Preparation And Characterization Of Graphene Oxide And Its Resistive Memory

With the development of information technology, non-volatile random access memory (NVRAM) with high speed and high density has become an inevitable trend. A variety of new NVRAMs have been developing rapidly in recent years as the traditional silicon-based memory is reaching its physical and technological limit. ReRAM becomes a most promising candidate in these novel memories due to its advantages of high storage density, low power consumption, fast reading and writing speed, strong endurance, long time data retention, compatible with CMOS process, and, more importantly, good scalability (non-charge storage mechanism).The unit cell of ReRAM has a simple "sandwich"(metal-insulator-metal, MIM) structure, and the middle layer plays a vital role in the device performance. Materials based on grapheme oxide (GO) have either the advantages of organic materials, such as simple and thin film fabrication, low cost, and large area solution processing, or the superiority of excellent device performance of inorganic materials, such as good stability and long life. Hence, this thesis focus on the study of preparation and characterization of GO and its resistive memory.In this thesis, GO sheets were firstly prepared by Modified Hummers’method and then characterized by Atomic Force Microscope (AFM), Raman Spectra, X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), and Thermo Gravimetric Analysis (TGA). Electrical measurements and mechanism analysis of our device were presented after GO film spin-coated on ITO glass and Al top electrode evaporated. Finally, the effects of annealing temperature on GO films and device performace were investigated. The research works and results are as follows:(1) Preparation and characterization of GO plateletsGood dispersion of GO in water prepared by Modified Hummers’ method confirmed the abundant oxygen functional groups attached to GO framework. And the AFM results indicated that its1.1nm thickness and hundreds of nanometers to a few micrometers lateral dimensions revealed the fully exfoliated of graphite. Compared to graphite, stronger D peak, weaker G peak, greater ID/IG and smaller I2D/IG in Raman spectra indicated that sp3average size increased in graphene oxide plane.0.93nm interlayer space of GO obtained by XRD data proved the successful intercalation of GO. XPS analysis indicated that there’re abundant hydroxyl, carboxyl, and carbonyl groups in GO base plane, and hydroxyl group occupies a high proportion. In addition, TGA data indicated that mass loss of GO occurred with the increased temperature, this further confirmed the existence of rich oxygen functional groups in GO.(2) Resistive switching characteristics and mechanism analysis(a) resistive switching characteristicsThe Al/GO/ITO device annealed at150℃exhibited excellent resistive switching characteristics with1.3V set voltage,-2V reset voltage, and about103ION/IOFF ratios.Endurance test indicated that the device has a good repeated reading and writing skills, namely ION/IOFF ratios still maintained constant after100cycles. And the retention time test indicated that the ION/IOFF ratios do not degradated up to104s, which revealed that the device is stable and nonvolatile at room temperature. In addition, compliance current test indicated that reset current density and reset voltage increases with the increased compliance current and the resistance in low resistance state is reduced with the increased compliance current, while no obvious relationship between set voltage and compliance current was observed.(b) mechanism analysisMechanism analysis by electroforming cycle direction indicated that the low resistance state and high resistance state of the devices were stable after positive electroforming cycle was performed, which is artributed to the conversion between sp3hybridization and sp2hybridization or the oxygen migration between the interface layer under voltage bias. While the devices enduring the negative electroforming cycle exhibited WORM characteristic. We considered that the asymmetrical electroforming cycle derived from the asymmetry of the device.The analysis of J-V curve indicated that our ReRAM devices meet the SCLC mechanism, which is in accordance with the mechanism of oxygen migration.Mechanism analysis of electrode cell area dependence indicated that there’s a linear relationship between the current and cell size in both high resistance state and low resistance state. This revealed that the resistive switching occurred in the entire interface. Hence, the mechanism of device isn’t the filament mechanism of transition between sp3hybridization and sp2hybridization, but rather the surface barrier changes derived from oxygen functional group migration under voltage bias.(3) The dependence of GO ReRAM characteristics on the annealing temperature(a) The dependence of GO film properties on the annealing temperatureAFM height analysis indicated that the GO film thickness decreased with the increased annealing temperature, and the roughness analysis revealed that the GO films had a good uniformity annealed at different temperature. The results of Raman spectra indicated that the sp2average area increased with increased annealing temperature. XRD data indicated that the increase in the crystallization degree in GO films after annealing. In addition, XPS analysis indicated that the number of C=C bonds increased gradually with the elevated annealing temperature, corresponding to an increasement of sp2area and a decreasement of oxygen groups.(c) The dependence of GO ReRAM characteristics on the annealing temperatureThe I-V measurements on the devices annealed at different temperature revealed that the original sample and the sample annealed at150℃had the same magnitude of ION/IOFF ratio, while the ION/IOFF ratios for the samples annealed at200℃and250℃decreased to the magnitude of10. This may be originated from the decreasement of oxygen functional groups.