Ferroelectric Resistive Switching Behavior And Regulation Of Pb(Zr,Ti)O₃/Nb:SrTiO₃ Heterostructures

Ferroelectric/semiconductor heterostructures have received intriguing attentions for the potential applications in ferroelectric photovoltaic,ferroelectric energy storage and ferroelectric resistive switching devices.As one of the most promising candidates of next-generation nonvolatile memories,ferroelectric resistive switching devices show high storage density,fast operating speed,low power consumption and large cycle endurance.In general,it has been widely accepted that the regulation of depletion layer width at the interface of ferroelectric film and semiconductor electrode plays an important role in ferroelectric resistive switching.However,owing to the existence and interactions of many effects,such as discontinuity of ferroelectric polarization,lattice mismatch and segregation of defects,the physical mechanism of ferroelectric resistive switching in ferroelectric/semiconductor heterostructures has not been fully understood.Besides,the performances of ferroelectric resistive switching devices are still too poor to satisfy the practical application.In this work,epitaxial Pb(Zr0.4Ti0.6)O3(PZT)thin films were grown on Nb-doped SrTiO3(NSTO)single-crystal substrates by chemical solution deposition,and their ferroelectric resistive switching behaviors were investigated.In addition,the modulations of ferroelectric resistive switching were successfully achieved by changing the Nb doping concentration of NSTO,the thickness of PZT thin films and the incorporation of gold nanoparticles.The main results are summarized as follows:PZT/NSTO heterostructures with different Nb doping mass fractions of NSTO were investigated.By analyzing the P-E hysteresis loops and C-V curves,the maximum tunability of depleiton layer width(DLW)is achieved with the Nb doping mass fraction of 0.5%.On the basis of the modified capacitance model and the ferroelectric semiconductor theory,we suggest that improving the tunability of DLW demands for the less depletion layer charges and the more ferroelectric polarization charges.However,the Nb doping mass fraction is proportional to depletion layer charges in NSTO and ferroelectric polarization charges of PZT.Thus,the tunability of DLW can be attributed to a delicate balance of the depletion layer charges and ferroelectric polarization charges.Our results reveal that the performances of ferroelectric/semiconductor devices can be improved by an appropriate doping mass fraction of semiconducting electrode materials.PZT thin films with different thicknesses were deposited on NSTO substrates,and their ferroelectric resistive switching behaviors were investigated.The results showed that the maximum ON/OFF ratio up to 850 can be obtained in the PZT/NSTO heterostructure with the 150-nm-thick PZT film.Based on the Schottky-Simmons model and the modified semiconductor theory,we also evaluated the interfacial build-in field and the depletion layer at the PZT/NSTO interface.The interfacial build-in field and the depletion layer can be modulated strongly by the ferroelectric polarization,but are independent with the thickness of the PZT thin films.It is clear that the ferroelectric resistive switching is related to the ferroelectric polarization and is modulated by the thickness of ferroelectric films.When the PZT film is too thin,the depletion layer occupies a considerable proportion in the PZT film,so that the modulation effect of ferroelectric polarization on the depletion layer becomes smaller.On the contrary,although the modulation effect of ferroelectric polarization on the depletion layer may become powerful in the thicker films,the conductive property of PZT films tends to decrease with increasing their thickness.In this situation,the volume resistance becomes crucial than the interfacial effect.Therefore,there is an optimal thickness of the PZT film for the maximum ON/OFF ratio due to the ferroelectricity and conductivity restrict mutually.These results are of great significance for the devices of ferroelectric/semiconductor heterostructure to further improve their resistive switching properties.Au-PZT nanocomposite thin films were prepared by a simple one-step chemical solution deposition(CSD)method,and the effect of Au NPs on the ferroelectric properties was systematically studied.The experimental results show that the ferroelectric remanent polarization of the Au-PZT films with Au mole fraction of 1.2%is about 80 ?C/cm2,which is 50%higher than that of the pure PZT thin films.The dielectric constant is also increased by 120%.Based on the analysis of chemical valence,the enhancement in electric properties can be ascribed to the charge transfer at the PZT/Au interfaces,which makes the PZT lattice tend to be perfect.Our results suggest that adding an appropriate amount of gold nanoparticles is an effective way to enhance the ferroelectric properties.Au-PZT nanocomposite films were fabricated on NSTO substrates to form the PZT/NSTO heterostructure.Comparing the PZT/NSTO heterostructure,it is found that the switching ratio of Au-PZT/NSTO heterostructure is increased by 10 times and the conduction current density is increased by one thousand times larger.Through the fitting of ?-? curves of these two heterostructures,and the XPS and UPS characterization,it is found that the enhancement of switching ratio in the Au-PZT/NSTO heterostructure is due to the increase of the ferroelectric properties of the nanocomposite thin films and that the Au nanoparticles can effectively regulate the band structure of the heterostructure,thus changing the conduction mechanism of the heterostructure.