隨著AI、大數據、5G時代等等革命性科技的來臨，對記憶體相關電子元件的需求量空前絕後，為了解決電子元件尺寸與製程技術的微縮困境，促使來自不同背景的工程師與科學家不斷提出創新、可行的解決方案，提供具有快速操作、低功耗以及高密度等等優點的記憶體元件，已然成為記憶體相關研究的重心。
本實驗使用磁控濺鍍與電子束蒸鍍等技術，製備了以鋁、銀、白金及氧化銦錫等材料作為電極之電阻式記憶體(ReRAM)元件，並以多鐵性材料鐵酸鉍(BiFeO3, BFO)為介電層，並藉由量測各個不同電極元件的電性，來觀察電極材料對元件特性之間的關係，結果顯示，製備的Ag/BFO/Pt以及Ag/BFO/ITO電阻式記憶體元件具備優秀且穩定的特性，其中又以Ag/BFO/ITO元件表現優異，超過1200次的操作次數以及優良(>102)且穩定的電流或電阻開關比(on/off ratio)，高、低阻態的維持時間也輕易地達到104秒，除了比較各個元件電性以外，也進一步探究電阻的切換機制，其中分別以金屬離子遷移主導的電化學金屬化機制以及氧空缺傳導主導的化合價變化機制為探討重點，兩者皆與導電燈絲的建立、破壞密不可分，除了製備典型的金屬-絕緣層-金屬(MIM)三層結構之外，也製備了由鐵酸鉍與氧化鐵(α-Fe2O3)所組成的雙層介電層元件，上下電極則採用氧化銦錫及銀，並於量測結果發現具備特殊的「兩段式Reset」現象，並透過燈絲理論作出解釋，此結果展現了電阻式記憶體元件的在內存與神經形態計算方面的應用與發展潛力。
除了元件特性與電阻切換機制之外，透過對電流、電壓的線性擬合，確定在高、低阻態與不同施加電場下參與的電流傳導機制，例如歐姆傳導、空間電荷限制電流(SCLC)、穿隧傳導及其他等等機制。

關鍵字: 電阻式記憶體、鐵酸鉍、雙層介電層電阻式記憶體、燈絲理論、電化學金屬化機制、化合價變化機制

With the advent of revolutionary technologies such as AI, big data, and the 5G, the demand for memory-related electronic devices has never been greater, which has prompted engineers and scientists of various backgrounds to propose innovative and viable solutions to mitigate the technical bottleneck of scaling down the electronic devices and continuously improving the manufacturing process. Delivering memory devices with appealing features such as fast operation, low power consumption, and high density has forcefully occupy the center of memory research.
In this work, we use magnetron sputtering and electron beam evaporation techniques to fabricate resistive random access memory (ReRAM) with aluminum, silver, platinum, and indium tin oxide (ITO) electrodes and the multiferroic bismuth ferrite (BiFeO3 or BFO) as the active dielectric layer. By measuring the electrical properties of various devices with different electrodes, the interrelationship between the electrode materials and the ReRAM characteristics is investigated. The results evince that the prepared Ag/BFO/Pt and Ag/BFO/ITO ReRAM devices have reflected excellent and stable electrical properties, especially for the Ag/BFO/ITO device. The Ag/BFO/ITO device exhibit great endurance with more than 1200 switching cycles and an excellent (>102) on/off current or resistance ratio. The retention time in both high and low resistance states can comfortably reach 104 seconds and beyond. In addition to comparing the electrical properties of each device, the resistive switching mechanism is also further explored. We mainly focus on the electrochemical metallization mechanism dominated by metal ion migration and the valence change mechanism dominated by oxygen vacancy conduction. Both of which are inseparable from the establishment and destruction of the conductive filament. Moreover, we not only fabricated ReRAM devices with a typical MIM structure but also prepared a bilayer device with a double dielectric layer composed of bismuth ferrite stacked on iron oxide (α-Fe2O3). The indium tin oxide and silver are deposited as top and bottom electrodes. A special "two-step reset" phenomenon is observed from the measurement results of this bilayer device, which could be explained through the filament theory. The outcome of measurements manifests the potential for applying ReRAM devices in in-memory and neuromorphic computing.
In addition to the device characteristics and resistive switching mechanism, the current conduction mechanisms in high and low resistance states under different electric fields applied are ascertained through the linear fittings of current-voltage characteristics including ohmic conduction, space charge limited current (SCLC), tunnel conduction, and other mechanisms.

Keywords: resistive random access memory, BiFeO3, filament theory, electrochemical metallization mechanism, valence change mechanism

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