Book Description

Resistive Random Access Memory (ReRAM) has attracted significant attention recently, as it is now considered as the promising candidate for the next generation of non-volatile memory device, due to its high density, low operating power, fast switching speed, and compatibility with conventional CMOS process. Among many resistance switching materials, TiO2 has been widely studied. However, the most challenging issue is that the underlying switching mechanism is lacking in-depth understanding. It has been proposed that the resistance switching is strongly coupled with the presence and a preferential distribution of oxygen vacancies involving the formation of a conductive filament. Although many experiments have been done to address the switching mechanism during the last decade, it is hard to figure out what happens at microscopic level. Therefore, systematic interpretation about the microscopic details of the role of oxygen vacancies in the formation of a conductive filament is essential. To address the conduction and the resistance switching mechanism, the effect of oxygen vacancies on the electronic structures in TiO2 has been investigated using first principles calculations based on density functional theory. In this dissertation, we report "ON"-state (Low Resistance State) conduction mechanism of rutile TiO2 including oxygen vacancies, and then the transition from "ON" to "OFF"-state (High Resistance State) is investigated. Although it is known that TiO2 exhibits n-type semiconducting property with extra electrons generated by the formation of oxygen vacancies, "ON" and "OFF"-state conductivity during resistance switching cannot be explained by isolated single oxygen vacancy. We calculated electronic characteristics such as density of states, electron localization function, band decomposed charge density distribution, and energy band structure, and show the influence of oxygen vacancy configurations on these properties and on the resistance change. Oxygen vacancy ordering and diffusion of either oxygen vacancy or hydrogen impurities have a significant impact on both the formation of the conductive filament and the transition from "ON" to "OFF"-state. Results from this study indicate that the "ON"-state conduction and resistance switching model that can be ascribed to the formation and rupture of conductive filament consisting of oxygen vacancy-ordered structure.