Synthesis And Switching Properties Of Zinc Oxide Nanorods

Resistive-random access memory (RRAM) has attracted much attention due to its high storage density, simple structure and good compatibility with CMOS process. It is well known that the storage performance of RRAM is closely related to the defects in material. And the storage performance could possibly be improved by controlling defects. Zinc oxide (ZnO) has a large number of intrinsic defects with outstanding storage performance. With the downscaling of electronic devices, the storage density can be drastically improved by using nanoscale material as building blocks. Therefore, it is significant to study ZnO-based RRAM. As an effective method of changing the defects, doping is widely applied in the semiconductor industry. So far, there are only some researches about the switching properties of pure ZnO nanorods, while few of them concern about the influence of doping on the defects and storage performance of the ZnO nanorods. In this thesis, the ZnO nanorods were synthesized with zinc acetate. The influences of seed layers and annealing on the physical properties of the ZnO nanorods were studied. The Cu-doped ZnO nanorods and Fe-doped ZnO nanorods were synthesized with zinc acetate and zinc nitrate respectively. The effects of doping on the physical properties of the ZnO nanorods were also studied. Additionally, the single nanorod based RRAM devices were also fabricated. The effects of doping on the switching properties were investigated and the switching mechanism was qualitatively analyzed.The ZnO nanorods possess hexagonal wurtzite structure with a preferential c-axis orientation. The seed layer can effectively control the alignment and diameter of the ZnO nanorods. The nanorods are randomly aligned on the bare silicon substrate. With the increase in the thickness of seed layer, the diameters of ZnO nanorod become greater with a better vertical alignment. The defects in the nanorods can be reduced by annealing process. Annealing increases the intensities of ultraviolet emission but decreases the visible emission.The Fe-doped and Cu-doped ZnO nanorods are hexagonal wurtzite structure. Copper and iron exist mainly as a secondary phase in the nanorods. The iron addition promotes the growth speed of the nanorods and enhances the UV emission, but weakens the visible emission and the emission peak shifts toward high frequency direction. Furthermore, the iron addition removes the Raman scattering peak related to the defects such as oxygen vacancies. The copper addition accelerates the radial growth of the ZnO nanorods. The lattice constant is decreased, the UV emission is suppressed, and the green emission is increased with the copper addition. According to HRTEM image, the lattice fringes are damaged which indicates that there are many defects in the ZnO nanorods caused by copper addition. XPS results indicate that the percentage of each oxygen component is changed by the copper addition, which produces oxygen vacancies but hampers the chemisorption of oxygen species.Switching properties of single ZnO nanorod indicate that both pure and doped ZnO nanorods exhibit reversibly bipolar resistive switching. For the device with the pure ZnO nanorods, its conduction in the low resistance state is associated with the oxygen-vacancy-assisted filaments, while that in the high resistance states controlled by space charge limited current (SCLC) mechanism. For the Cu-doped ZnO nanorods, the conduction in both the low resistance state and the high resistance state are predominated by SCLC mechanism, in which the oxygen vacancies and the copper-related defects play an important role. The retention performance and the uniformity of programming voltages are thereby improved by the introduced defects.