Research On Resistive Switching Memory Based On Zinc Oxide Film

With the popularization of portable equipments,such as mobile phone,laptop,and with the fast development of Internet technology and new intelligent electronic products,the nonvolatile memory(NVM)plays an irreplaceable key role in the semiconductor industry.Flash memory plays a main role and occupies a large share of the current NVM markets.As the advancement of semiconductor technology,flash memory is facing technical bottleneck.The scientific and industry community have made a lot of research in NVM.Lots of research results indicated that resistive switching random access memory(RRAM)is one of the promising candidates for the next generation NVM application.RRAM has many quilities at the same time:high-speed,high-density,low power consumption,etc.The researchers focus on three areas:designing appropriate structure of RRAM,seeking for compatible switching media and finding out the physical mechanism of resistive switching.In this dissertation,some oxide films such as ZnO,CuO,Ni O thin film were deposited by radio-frequency magnetron sputtering and two types of RRAM devices,anion-migration type and cation-migration type,were fabricated with these materials.In addition,ZnO nanorods and microwires were synthesized via thermal evaporation.Based on this,unipolar RRAMs were fabricated,too.Resistive switching(RS)performance was tested on all RRAM devices and resistive mechanism was explored.The results show that the formation/rupture of Ag nanofilaments in single layer amorphous ZnO thin film based on the electrochemical metallization mechanism are responsible for the bipolar resistive switching(BRS)in the device of Ag/a-ZnO/Pt.During switching,the current shows an abrupt increasing process during the set process while a gradually continuous Low-Resistance-State(LRS)to High-Resistance-State(HRS)transition during the reset process.the resistance ratio of high to low resistance states(HRS/LRS)more than 10⁷.The BRS behavior exhibits a large R_OFF/R_ON ratio(>10⁷),stable endurance,and long retention(>10⁶s).Secondary resistive switching behavior is also be found while the compliance current is increased.We also fabricated the two kinds of RRAM devices(Pt/a-ZnO/Pt and ITO/a-ZnO/ITO),different in electrode materials,the same in switching media,which are based on anion-migration type.In addition,the structure of Ag/a-ZnO/Pt devices is to be improved with crossbar electrodes and flexible substrate.The RS parameter of RRAM device with single-layer storage medium is often discrete.We carry out a comparative study on resistive switching in CuO/ZnO bilayer films;both samples grown Pt and Ag electrodes show BRS behaviors.We conjecture that the formation and rupture of conducting filaments are responsible for the switching effect in both Pt/CuO/ZnO/Pt and Ag/CuO/ZnO/Pt.Filaments induced by migration of oxygen ions are responsible for RS with the Pt electrode.In contrast,RS with the Ag electrode is attributed to the migration of metal cations and the corresponding electrochemical metallization.The two kinds of physical mechanisms are the same with Pt/a-ZnO/Pt and Ag/a-ZnO/Pt,respectively.But the RS characteristics of CuO/ZnO-bilayer-devices were improved obviously.Better repeatability on cycling tests and smaller discreteness of HRS and LRS.It is also found that Ag/Ni O/ZnO/Pt devices have equal performance with Ag/CuO/ZnO/Pt devices via replacing CuO by Ni O.In addition,the Ni/ZnO/Cs Pb Br3/FTO device shows nonvolatile bipolar resistive switching with high on/off ratio(>10⁵),low operation voltage(<1 V),long data retention(>10⁴s)and high stability.Furthermore,light induced resistive switching effect of more than 10³ with a rapid response speed(<1 ms)makes information storage and processing be conducted simultaneously,indicating promising potentials for future better computing that goes beyond traditional architecture.The RS behaviors of nanomaterials were investigated on Cu/ZnO nanorods/Cu,Ag/ZnO single-microwire/Ag and Ag/ZnO NWs/ZnO TF/Pt devices.Both Cu/ZnO nanorods/Cu and Ag/ZnO single-microwire/Ag devices exhibit unipolar resistive switching(URS)characteristics.ZnO nanorod memory is stable,rewritable,and nonvolatile with on/off ratio up to 100.Both V_set and V_reset are less than 5V.As for ZnO single-microwire memory,both voltages are less than 1V,indicating the smaller operating voltage the smaller power consumption.The resistance ratios of HRS to LRS reached 10³ with programming speed of 110?s.The bistable URS behaviors were entirely reversible and steady within 100 cycles.Based on TEM results,it was found that the dominant conduction mechanisms in LRS and HRS were Ohmic behavior and space-charge-limited current(SCLC),respectively.Also,ZnO single-microwire memory exhibits multilevel resistive switching,it should have a great potential in multilevel storage applications.For Ag/ZnO NWs/ZnO TF/Pt memory device,bipolar resistive switching behaviors with memory window of 10⁴ were observed.A mechanism based on the joint contributions of oxygen vacancies and Ag atoms in the conducting filament explains the resistive switching behavior.Additionally,we prepared Ag/Ag:ZnO/Pt and Ag/BN/Pt memristors.The two kinds of memristors emulate several essential synaptic behaviors,including nonlinear transfer,short-term plasticity,pair-pulse facilitation,long-term potentiation or depression.These results suggest the two kinds of memristors can be a promising key component for artificial synapses devices.