The Study Of Graphene Oxide Based Flexible Resistive Switching Memory And Spin Transport

Flexible electronics is a very hot topic recent few years, and has a promising future of application. For example, wearable devices, e-skins, skin sensors and flexible circuits. Most plastic flexible substrates cannot stand very high temperature, however, high temperature is almost necessary for the deposition process of many functional materials. This contradiction hugely limits the development of flexible electronics. As for memory devices, conventional FLASHs have to inject hot carriers under the stress of high voltage (high power consumption), have a slow writing and erasing speed, and bad endurance performance. Therefore, it is necessary to find an alternative. Recently, new non-volatile random access memory attract intensive research attention. As a promising member, resistive switching random access memory (RRAM) gain more and more research efforts because of simple structure, low power consumption, fast writing/erasing speed, good retention performance and scaling down ability. In order to demonstrate a flexible RRAM device, researchers should find a kind of material, which serves as the functional layer and doesn’t need a high temperature deposition process.On the other hand, the geometry configuration of edge carbon atoms in graphene nanoribbon could have a significant impact on the Ⅰ-Ⅴ transportation properties, which inspires us to think that connecting oxygen atoms to edge carbon atoms to form a graphene oxide nanoribbon whether could induce interesting physical phenomenon and application directions.The three main works in this thesis are outlined as followed:1. Through the spin-coating method at room temperature, we successfully fabricated a flexible RRAM with Al/GO/ITO/PET structure. Raman spectrum, X-ray phonon-electron spectroscopy, Atomic force microscopy and scanning electron microscopy were used to characterize the as-fabricated GO film. With Agilent B1500A electrical testing platform, we characterized the electrical performance of as-fabricated GO RRAM. The results show that Al/GO/ITO/PET flexible RRAM has a ON/OFF ratio up to 280, with a high resistance value 224 kΩ and a low resistance value 774 Ω, can stand>100 writing/erasing operation without obvious degeneration and the data can maintain more than 104 second. Varied time-width pulse voltage writing/erasing results indicates that RESET speed is three order of magnitude faster than SET speed, which is opposite to the conventional transition metal oxide. The speed difference could be theoretically illustrated by the carrier injection and the modification of oxygen migration barrier change.2. Using Atomistix TooKit (ATK) software package, which bases on non-equilibrium green function (NEGF) and density function theory (DFT), this thesis construct an atomic scale model to simulate the GO RRAM. The Ⅰ-Ⅴ curves are calculated at three different configuration, such as fully oxidized graphene oxide, oxygen vacancies filament formed and filament ruptured. When the oxygen vacancies formed, the current will increase a lot. However, the break of filament will force the current back to fully oxidized state. This result theoretically demonstrates the current is highly sensitive to the oxygen configuration, and the theory of oxygen vacancy filament is suitable for the explanation of GO RRAM switching mechanism. Moreover, Tunneling-AFM results show the intrinsic resistive switching of GO film with ultra-low pA current and the switching area size under different compliance current limit. Then, the thesis model the resistive switching layer into micro resistance switch unit network, and construct a MATLAB script to calculate the voltage distribution and simulate the switching process. The Ⅰ-Ⅴ curves obtained from model simulation can fit the experimental result well.3. The spin transportation properties of zigzag graphene oxide nanoribbon was investigated using ATK. The modified edge carbon atoms can impact the space resolved density of states and influence the transport of electrons with different spin direction. Theoretical finding show the device can achieve a magneto-resistance up to 104% and can be utilized as spin filter at parallel configuration. The spin polarization ratio could be more than 80% under low bias voltage. The space distribution of density of states and band selection rule could explain the spin-valve and spin-filter mechanism very well.