Study of Electrical Properties and Switching Behaviors in Ag/Ta2O5-Based RRAM devices

Autor: Chang, Chia-Fu, 張家富
Rok vydání: 2018
Druh dokumentu: 學位論文 ; thesis
Popis: 106
Resistive random access memory (RRAM) has become an important development of next-generation nonvolatile memory, which had been widely researched. In particular, electrochemical metallization mechanism (ECM) -based RRAM composed of the active metal (Ag, Cu) electrode received great attention. The reason was that this device showed highly conformability to CMOS process; however, the durability of this device was lower in the process of repetitive switching operation. It is an important issue that how to improve the reliability of this device. In addition, many reports had confirmed that the storage/erase method of RRAM was based on the conducting filament (CF) theory. However, the ECM-RRAM operated in different humidity environment, the CF formation is still not fully understood. In this work, we study the Ag/Ta2O5/Pt RRAM device in atmosphere and vacuum, which discussed the switching behavior and the reliability improvement. Moreover, we further observed the filament formation after the device scale down. In the first part, we measured the switching properties of the Ag/Ta2O5/Pt RRAM in atmosphere and vacuum environments. The absence of hydroxide (OH-) will restrict the redox reaction of Ag, resulting in Ag filament formed difficult and the Forming and Set voltages increasing in vacuum environment. Furthermore, we observed the Forming phenomenon via in-situ transmission electron microscopy (TEM) in Ag/Ta2O5/Pt system. The device was switched to low-resistance state (LRS) as the dual-filament was connected to the electrodes. The results of energy dispersive spectrometer (EDS) and electron energy loss spectroscopy (EELS) analysis demonstrated that the filament was composed by a stack of oxygen vacancies and Ag metal. This study indicated that the humidity played a crucial role, affecting the filament evolution and electrical characteristics, and provided new viewpoints of RRAM electrodes structure composing in the future. In the second part, we used the polycrystalline-Fe3O4 to improve the reliability of the Ag/Ta2O5/Pt Resistive random access memory (RRAM). In both the Ag/Ta2O5/Fe3O4/Pt and Ag/Fe3O4/Ta2O5/Pt structures, the switching properties for these bilayer RRAMs were measured in atmosphere and vacuum environments. The results demonstrated that the humidity would affect the Ag filament formation in different environments, and the Ta2O5 and Fe3O4 interface in a different sequence would change the performance of the device, particularly the Forming voltage. Furthermore, the reliability of these bilayer RRAMs was better than single-layer RRAM device, which significantly increased endurance, especially in the Ag/Fe3O4/Ta2O5/Pt device. This study provided detailed switching knowledge of the bilayer RRAM for improving the reliability of the device and new design viewpoints of the RRAM structure in future applications. In the third part, we also observed the conducting filament shape and evolution during Forming via in/ex-situ transmission electron microscopy (TEM) in the Ag/Fe3O4/Ta2O5/Pt system. In low humidity, the CF was composed of many weak filaments in a low-resistance state (LRS), where the grain boundaries in the Fe3O4 layer limited filament size. The results of energy dispersive spectrometry (EDS) analysis demonstrated that the filament was composed of Ag metal.
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