氧化鎂型元件應用於磁阻式及電阻式非揮發性記憶體之研究

Magnetic and Resistive Switching in MgO-based Devices for Nonvolatile Memory Applications

黃鑫泓(Hsin-Hung Huang)

指導教授 ： 賴志煌

國立清華大學/工學院/材料科學工程學系/博士(2010年)

https://doi.org/10.6843/NTHU.2010.00303

摘要

ABSTRACT

Modern memory technologies, including the magnetic random access memory (MRAM), the resistive random access memory (RRAM), the ferroelectric random access memory, and the phase-change random access memory, have attracted a great deal of attention for the development of the next-generation nonvolatile memories recently. In this dissertation, the main research focuses are magnetic switching and resistive switching in MgO-based nonvolatile memories for the applications of the MRAM and the RRAM, respectively. In addition, the exchange-bias (EB) scheme, which is indispensable to MRAM devices, is also investigated.
The first topic of this dissertation is focused on the EB effect and the thermal stability of the epitaxial (002) Ir20Mn80/Co50Fe50 bilayers, which exhibit the unique double-shifted (DS) characteristics. For IrMn/CoFe samples with thin IrMn layers set in the negatively-magnetized state at room temperature, transitions of the EB field from a negative field to a positive field with the increasing time scale occurred. This relaxation process of IrMn interfacial spins can be further enhanced by placing IrMn/CoFe samples in the setting field applied along various orientations for a period of time at room temperature and can be investigated by studying DS characteristics of this epitaxial (002) system. The DS characteristics of this epitaxial (002) system can be used as a good indicator for the relaxation of IrMn interfacial spins and verify the orientation changes of IrMn interfacial spins.
The second topic of this dissertation is focused on the device fabrication and performances of highly textured (002) Fe/MgO/Fe magnetic tunneling junctions (MTJs) for MRAM applications. The ultrathin (002) ZnO/MgO composite buffer layers, which were grown on Si/SiO2 substrates at room temperature by using the ion beam deposition system with the high ion beam energy, can be used as the structure template for the growth of the (002) Fe/MgO/Fe texture. The growth mechanism of the ultrathin ZnO/MgO composite buffer layers is also discussed. On the other hand, we developed the micro-fabrication process for MTJ devices and demonstrated the tunneling magnetoresistance performance of the micro-fabricated (002) Fe/MgO/Fe MTJ devices.
The third topic of this dissertation is focused on resistive switching (RS) characteristics of the Pt/MgO/Pt memory devices, which reveal the novel voltage-polarity-independent nonpolar RS behavior, for RRAM applications. Robust RS characteristics in terms of the endurance and retention measurements are demonstrated. Furthermore, the MgO memory device exhibits the non-forming nature, which can be further manipulated by the deposition atmosphere, demonstrating the stoichiometry-controlled forming process. Electrical investigations reveal that the formation and the rupture of localized conducting filaments in the MgO film are responsible for the RS behavior. Further studies of Auger electron spectroscopy and x-ray photoelectron spectroscopy analyses combining with the temperature dependence of resistance suggest that metallic Mg filaments are formed in the low resistance state during the SET process. In addition, the voltage-polarity-independent RESET process implies that filaments may be ruptured by local Joule heating, leading to nonpolar RS characteristics.

關鍵字

氧化鎂 ； 交換異向性 ； 穿隧式磁阻元件 ； 電阻轉換

並列摘要

論文摘要 (ABSTRACT IN CHINESE)

近年來，新世代非揮發性記憶體應用於新穎儲存科技之研究與發展受到與日俱增的重視，其中主要包括了磁阻式記憶體、電阻式記憶體、鐵電式記憶體以及相變式記憶體。本論文主要為探討氧化鎂型元件應用於磁阻式記憶體以及電阻式記憶體之研究。此外，磁阻式記憶體當中不可或缺的鐵磁/反鐵磁交換異向性亦是本論文研究之重點。
本論文的第一部分為探討(002)銥錳/鈷鐵磊晶系統之交換異向性與其熱穩定性。此(002)銥錳/鈷鐵磊晶系統在磁滯曲線量測上展現出獨特之雙偏移特徵。當我們把具有較薄銥錳厚度的銥錳/鈷鐵樣品設定在負磁化狀態並將其放置在室溫下一段時後之後，我們可以發現其交換異向性場隨著時間增加而展現出由負至正之轉變。此轉變主要是由於銥錳界面自旋的鬆弛行為所造成。藉由將銥錳/鈷鐵樣品沿著不同方向放置在設定磁場中並將其保持在室溫下一段時間之後，銥錳界面自旋的鬆弛行為則更為顯著。另一方面，此銥錳界面自旋的鬆弛行為亦可藉由銥錳/鈷鐵樣品之雙偏移特徵來深入地了解。本部分的實驗結果顯示了此(002)銥錳/鈷鐵磊晶系統之雙偏移特徵可以做為銥錳界面自旋鬆弛行為的指標以及驗證銥錳界面自旋方向的改變。
本論文的第二部分為探討(002)鐵/氧化鎂/鐵穿隧式磁阻元件之製程與效能。我們利用高能量離子束濺鍍之方式在二氧化矽基板上以室溫製程成功製備出具有(002)優選方向的極薄氧化鋅/氧化鎂複合式緩衝層，並且在此緩衝層上成長具有(002)優選方向之鐵/氧化鎂/鐵結構。氧化鋅/氧化鎂複合式緩衝層的成長機制在此部分研究中亦有詳細的探討。另一方面，我們發展了磁阻式記憶體元件之細微化製程並且成功展示了(002)鐵/氧化鎂/鐵穿隧式磁阻元件之磁阻表現。
本論文的第三部分為探討鉑/氧化鎂/鉑電阻式記憶體元件之電阻轉換行為。此氧化鎂型電阻式記憶體元件展現出新穎且與電壓極性無關之無極性電阻轉換行為並且具有良好的耐久度特性與電阻態保存時間。此外，此氧化鎂型電阻式記憶體元件無需形成製程來初始化其電阻轉換行為，並且可以藉由不同的氧化鎂製程氣體來達到以氧化鎂之化學劑量比來控制形成製程之特性。另一方面，從氧化鎂型電阻式記憶體元件的電性分析結果可以推斷，其電阻轉換行為主要是源自於氧化鎂膜層內局部導電燈絲的形成與斷裂。進一步藉由歐傑電子能譜儀分析以及X射線光電子能譜儀分析並搭配氧化鎂型電阻式記憶體元件之低電阻態溫度相依性，我們可以推測氧化鎂型電阻式記憶體元件之電阻轉換行為主要是藉由金屬鎂導電燈絲之形成與斷裂。此外，由於金屬鎂導電燈絲之斷裂過程與電阻轉換電壓之極性無關，此結果暗示金屬鎂燈絲周圍產生之局部焦耳熱效應為其斷裂之主要原因。

並列關鍵字

MgO ； Exchange Anisotropy ； Magnetic Tunneling Junction ； Resistive Switching

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