二元金屬氧化物電阻式記憶元件之界面效應研究

Abstract

隨著數位行動生活的到來，非揮發性記憶體在可攜式電子產品，如：手機、數位相機跟筆記型電腦扮演著重要的角色，且必須具有於無電源供應時仍能維持其記憶狀態。快閃記憶體是現今非揮發性記憶體的主流，但是它有著許多缺點，包含：高的操作電壓、低的操作速度與較差的耐久力。近年來因傳統快閃式記憶體在不斷微縮下，面臨了許多急欲克服之難題，例如儲存在懸浮閘極中之電荷，因穿遂氧化層過薄而隨時間漸漸流失，造成資料流失；此外，在長時間操作之下，易在穿遂氧化層內產生缺陷以及超作電壓過高…等，如此瓶頸，加快了下世代非揮發性記憶體之研究腳步。下世代非揮發性記憶體有：鐵電記憶體、磁記憶體、相變化記憶體與電阻式記憶體等，其正如火如荼地發展。而其中電阻式記憶體具有操作電壓低、功率消耗低、寫入抹除時間短、可微縮、耐久力長、記憶時間長、非破壞性讀取、製程簡單及製作成本低等優點，因此電阻式非揮發性記憶體是目前新興非揮發性記憶體元件中的研究重點。 本論文第一章根據已發表之論文，把現今電阻式記憶體研究之重點、現況與理論做一整理、歸納與比較。第二章為實驗步驟，介紹元件製作、材料分析儀器與量測方法。第三章我們利用鎢探針把氧化鋯薄膜掃至崩潰狀態，其後鍍上鈦上電極，又有電阻轉態特性的發生，稱為復活的現象，有別於鋁和白金電極，並且這個現象為界面層產生所造成。第四章我們在鈦上電極與氧化鋯薄膜之間嵌入氧化鈣摻雜於氧化鋯之氧導體緩衝層，來改善單邊電阻轉態的特性。在第五章我們改變氧化鎵薄膜厚度、上電極面積和快速退火溫度，發現利用熱產生電阻轉態現象與利用電場的形成過程非常相似，並且其導電細絲由氧空缺所組成。 第六章我們用黃金奈米點來改善白金底電極，來探討氧化鋯記憶元件之電阻轉態特性，由於黃金奈米點的尖端放電效應與較小等效厚度，所以最大電場都會集中黃金奈米點附近，因此更能控制導電細絲產生的位置與抑制操作上的差異性。第七章我們為了實現1D1R結構的應用，我們探討電阻式記憶元件與氧化物二極體連接的電特性關係，以達到實際上的應用。最後對全文作一總結，並對未來可行的研究工作做一建議。

With the arrival of the Digital Age, nonvolatile memory (NVM) plays an important role for portable electronic products, such as the mobile phone, digital camera, and notebook computer. Flash memory is the mainstream among the nonvolatile memory devices nowadays, but it has many drawbacks, including high operation voltage, low operation speed, and poor endurance. In addition, when the device dimensions are continuously scaled down, the flash memory faces the challenge of thin tunneling oxide that causes an unsatisfactory retention time. Consequently, there are many proposals for new nonvolatile memories such as the ferroelectric random access memory (FeRAM), the magnetic random access memory (MRAM), the phase change random access memory (PCRAM) and the resistive random access memory (RRAM). Among them, RRAM possesses the excellent advantages, including low operation voltage, low operation power, high operation speed, high scalability, good endurance, long retention time, nondestructive readout, simple structure, and low cost. As a result, RRAM has been investigated for the commercial NVM applications. A review of various types of novel memory devices and current status of the resistive switching memory are presented in Chapter 1. Some important effects and possible resistive switching mechanisms are discussed. The detailed experimental procedures of fabrication of resistive switching devices, the principles of material analyses, and the related electrical measurements are presented in Chapter 2. In chapter 3, we use W-probe to contact as-deposited ZrO2 films to study the resistive switching phenomenon, and the ZrO2-based device finally come to breakdown (defined as BD-ZrO2/Pt device). A remarkable phenomenon named “recovery” is observed, which the resistive switching phenomenon appears again in a broken ZrO2-based device after Ti top electrode deposition. Oppositely, there is no such phenomenon when the Pt and Al top electrodes are deposited on the BD-ZrO2/Pt devices. The Ti-induced recovery phenomenon of resistive switching could be explained by the effects of the interface layer formation. In chapter 4, a calcium oxide doped zirconium oxide oxygen ion conductor buffer layer is introduced between the Ti/ZrO2 interface of conventional Ti/ZrO2/Pt memory devices to improve their unipolar resistive switching properties. In chapter 5, by modifying the thickness, area, and RTA temperature of the device, the thermal-induced resistive switching is similar to those induced by the electrical forming process. The conducting filaments composed of oxygen vacancies are created by the Cr diffusion and oxidization during RTA. The resistive switching properties of the ZrO2 memory devices with bottom electrode modification by using Au nanodots are investigated in chapter 6. Due to the tip of the Au nanodots on the Pt bottom electrode, it causes a higher electric field within the ZrO2 film above the nanodots due to reduced effective film thickness and induces the localized conducting filaments easily. The operation parameters’ variation for switching devices is, therefore, suppressed with lower operation voltage and resistance ratio. To fulfill one diode and one resistor (1D1R) structure, the electrical relation between the RS device and the diode is investigated in chapter 7. The conclusion and the suggested future work are presented in chapter 8.