Modeling and Simulation of an Oxygen Exchange Induced Resistive Switching in a Two Layer Valence Change Memory Cell

Some performances of the resistive switching mechanisms in Redox-based Random Access Memory (ReRAM) Valence Change Mechanism (VCM) devices are listed to make it one of the candidates for the future innovation of non-volatile memory application. There is a need for detailed and precise explanations of their microscopic and nanoscopic behaviors. Hence, in this thesis, features of the complex dynamics of bipolar resistive switching of ReRAM devices, more specifically VCM resistive memory devices with HfOx/TaOx bilayer stacks, are broughtto light. A series of one-dimensional simulations with the finite element method is carried out using COMSOL Multiphysics software to investigate the impact of applied external biases on internal prop- erties such as oxygen vacancy concentration and conduction band energies. These are done by two different approaches direct implementation differential equations and using built-in physical models from COMSOL Multiphysics software. From the results, it can be seen that there is a great change in the internal properties of the device with the application of the pulsed voltages, and the changes depend on the magnitude, polarity, and duration of the applied bias. In other words, the effects of magnification of the device’s conductivity on voltages and duration, respectively, bring distinct resistance states to the device which may be either a high resistance state or a low resistance state. More specifically, this can be seen through the characteristics of the plotted device resistance versus the oxygen vacancy concentration. It is possible to observe that the increase in oxygen vacancy concentration in the conducting oxide layer is always accompanied by an increase in device resistance. In this work it is tried to reproduce these findings using the Semiconductor Module of COMSOL Multiphysics. Yet, there are certain problems in modeling the ions as charge carriers and the phenomenon of direct tunneling. In order to prevent these problems, the possibility of enhancing the methodology of simulation through the development of new advanced physical models and modules is worked out. It goes on to examine the effect of different electrode materials on device behavior in such a manner that symmetric compositions of electrodes would cause symmetrical oxygen vacancy distributions within the device. The material selection and their corresponding dependence on electrode metal work functions further underscores the importance of material selection in device design. Overall, this thesis sheds light on the complicated nature of VCM ReRAM devices, in the context of simulation techniques and research in general, in order to exploit their potential to the hilt with rising memory technologies.