金屬氧化物奈米線異質結構:動態反應及其電阻轉換特性研究

Abstract

近年來，在微電子工業，為了增加生產效能及降低成本，電子元件尺寸的微縮成為主要的發展議題，在此趨勢下奈米結構因而受到產學界極大的重視；其中一維金屬氧化物因其製備容易又具物理、化學性質及材料形狀多變的優異特性而受到許多科學家的關注。在眾多的金屬氧化物材料中氧化鋅是最具有發展潛力的材料，被廣泛應用於光電、壓電等半導體元件，因此本研究以氧化鋅為主軸材料，探討氧化鋅與氧化鋁及二氧化鈦材料於奈米尺度下動態反應及電阻式記憶體之相關應用。 在第一部分我們選用氧化鋁材料來包覆氧化鋅奈米線，使形成一維核殼奈米結構，並將其結構放置於穿透式電子顯微鏡下，臨場加熱至600℃觀察一維核殼奈米線受溫度的影響並探討其結構變化，氧化鋁是半導體製程內相當受到重視的氧化物材料，經常使用於介電層等…故當氧化鋅材料引入元件製備時，在奈米尺度下與氧化鋁之關係就值得深入研究。我們利用原子層磊晶技術沉積氧化鋁來包覆氧化鋅奈米線，對核殼-氧化鋅/氧化鋁做溫度與電子束影響的觀察，發現在600℃的實驗條件下，氧化鋁材料受電子束的影響，氧化鋅於氧化鋁的界面開始融化分解，最後由真空系統抽離，由原本的核殼結構轉變為具有結晶性氧化鋁的奈米管，其轉變過程則由及時影像擷取系統截錄，而元素分佈則由能量分佈圖譜(Energy-dispersive spectroscopy, EDS)來分析。 第二部分我們成功製備且觀察氧化鋅/二氧化鈦奈米線異質結構的動態反應。此研究中我們利用電子微影技術及電子束蒸鍍的方式，對氧化鋅奈米線做特定位置的鈦金屬鍍覆，並將此元件放入臨場穿透式電子顯微鏡下升溫700℃使之反應，因整個系統處於高真空的環境，而鈦與鋅元素本質上電負度又有一定的差異，使得氧化鋅奈米線結構中的鋅元素漸漸被鈦元素取代，讓氧化鋅奈米線結構變成軸向異質結構氧化鋅/二氧化鈦的一維奈米線，我們除了探討整個過程反應、形成機制，也對其元素分佈成分結構，做完整的研究。 最後我們將合成出的氧化鋅/二氧化鈦異質結構奈米線，做電阻式記憶體的應用。金屬氧化物材料其結構易離子遷移且擁有多變相的結構，近期於電阻式記憶體上的應用開始受到重視，但因尺度的降低，於存讀的過程中易產生漏電，而有誤判資料的可能性，而本論文中第二部分所製備出的氧化鋅/二氧化鈦/氧化鋅異質奈米結構，正好形成互補式電阻記憶體(Complementary Resistive Switching, CRS)的結構。CRS有其優點:一.在尺寸微縮的前景下，CRS可以免去電晶體或二極體的空間，可以有效增加元件密度；二. CRS元件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。故第三部分我們則針對材料本身特性、儲氧層的影響、電壓電流對內部構造的改變等深入探討。

Over the past decades, researchers are increasingly enthralled in one dimensional metal-oxide to develop and expand its functionality. Metal oxides not only inherit the remarkable properties from bulk such as electro-mechanical, and electro-chemical properties, but also exhibit special geometric/anisotropic property and size effect in the research of photodetectors, single-electron transistors, electron emitters, light-emitting diodes, biological and ultraviolet nanolasers. Among them, zinc oxide is П-VІ n-typesemiconductor which exibhits piezoelectric and semiconducting dual properties. The nanostructure of ZnO have novel applications in optoelectrics, sensor, transducers and biomedical sciences. The thesis will focus on the physical propertics and dynamic reactions of nanoscale 1-D ZnO heterostructure. In the first part, the melting behaviours of ZnO nanowires by heating of ZnO/Al2O3 core/shell heterostructures to form Al2O3 nanotubes in an in-situ ultrahigh vacuum transmission electron microscope (UHV-TEM) has been investigated. When the ZnO/Al2O3 core/shell nanowire heterostructures were annealed at 600 oC under electron irradiation, the amorphous Al2O3 shell turned to be single crystalline and then the ZnO core melted. The average vanishing rate of the ZnO core was measured to be 4.2 nm/sec. The thickness of the Al2O3 nanotubes can be precisely controlled by the deposition process. Additionally, the inner geometry of nanotubes can be defined by the initial ZnO core. The result shows a promising method to obtain the biocompatible Al2O3 nanotubes, which may be applied in drug delivery, biochemistry and resistive switching random access memory (RRAM). In the second part, we successfully transformed piezoelectric ZnO into photocatalytic TiO2 and formed TiO2/ZnO axial heterostructure nanowires with flat interface by solid to solid cationic exchange reactions in high vacuum (approximation 10-8 torr) transmission electron microscope (TEM). Kinetic behavior of the single crystalline TiO2 was systematic analyzed. The growth rate of TiO2 has been measured using in-situ TEM videos. On the basis of the rate, we can control the dimensions of the axial-nanoheterostructure. Finally, we utilized TiO2/ZnO axial heterostructure nanowires to form the unique Pt/ZnO/TiO2/ZnO/Pt heterostructures with complementary resistive switching (CRS) characteristic were designed to solve the important issue of sneak-peak current. The resistive switching behavior was attributed to the migration of oxygen and TiO2 layer served as reservoir, which was confirm by EDS analysis. This study not only supplied a distinct method to explore the transformation mechanisms, but also exhibited the potential application of ZnO/TiO2 heterostructure in nano-scale cross-bar array resistive random access memory.