Towards advanced flexible functional electrochemical metallization (ECM)-based reram devices
Date of Issue2018-03-13
School of Materials Science and Engineering
Driven by the innovation and growing demand for the information technologies, new non-volatile memory devices are needed to be scalable and capable with excellent performances. The electrochemical metallization (ECM)-based resistive switching random access memory (ReRAM) device, which is the most promising candidate of the conventional Si-based Flash memory devices, has attracted tremendous attention due to its proven scalability potential, high-performance, and industrialization maturity. Advanced flexible functional memories, which stand for lightweight and printability, are highly desirable for future wearable electronics. Besides the flexibility, the devices with other advanced functionalities such as transparency, portability, and stretchability also have significant advantages and added value in different areas for future electronics’ applications. Therefore, to realize an integrated flexible functional circuit, a very important component of the circuitry elements-the advanced flexible functional memory unit is also in great demand. In this thesis, the research is focused on feasible approaches and decent materials for the realization of advanced flexible multi-functional ECM-based ReRAM devices with high performances. To realize these, the research started from the design of memory devices structure. In the ECM-based ReRAM devices, the growth process of conductive filaments will be affected by the electric field. In order to realize the requirement of high electrical performance memory devices with low electroforming voltage and large ON/OFF ratio, Au nanoparticles (NPs) was inserted between amorphous Si switching layer and Au bottom electrode via facile self-assembly method at room temperature. The inserted Au NPs would modify the electric field distribution in the switching layer to affect the filaments growth, which was confirmed via ex situ TEM. The fabricated memory devices with Au NPs layer exhibited excellent electrical performances, i.e., low electroforming voltage and large ON/OFF ratio on both flexible and rigid substrates at the same time. This work offers a facile method and versatile structure for high electrical performance ECM-based ReRAM applications on both flexible and rigid substrates. In order to realize flexible functional memory devices with excellent bending stability and switching endurance, the two dimensional (2D) hexagonal boron nitride (hBN) with inherent flexibility and excellent thermal stability was exploited as the switching layer for the first time. The ultrathin hNB (~3-5 nm) film was grown on the Cu foil substrate via scalable chemical vapor deposition (CVD) method, where Cu foil can act as both bottom electrode and substrate. The final fabricated Ag top electrode/hBN/Cu foil memory devices exhibited long retention time, reliable switching endurance, and excellent bending stability under bend conditions. This work promotes the 2D materials applications in the flexible ReRAM in future computing technology and wearable electronics. Finally, the advanced flexible multi-functional ECM-based ReRAM devices were demonstrated, which possess flexibility, transparency, and portability. By utilizing the excellent properties of 2D hBN and graphene materials, the indium tin oxide (ITO) top electrode/hBN/Graphene memory devices was fabricated, which exhibited excellent performances in terms of transparency, ON/OFF ratio, retention time, and bending stability. The hBN/graphene film can be easily transferred onto different substrates for transferable memory devices fabrication, such as polyethylene terephthalate (PET) and soft polydimethylsiloxane (PDMS) substrates. The indium filaments were directly found for the first time via ex situ TEM, which broadens our understanding of the resistive switching mechanisms in the ECM-based memory device. In conclusion, the thesis together with the herein encompassed approaches and mechanisms are believed to be able to provide some insightful ideas on the emerging topic of flexible memories and meanwhile it serve as a good guidance for the realization of advanced flexible functional ECM-based ReRAM for the future integrated flexible electronics applications.