鋯酸鍶基電阻轉換記憶元件之特性研究

Abstract

隨著可攜式電子元件的蓬勃發展，例如手機及數位相機，非揮發性記憶元件在半導體產業的需求也日益增加。現今非揮發性記憶元件的主流為快閃式記憶體，然而快閃式記憶體存在一些必須克服的問題，諸如操作電壓較高、操作速度較慢以及記憶力隨著元件尺寸微縮而降低等。因此許多新型態的快閃式記憶體正被積極的研究開發以取代傳統的快閃式記憶體。此外許多研究機構更紛紛投入新世代非揮發性記憶元件的開發，希望能研發出一種具有動態隨機存取記憶體的高集積密度、靜態隨機存取記憶體的高操作速度及快閃式記憶體的非揮發性之記憶元件，其中以電阻轉換記憶元件最被看好。電阻轉換記憶元件具有操作電壓及耗能低、操作速度快、可微縮性高、記憶時間長、耐久力佳及尺寸小等。多種材料都被發現具有電阻之轉換特性，如微量摻雜的鋯酸鍶及鈦酸鍶、過渡金屬氧化物、鈣摻雜的鐠酸錳及鑭酸錳、有機材料及高分子材料等。本論文對鋯酸鍶基電阻轉換記憶元件之電阻轉換特性作一深入的探討，並提出可能的電阻轉換機制。 本論文之第一章為記憶體簡介，並深入的介紹電阻轉換記憶元件。第二章為實驗步驟，本論文所述之所有元件製備及量測分析方法，均在此章有詳細的說明及介紹。第三章介紹以鎳酸鑭為底電極及以鋁為頂電極之鋯酸鍶基電阻轉換記憶元件之電阻轉換特性，此結構之元件具有電壓極性之電阻轉換特性，施加負電壓於頂電極可將元件之電阻值由高電阻狀態轉換至低電阻狀態，而施加正電壓於頂電極可將元件轉換回原本的高電阻狀態。本章亦對摻雜濃度對電阻轉換記憶元件之特性作一探討，發現適當的摻雜濃度可將電阻轉換記憶元件之特性最佳化。研究的結果顯示，具有百分之0.3釩摻雜的鋯酸鍶基電阻轉換記憶元件具有最佳的電阻轉換特性，例如兩個電阻狀態的比值高達一萬倍；元件的記憶能力長達一年以上。此外該元件在攝氏100度下操作仍具有極佳的穩定性。 第四章介紹以白金為底電極、鎳酸鑭為緩衝層及以鋁為頂電極之鋯酸鍶基電阻轉換記憶元件之電阻轉換特性，此結構元件之電阻轉換沒有電壓極性，亦即該元件可藉由施加正電壓或負電壓使它改變其電阻值。由研究結果顯示，無電壓極性之電阻轉換為鋯酸鍶基電阻轉換記憶元件之本質特性，而電極的選擇將限制某個電壓方向之電阻轉換，進而成為有電壓極性之電阻轉換行為。此元件利用高導電率的白金作為底電極，並成長具有(100)及(200)方向之鎳酸鑭作為緩衝層，而成長於上的鋯酸鍶電阻層亦具有(100)及(200)之優選方向，有該優選方向之鋯酸鍶薄膜已被研究具有極佳的電阻轉換特性。此外由於底電極的導電率佳，因此該元件之操作電壓可小於7伏特，且操作速度可高達10奈秒。該元件的電阻比值更高達一千萬倍；元件的記憶能力長達八個月以上。此外該元件在攝氏150度下操作仍具有極佳的穩定性。 由各方的研究得知，鋯酸鍶基電阻轉換記憶元件之電阻轉換機制為導電路徑的形成與破壞，當導電路徑形成時即為低電阻狀態，而當導電路徑被打斷時即為高電阻狀態，而導電路徑的形成與破壞與鋯酸鍶薄膜中的缺陷有關。低電阻狀態及高電阻狀態的導電機制分別為Ohmic及Frenkel-Poole機制，由此可證明電阻的轉換發生於鋯酸鍶薄膜內部，而非發生於界面層。 鋯酸鍶基電阻轉換記憶元件具有良好的電阻轉換特性，例如操作電壓及耗能低、操作速度快、記憶時間長及結構簡單等。因此鋯酸鍶基電阻轉換記憶元件是有可能取代動態隨機存取記憶體、靜態隨機存取記憶體及快閃式記憶體之新世代非揮發性記憶元件。 最後為本文之總結，並對未來可行的研究工作作一具體之建議。

Due to the popularity of portable equipment, such as mobile phone and MP3 player, the requirements of nonvolatile memory (NVM) increase significantly in the semiconductor industry. The mainstream of NVM nowadays is the Flash memory; however, the Flash memory has some issues such as high operation voltage, low operation speed, and poor retention time and coupling interference effect during the memory scaling down. Therefore, some new-type Flash memories, such as charge-trapping (SONOS) Flash and band-engineered SONOS Flash are studied to replace the traditional Flash memory. Besides, researchers are eagerly finding one kind of next-generation NVM possessing the advantages of high density, high speed, and nonvolatility of DRAM, SRAM, and Flash memory, respectively. One of the promising candidates of next-generation NVMs is the resistive random access memory (RRAM) owing to its low operation voltage and power, high operation speed, high scalability, good endurance, small size, etc. Resistive switching properties have been observed in many kinds of materials, such as doped SrZrO3 (SZO) and doped SrTiO3, transition metal oxides, Pr1-XCaXMnO3 and La1-XCaXMnO3, and organic and polymer materials. In this dissertation, the resistive switching properties and mechanisms of the SZO-based memory devices are studied in depth. In this dissertation, Chapter 1 introduces the memories, especially for the RRAM. Chapter 2 shows the experimental procedures of the devices indicated in this dissertation. Chapter 3 presents the properties of the SZO-based memory devices with LaNiO3¬¬ (LNO) bottom electrode and Al top electrode (Al/SZO/LNO, ERE devices), which have bipolar resistive switching characteristics. The doping effects of the SZO-based ERE devices are investigated, indicating that a proper concentration of doping into the SZO film can improve the resistive switching properties, such as resistance ratio and stability. The resistance ratio between high resistance state (HRS) and low resistance state (LRS) of the 0.3%-V:SZO ERE device is over 104, and retains 1000 after applying 100 voltage sweeping cycles. This device has stable resistive switching properties even when the measurement is performed at 100oC. The retention time of the device is longer than 107s, and the nondestructive readout property of the device is also examined. Chapter 4 presents the properties of the SZO-based memory devices with Pt bottom electrode, LNO buffer layer, and Al top electrode (Al/SZO/LNO/Pt, ERBE devices), which possess nonpolar resistive switching characteristics. The nonpolar switching is considered as an intrinsic property of the SZO-based memory devices, while the electrode materials employed in the devices would determined the resistive switching polarities. The operation voltages of the 0.3%-V:SZO ERBE device are less than 7V, and the resistance ratio of the device is higher than 107. This device with Pt bottom electrode has lower resistive switching voltages and higher resistance ratio comparison with the 0.3%-V:SZO ERE device with LNO bottom electrode. The resistive switching speed of the 0.3%-V:SZO ERBE device is 10ns, which is the fastest speed in comparison with that of the previous reports. The device has stable resistive switching properties even when the measurement is performed at 150oC. The nondestructive readout property of the device is also demonstrated, and the retention time of the device is longer than 107s. The resistive switching mechanism of the SZO-based memory devices are considered as the formation and disruption of the current paths, which possibly attributed to the storage and release of electrons in the trap states of the SZO film. The conduction mechanisms of both LRS and HRS currents of the SZO-based memory devices are dominated by Ohmic conduction and Frenkel-Poole emission, respectively. Consequently, the SZO-based memory device with good resistive switching characteristics including low operation voltage, low power consumption, high operation speed, long retention time, nondestructive readout, and simple structure is a promising candidate for next-generation NVM applications. Finally, the experimental results and discussion are summarized in chapter 5. Some suggestions for future work are also provided in this chapter.