Resistive random access memory [electronic resource] : doping technology, pulse characterization and scalability

Recently, the rapid development of big data and internet-of-things has fueled a surge of demand for semiconductor memories. As the scaling of NAND flash is hitting its physical limits, several emerging non-volatile memory technologies are being studied intensively to enable higher memory density and better performance. Among them, resistive random access memory (RRAM) has attracted tremendous interests due to its ability to overcome the inherent limitations of flash memory, while also delivering cost-effectiveness, robust performance and small footprint. Despite its promising features, several challenges remain to be addressed for the future development and commercialization of RRAM technology. First of all, the physical mechanisms behind resistance-change phenomena have not been fully understood, making it difficult to optimize the device performance. Secondly, the reliability of RRAM should be improved in several aspects, such as the variability of switching parameters, the retention/endurance failures caused by the random nature of filament formation, as well as the requirement for a high-voltage forming process. Moreover, it is also highly desired to stack RRAM devices in a 3D architecture and/or develop multi-level storage capability to reduce the cost-per-bit and compete with NAND flash. This thesis presents an in-depth analysis of some state-of-the-art techniques to tackle these challenges from three aspects: the materials, the device structure, as well as the characterization methods. From the material's perspective, doping technology of RRAM is investigated as an approach to improve RRAM performance. Ab initio modeling and simulations are applied to study the effects of dopant types, dopant concentrations, oxide phases, and oxide stoichiometry on the electronic and thermodynamic properties of oxygen vacancies in HfO2. The physical insights derived from the calculations provide guidelines to achieve desirable RRAM characteristics through doping. In the aspect of electrical characterization, the pulse-train characterization techniques are developed for the multi-level control and in-depth physical understanding of conductive filament evolution. By adopting pulse-train operation for an RRAM device with 3-bit potential, the relative standard deviations of resistance levels are improved up to 80% compared to the single-pulse scheme. The observed exponential relation between the saturated resistance and the pulse amplitude provides supporting evidence for the gap-formation model during the RESET process of RRAM. From the device-structure point of view, the feasibility of ultra-thin HfO2 RRAM is investigated, which helps to achieve the forming-free property and low-power operation. The theoretical scaling limit of HfOx thickness is first estimated using density functional theory within the non-equilibrium Green's function formalism. The feasibility of 2-nm HfOx RRAM is predicted for large-area devices, and verified by fabricating both planar and 3D vertical RRAM devices. The 3D ultra-thin devices demonstrate promising characteristics including ON/OFF ratio (~100), switching speed (~20 ns), endurance (108 cycles) and data retention (> 10 years at room temperature). In contributing to these areas, this thesis aims at advancing both the fundamental understanding and practical implementation of RRAM technology, towards the vision of high-density mass-storage applications.

Keywords: Electrical Engineering.
Advisor: Nishi Yoshio
Wong Hon-Sum Philip
Wong S.
Yu Zhiping