Electron microscopy observation of resistive switching mechanism in graphene oxide memory

Abstract

Resistive random access memory has emerged as an excellent candidate among next-generation non-volatile memory devices for overcoming the physical limitations of conventional charge-based memory devices due to its simple structure, high density, low power consumption, and fast switching speed. To date, versatile resistive switching phenomena have been reported in various insulating binary transition metal oxides, perovskite oxides, and amorphous Si. Recently, carbon-based materials have been considered as a potential element for resistance-change materials. Among these, electrically insulating graphene oxide with various oxygen-functional groups is a novel material as an active layer in resistive switching memories via reduction process. The electrically insulating graphene oxide could be tuned suitably by thermal or chemical reduction process which is able to detach oxygen functional groups from graphene oxide sheet. Although many research groups have reported on graphene oxide-based resistive switching memories, revealing the origin of conducting path in a graphene oxide active layer remains a critical challenge. In this research, three big categories of memory devices were characterized to reveal the mechanism of resistive switching with electrical path in graphene oxide films. One is the direct observation of Al metallic filament within the top interface layer in Al/graphene oxide/Al memory. The second is graphitic channel in graphene oxide films induced by Al metallic protrusion in the bottom interface layer. Lastly, graphitic filament with highly crystallized graphene sheet is clearly observed under electrical bias using in situ HRTEM. First, we have demonstrated that the formation of Al metallic nano-crystal within the TIL, which is in-duced by the movement of oxygen ions into GO thin films, can be attributed to the microscopic origin of the bistable resistive switching behaviors of our Al/GO/Al memory devices. From cross-sectional atomic resolu-tion TEM images and a quantitative analysis of the corresponding FFT patterns, we showed that the GO thin films are reduced and ordered by the desorption of oxygen ions after Al top electrode deposition. The EFTEM oxygen mapping and atomic resolution TEM images of a sample in the ON state clearly show the formation of an oxygen-deficient region at the amorphous TIL and the growth of Al metallic filaments from the top Al surface. These contribute to the electrical conducting path in an Al/GO/Al memory device. Our findings provide crucial evidence to explain interface-dominant switching models in GO-based resistive memory. Second, We demonstrated nanoscale conductive channels with highly reduced GO platelets induced by local oxygen ion diffusion in Au/GO/Al resistive switching memory devices, using atomic-resolution TEM images and EELS spectra. Supportively, we visualized the chemical transition of GO films during the set and reset process from a Raman intensity ratio map of GO films, confirming the formation of highly reduced GO regions. It was also unambiguously revealed that Al metallic protrusions, which are generated in the BIL at the early stage of the set process, can assist the local formation of conductive graphitic channels directly onto GO films by generating a local strong electric field around the Al protrusions. We believe that our resistive switching mechanism, based on the migration of oxygen ions in GO-based memory, could shed light on the understanding and improvement of future carbon-based nanoelectronic devices. Lastly, The dynamic motion of oxygen ions related to the crystallization of GO layer under electric field was clearly characterized using in situ TEM technique. Although Pt/GO/Pt devices show bubble destruction applying the negative bias without resistive switching due to the supersaturation of oxygen ions, similar trend was not occurred in Pt/GO/Pt TEM-sized nanodevices during repeated experiments. This phenomenon is gen-erally explained by dimension of the device and characterization condition. Pt/GO/Pt and Pt/bilayer-GO/Pt devices show the resistive switching behavior under negative polarity, although the reversible switching was not occurred. The thickness of GO films in Pt/GO/Pt device was decreased, and the crystallinity of GO sheets was improved dramatically after resistive switching due to the drift of oxygen ions. In addition, we clearly observed the formation of graphitic filament composed of highly reduced GO sheets using in situ HRTEM technique. The crystallization of GO sheets is closely related to the drift of oxygen ions bonded to GO sheets. The reduction of GO films starts from the top region because the negative bias repels the negatively charged oxygen ions. The reduced top region is expanded and the locally reduced bottom region occurred. Finally, the connected graphitic filament was formed between two electrode s after both regions grow at each other. We believe that real time observation of GO filament growth directly related to oxygen movement could shed light on the future carbon-based nanoelectronic devices.