Investigation of switching mechanism in HfO_2-based oxide resistive memories by in-situ Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy (EELS)

摘要

To meet the increasing need for data storage, several kinds of memories called random access memories (RAM) are being developed. Oxide resistive RAM (OxRRAM) with metal oxides are considered as one of the most promising candidates for replacing FLASH technology in the next memory generation. OxRRAM are based on resistance changes under an applied electrical stress. Their principle relies on the ability of the oxide to reversely change its resistivity from a High Resistance State (HRS) to a Low Resistance State (LRS) during SET and RESET operations after an initial forming step. Forming and breaking a nanometer-sized conductive area is commonly accepted as the physical phenomenon involved in the switching mechanism of OxRRAM [1]. Nevertheless, the nature of this filament is still highly debated as switching from both oxygen vacancies [2] and metallic migrations from the electrode [3] have been experimentally shown. In this study, we propose to investigate a state of the art OxRRAM device by in-situ Transmission Electron Microscopy (TEM). Combining high spatial resolution obtained with a very small probe scanned over the area of interest of the sample (STEM) and chemical analyses with Electron Energy Loss Spectroscopy (EELS), we were able to compare the local chemical state of the device before and after applying an electrical bias. This in-situ approach allows simultaneous TEM observation and memory cell operation. In situ observation is important as the study of devices that have been switched ex-situ can be misleading, for example, the filament may be modified during specimen preparation or the filament may not even be within the thin TEM lamella that is observed.