Research On Resistive Switching Mechanism And Performance For Non-volatile Resistive Switching Memory

With the development of semiconductor technology node, the Flash memory confronts many problems, such as the coupling between floating gates, charge leakage, cross-coupling between cells and so on. Therefore, it is essential to explore the next generation of non-volatile memory. At present, the explorations mainly concentrate on the followings:phase random access memory (PRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM) and resistive random access memory (RRAM). Moreover, due to the advantages including simple structure, low power consumption, high density, fast program/erase speed and compatibility with CMOS technology, RRAM is regard as the most promising candidate for the next generation non-volatile memory to replace Flash memory.Now, the uncertainty about the resistive physical mechanism becomes the bottleneck impediment to the development of RRAM. For the device, there are still some aspects that could be enhanced, such as stability, speed and so on. The study on RRAM is mainly based on the experimental research, microstructure characterization and real-time monitoring. It is obvious that the analytical tools above are time and money consuming. In this paper, we adopt first-principle method to have an intense simulation about RRAM device. First, we study the physical mechanism of carrier transportation and storage in the resistive material. Then we discuss the performance improvement by doping, making defect and controlling valence state. Finally, we quantify the correlation between material micro-parameter and RRAM device macroscopic property, thus guiding the design and optimization of RRAM device.With a comprehensive review of the developments in RRAM research, we have got an overview about RRAM. In this paper, we explore the role of oxygen vacancy (Vo) and doped Ag when conductive filaments form in HfO2. The results show that Vo and Ag can form conductive filaments alone by inducing impurity levels in the gap. Then we study the composite system, i.e. Vo and Ag co-exist in HfO2, it is found that both conductivity and stability are enhanced. At the same concentration, Ag can form filament with the of Hf; however, Vo fails. For the composite system, the performance enhancement benefit from the existence of Vo. One step further, Vo assist Ag to migrate in HfO2, thus improving the conductivity and stability.Electrode and resistive materials will directly affect the performance of the composite material and the interface is especially crucial to the performance of RRAM device. So we set up different Cu/HfO2interface models and compare their properties. The results show that different interface will have a great impact on the performance of RRAM devices. For all potential interface models, the mismatch ratio and the interface adhesion energy implying that Cu (111)/HfO2(010) is the most stable. Furthermore, Cu(111)/HfO2(010) is the only case that can form connective electronic channel along the vertical direction of the Cu electrode. And the electrons transfer mutually and bond at Cu(111)/HfO2(010) interface indicate that electrons possess the localizability and connectivity along the direction, which corresponds to the switching-on direction of the RRAM device. For Cu(111)/HfO2(010), we research the impact of electrode material on the interface and resistive material. The results show that the closer Cu or Vo defect is to the interface, the easier it is to form. And Cu atoms can migrate into HfO2more easily. This indicates that the electrochemical reaction takes place more easily under the biased voltage, resulting in the formation and rupture of Cu conductive filaments.The fact that doping can improve the electrical characteristics of metal oxide resistive materials is widely understood. But the effects of the defect valence state are still unknown. Therefore, we pay a close attention to the defect valence state in HfO2. Firstly, based on the analysis of electron affinity and formation energy, we grasp the gain and loss of electrons in the doped element; secondly, we focus on the electrons location by charge density difference and modified Bader charge. Finally, we draw an instructional conclusion about the influence of different valence state on the performance of resistive material.The results will provide theoretical guidance to design and improve the performances of RRAM devices. It definitely will help to understand and develop RRAM toward a higher stage.