電阻式記憶體傳導機制與轉態行為之動力學研究

Abstract

非揮發性電阻式記憶體(RRAM) 因其極有可能取代現今最廣泛使用的快閃記憶體(Flash Memory) 成為新世代的奈米記憶體而受到極大的重視。由於電阻式記憶體性能優越且富有市場潛力，如具有低的操作電壓、快速讀寫、低消耗功率、高讀寫次數等優點。此外，因電子元件尺寸的微縮為產業界主要的發展議題，而具有簡單金屬/絕緣體/金屬(Metal/Insulator/Metal, MIM) 的三明治結構的電阻式記憶體，除了能有效增加生產效能及降低成本外，也具有奈米微縮化的潛力。本研究以電阻式記憶體為主軸元件，探討以氧缺為主體的氧化鋅與氧化鉭於奈米尺度下動態轉態的行為研究及其電學特性之相關應用。 在第一部分我們利用臨場穿透式電子顯微鏡(TEM) 觀察三明治結構Pt/ZnO/Pt的電阻高低轉態行為，臨場觀察燈絲的動態生成和斷裂。其利用原子級解析度的影像對燈絲結構做鑑定，用電子能量損失能譜儀(EELS) 分析燈絲的元素成份及化學鍵結，且量測溫度對於燈絲的影響，證明傳導路徑的燈絲為金屬鋅原子而氧離子的遷移居於主導地位。我們成功用材料分析方法直接觀察到電阻式記憶體轉態的結構動態變化過程，對電阻式記憶體元件的操作機制及物理模型提供了良好的材料分析與驗證機制。 第二部分我們則以電阻式記憶體性能較好的氧化鉭作為主軸，針對其進行電學性質的探討。而在電阻式記憶體製備為3D陣列(crossbar) 元件中易產生漏電(sneak path current)，而有誤判資料的可能性，而本論文製備出二種元件：PN二極體氧化鋅/氧化鉭/金結構(1 diode and 1 resistor,1D1R)；以及反向對接的金/氧化鉭/氧化鈦/鉑/氧化鈦/氧化鉭/金的互補式電阻記憶體(Complementary Resistive Switching, CRS)的結構。1D1R的二極體的整流效應可有效防止逆向電流的產生，而CRS則是的結構簡單且製程容易，僅需在原本RRAM的元件鍍附上對稱的金屬半導體結構其優點，此二者都可免去電晶體的空間，可以有效增加3D陣列元件的密度。 最後我們將氧化鉭做臨場TEM的電阻高低轉態的研究，以第一部分中所提到的材料分析手法，包含X光微區分析(EDS)和電子能量損失能譜儀(EELS)，得知其奈米燈絲是由具金屬特性的氧化鉭氧缺相所組成，由原本介電性質的五氧化二鉭(Ta2O5) 中生成二氧化鉭(TaO2) 的燈絲。此外，我們設計出小尺寸電極的元件來侷限燈絲生成的位置，實際觀察元件是否微縮化的潛能，發現燈絲的確可以有效的被限制在電流集中的位置。也發現了在材料內部有結構缺陷時，燈絲生成的位置及生長的方向會不同於一般元件，與氧空缺及電子傳輸有密切的關連。

Non-volatile resistive random-access memory (RRAM) is consider to be a promising candidate to replace flash memory as a new generation of nano-memory. Because RRAM shows outstanding performance and potential for market opportunities, such as low operating voltage, fast read and write speed, low power consumption and high retention and endurance. Besides, the simple metal/ insulator/ metal (MIM) sandwich structure exhibits the potential for miniaturization, which increases the production efficiency and reduce the costs. In this work, we study the RRAM switching behavior and electrical characteristics in zinc oxide and tantalum oxide, which based on valance change mechanism(VCM) that the oxygen vacancies act as mobile donors. In the first part, We designed an innovative sample structure for in-situ transmission electron microscopy (TEM) to observe the formation of conductive filaments in the Pt/ZnO/Pt structure in real time. The corresponding current-voltage measurements help us to understand the switching mechanism of ZnO film. In addition, high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) have been used to identify the atomic structure and components of the filament/ disrupted region, determining that the conducting paths are caused by the conglomeration of zinc atoms. The behavior of resistive switching is due to the migration of oxygen ions, leading to transformation between Zn-dominated ZnO1-x and ZnO. In the second part, We chose tantalum oxide as RRAM material, and measure its basic electrical characteristic. However, when developing 3D RRAM system by crossbar technology, the sneak path current became an importance issue, where the current sneak through other cells, resulting a wrong state of the memories. We developed two kinds of devices to restrain the problem: p-n junction diode zinc oxide/ tantalum oxide/ platinum (1 diode and 1 resistor, 1D1R); and reverse stacked device gold/ tantalum oxide/ titanium oxide / platinum / titanium oxide/ tantalum oxide/ gold as complementary resistive switching, (CRS) structure. The diode with rectifying effect in 1D1R device can effectively prevent the reverse current. While, CRS is simple structure and process easily. Both of device reduce the space of transistor, effectively increased the density of the 3D array elements. Finally, we used in-situ TEM to observe the switching behavior in Au/Ta2O5/Au system, which exhibited a large ON/OFF ratio (>106). The shape and size of the conducting filament were obtained, and the composition of the filament was identified by EELS and energy dispersive spectroscopy (EDS). The switching was affected by the evolution of the conducting filament in a Ta2O5−x insulator layer, which consisted of nanoscale TaO2−x filaments. Moreover, the filament were shown to grow in structural defects, and the evolution of the filament in vacuum conditions was demonstrated. The shrinking electrode and the defect boundary led to the accumulation of oxygen vacancies, localizing the formation of the filament. Knowledge of the filaments would provide a foundation for clarifying VCM, improving its stability and scalability in oxide metals for commercial applications.