-capacity and high write/read speed is urgent needed with the development of portable mobile devices and high-definition video/image technology.The switchable ferroelectric diode effect found in BiFeO 3 crystal could not only avoid the instability of the resistive random access memory(RRAM)based on the conductive filament,but also inherit the high speed of ferroelectric random access memory(Fe RAM)and the non-destructive read-out of RRAM.Although the density functional theory(DFT),phase fie ld calculation and self-consistent model have predicted that the switchable ferroelectric diode effect should be found in all ferroelectrics,this novel read-out method is only made out in BiFeO3,even some scientists deduce that the switchable diode-like behavior is unique effect belonged to BiFeO3 because its band gap is smaller than Pb(Zrx Ti1-x)O3(PZT).In this paper,PZT thin film was deposited on Pt substrate prepared by Sol-Gel method,then the top electrode Au was deposited on the as-grown thin film.Next,the current-voltage(J-V)after pre-polarization and the filping the polarization would be investigated,then the polarization flipping details would be researched.Finally,the physical mechanism of switchable ferroelectric diode e ffect in PZT and the effect of remnant polarization and temperature on switchable diode-like electrical conduction have been discussed.In the last chapter,a novel deposit ion of ZnO /ZnO :Ov nanowire array with high aspect ratio induced by polar-surface dominated growth would be introduced and the resistive switching induced by oxygen vacancies has been studied.The detail results is shown as following:PZT thin film was prepared by Sol-Gel method on Pt substrate.X-Ray Diffraction(XRD),scanning electron microscope(SEM)and atomic force microscope(AFM)were used to analyze the crystal structure and surface morphology of as-grown PZT thin film.After excluding the effect of leak current on ferroelectric polarization by ferroelectric tester,the results showed that polarization reach saturation and the remnant polarization is 30.2μC/cm2 while the coercive field is 3.2V(53.3k V/cm),which indicate that the as-grown PZT thin film have perfect ferroelectricity.The electrical conduction measured in coercive field shows diode-like behavior after the PZT thin film is pre-polarized by +5V(Pdown)and-5V(Pup).When the polarization is in the direction to Pt electrode,the threshold voltage of diode is at negative voltage.Nevertheless,when the polarization is flipped to Au direction,the threshold voltage of diode is at positive voltage.Since the direction of threshold voltage is always opposite to the direction of polarization,the switching current measurement shows that the polarization is not flipped at the moment of JV test.The switchable ferroelectric diode effect is determined by the polarization reversal,which induces the conversion between schottky contact and metal contact.With the increases of remnant polarization,the forward current decrease and the polarization dependence of Pt/PZT/Au conduction act as a combined effect of domain wal s conductivity and barrier height modulated by polarization.With the increases of temperature,the forward current increases due to the amount of carries emitted by the heat goes up.When the temperature reaches 400 K,the J-V curves are almost symmetric no matter the sample is poled at positive voltage or negative voltage.The conduction mechanism analys is indicates that the current follows the O hm’s law at 150 K and 200 K.However,the electrical conduction conforms to space charge limited conduction model when the temperature is near room temperature(250K,300 K and 350K).Shallow hole trap levels induced by Pb3+ is discrete and about 0.28 eV above the valence-band edge.At 400 K,the schottky emission dominates the conduction in PZT thin film.Growth of single crystal ZnO /ZnO :Ov nanowire array with the aspect ratio of 75 via carbon reduction method with nanorods in c axis direction on Si substrate is induced by polar surface on the top of nanorod due to minimum electrostatic energy.The oxygen density of ZnO nanowire via carbon reduction is much higher than the nanorod via hydrothermal method.Owing to the valence change memory e ffect,resistive hysteresis is found in Si/ZnO /Ag structure.The resistive ratio is 106.8 and 21.9 when the voltage of read-out and write is 1V and 2V,respectively.Switchable ferroelectric diode effect found in PZT thin film proves that this novel read-out technology is not unique.Based on the research on the effect of remnant polarization and temperature on switchable ferroelectric diode effect in this paper,the result indicates a nother direction for designing me mory.The study on the deposition method of ZnO nanowire induced by polar surface offers a method with low cost,high growth efficiency and high repeatability for the integration of micro-nano device based on ZnO nanowire.The research on RRAM with Si/ZnO /Ag structure have offered enough experiment details for the investigation of single ZnO nanowire based RRAM.