Study On The Resistive Switching And Photovoltaic Effects Of Ferroelectric Films

At present,the researches of new photoelectric devices and resistive switching random access memory(RRAM)are the advanced hot research subject in the field of functional materials.The discovery of ferroelectric photovoltaic and resistive switching(RS)effects in multiferroic material extends its physical connotation and application fields,and its intrinsic ferroelectric polarization can effectively modulate photovoltaic effect,RS effect and their coupling effects,which has important scientific significance and application prospect.In this dissertation,we have made in-depth research on some key issues of materials and device application in ferroelectric RS and photovoltaic effects,and explored the effects of the composite materials,doping or structure optimization on the performance of the films and devices.The relationship between the ferroelectric polarization and the photovoltaic and RS properties of the ferroelectric films were studied,and the ferroelectric polarization modulating the photovoltaic and RS properties of the heterostructure were demonstrated.The interaction between the RS and photovoltaic effects in the ferroelectric films has been studied,and the coupling of ferroelectric/RS/photovoltaic and their physical mechanism were explored,which provided the research basis for developing the new generation of ferroelectric optoelectronic devices and non-volatile RRAM.The main results of this study are listed as follows:1.BiFeO3(BFO)and Au nanoparticles composite BFO films were deposited on Pt substrates by sol-gel technique.Studies indicate that the surface plasmon resonance(SPR)effect of Au nanoparticles can be used to convert the far field excitation of solar light into near field excitation,which can effectively enhance the light absorption efficiency of BFO films and the separation efficiency of photogenerated carriers,and finally realize the obvious enhancement of BFO ferroelectric photovoltaic performance.Furthermore,the effect of ferroelectric polarization on the ferroelectric photovoltaic was studied,and the corresponding mechanism was analyzed.This research provides a new way for the application of ferroelectric photovoltaic cells and optoelectronic devices.2.The Bi3.15Nd0.85Ti3O12 films with different thickness and Nd doped Bi(4-x)NdxTi3O12(BNxTO,x=0,0.15,0.5,0.85 and 1.0)were deposited on Pt substrates by sol-gel technique,and the effects of film thickness and Nd doping content on the photovoltaic characteristics were studied.The results indicate that the photovoltaic performance of BNxTO film is best when the thickness is about 400?450 nm and the Nd content is x=0.85,and the photovoltaic performance can be regulated by ferroelectric polarization.At the same time,ITO/BN0.85TO/Pt heterostructure exhibits typical stable bipolar RS effect and the Electro-Optical dual modulation on the RS.Analysis proposed that the interface Schottky barrier controlled by ferroelectric polarization is the reason for the generation of the ferroelectric control of the photovoltaic and RS effects.The results of this study show that the appropriate film thickness and doping content are effective ways to improve the photovoltaic performance of ferroelectric films,and also proved that BNTO thin films have potential applications in non-volatile memory;3.BFO films and Ag2O nanoparticles composited BFO films were deposited on Pt substrates by sol-gel technique.The results indicate that the two films have superior photovoltaic effect,and wide band gap BFO based composite narrow band gap Ag2O nanoparticles can effectively broaden the range of light absorption and enhancement the absorption efficiency of visible light,so as to realize the photovoltaic effect significantly enhanced.Further studied on the RS characteristics of ITO/BFO/Pt heterojunction device,the device exhibit typical bipolar RS effect with multi-level resistance states,good retention and endurance.Analysis shows that the RS characteristics can be ascribed to the Ohmic conductance controlled by the migration of oxygen vacancies in BFO films and the bulk effect of SCLC conductance.The results extend the application of BFO as multiferroic material,increasing the additional freedom of BFO based multi-functional devices.4.Pt/BFO/FTO and Pt/BTO/BFO/TiO2/FTO heterostructures was fabricated on FTO substrates by sol-gel technique.The comparative study of the photoelectric effect of the two heterostructures was carried out at different polarization states with light On/Off.The results show that the multilayer BTO/BFO/TiO2 heterostructure can enhance the light absorption efficiency and improve the ferroelectric property compared with that of the BFO film.Meanwhile,the TiO2 and the BTO layer act respectively as electron and hole collection/transferring layers,which effectively separate the photo-generated electron-hole pairs,which contribute to comprehensive enhancement of the photovoltaic performance.The results indicate that the Pt/BaTiO3/BiFeO3/TiO2/FTO heterostructure exhibit stable bipolar resistive switching behavior,good retention and fatigue characteristics.Four stable resistance states are realized under different pulse voltage and light field modulation.Analysis shows the SCLC is the main conduction mechanism of the multilayer heterostructure,and we speculated that the SCLC effect controlled by oxygen vacancy defect and modulation effect of different polarization fields on the depletion layers and the barrier height of the film interfaces can form conductive filaments combined with the P-E properties and band structures characteristics,while the new additional RS formed by optically controlled photo-induced carriers increase new dimensions for resistance state modulation.Therefore,the research provides a promising way for the following research of ferroelectric photoelectric devices and should have potential applications value to develop multi-state nonvolatile RRAM devices based on ferroelectric materials.