Investigations On Resistive Random Access Memory Devices Based On TiO₂ Nanowires

Memristor is considered as one of the potential candidates for the next-generation nonvolatile memory applications,due to its merits of low power,high speed,high density and nonvolatility.In this dissertation,the single-crystalline rutile Ti O2 nanowire,the single-crystalline anatase TiO2 nanowires and the anatase/rutile TiO2 heterostructures have been prepared on the FTO substrates by hydrothermal method,respectively,and the TiO2 nanowires-based memristors with sandwiched structures have been fabricated with these materials.In addition,the resistive switching characteristics and resistive switching mechanisms have been investigated and the resistive switching models of the as-fabricated devices have been proposed.Firstly,the single-crystalline rutile TiO2 nanowire arrays have been prepared by a facile one-step hydrothermal method on the FTO substrates.The nonvolatile bipolar resistive switching behaviors have been observed in the as-fabricated Au/TiO2(101)NWAs/FTO and Au/Ti O2(002)NWAs/FTO sandwiched structures.Meanwhile,the resistance ratios of Au/TiO2(101)NWAs/FTO and Au/TiO2(002)NWAs/FTO devices are 1.08 ? 102 and 2.94 ? 102 at 0.1V,respectively.Compared with the Au/TiO2(101)NWAs/FTO device,the much higher resistance ratio and multilevel switching behavior have been observed in the Au/TiO2(002)NWAs/FTO device.In addition,the conduction mechanism of the low resistance state is dominated by the ohmic conduction mechanism,while the trap-controlled space charge limited current conduction(SCLC)mechanism and Frenkel-Poole conduction mechanism dominate the high resistance state.On the basis of the XPS observation and I-V measurement,we have proposed the resistive switching model in which the filamentary conduction mechanism associated with the redistribution of oxygen vacancies has been suggested to be responsible for the resistance switching behaviors of the as-fabricated devices.Secondly,a one-step low-temperature hydrothermal process has been reported to synthesize the single-crystalline anatase TiO2 nanowires on the FTO substrates.The as-fabricated Au/TiO2 NWAs/FTO device displays nonvolatile bipolar resistive switching behavior.Meanwhile,the resistance ratio between high resistance state and low resistance state exceeds 1.80 ? 102 at 0.1 V,which can be maintained over 103 s without significant degradation.It demonstrates that the Au/TiO2 NWAs/FTO device indicates excellent nonvolatile properties.Furthermore,the conduction mechanisms of high and low resistance states can be explained by the trap-controlled SCLC mechanism.From the I-V fitting and XPS measurement,the resistive switching model of the Au/TiO2 NWAs/FTO device has been developed,and the resistive switching mechanism could be attributed to the trap-controlled SCLC mechanism which is controlled by the localized oxygen vacancies.Finally,the resistive switching characteristics and device structures of the as-fabricated rutile and anatase TiO2 nanowires-based memristors have been further improved.A two-step hydrothermal method has been reported to fabricate the anatase/rutile TiO2 heterostructures on the FTO substrates.The as-fabricated Au/Anatase/Rutile TiO2/FTO device exhibits nonvolatile bipolar resistive switching behavior.The resistive ratio of the high resistive state to the low resistive state is more than 0.82 ? 103 at 0.1 V,which indicates that the as-fabricated device reveals excellent data retention characteristics.Furthermore,the conduction mechanism of the high resistance state is governed by the trap-controlled SCLC mechanism and Schottky emission mechanism,whereas the ohmic conduction mechanism dominates the low resistance state.According to the XPS observation and resistive switching behaviors,the resistive switching model of the Au/Anatase/Rutile TiO2/FTO device has been proposed,and the resistive switching mechanism could be ascribed to the formation and rupture of the conductive filaments related to oxygen vacancies and oxygen ions.In particular,the Au/Anatase/Rutile TiO2/FTO device exhibits higher resistance ratio,lower threshold voltage and lower power than the rutile and anatase TiO2 nanowires-based memristors.It indicates that the Au/Anatase/Rutile TiO2/FTO device may be a promising candidate for the future nonvolatile memory applications.