氧化銅奈米線電阻式記憶體之研究

Abstract

為了因應人們對於電子產品尺寸微縮的需求，奈米尺度下之非揮發性記憶體越來越受到重視，而其中的電阻式記憶體(RRAM)元件因其結構簡單、阻態轉換快速等優點而引起廣泛的研究。本研究利用VS(Vapor-Solid)機制，將銅箔在大氣氛圍下，於500℃退火兩個小時，合成一維結構之氧化銅(CuO)奈米線。並利用電子束微影製程，分別製作兩端皆以鈦／金作為電極，以及銀為上電極、鈦／金作為下電極的RRAM元件，藉此探討元件在奈米尺度下的電阻轉換機制。 在兩端皆以鈦／金作為電極的元件中，我們成功的量測到了電阻轉換特性，經由電性量測結果，確定RESET的機制來自於焦耳熱效應，以及低阻態下介電層的導通機制為空間電荷限制電流(SCLC)。在穿透式電子顯微鏡的觀察中，我們在陽極處發現了Cu2O相，以及能量散佈光譜儀(EDS)的元素分佈，證實電阻轉換是由氧擴散為主導的氧化還原反應 。並在奈米線的外側發現了Cu2O的結構，此結構連通了上下電極，此外，利用能量散佈光譜儀進一步分析CuO的化學鍵結，證實元件內部有大量氧空缺的，並導致元件的從高組態(HRS)轉換至低組態(LRS)。 在以銀為上電極、鈦／金作為下電極的元件中，我們量測到不同的單極性電阻轉換特性，元件在小於3伏特的負偏壓下就可以回到高組態，並大幅的提升元件的循環壽命(Endurance)，其主要原因為，銀離子在擴散進入氧化銅奈米線後生成銀導電路徑，取代兩端皆以鈦／金電極，以氧空缺為主導的電阻轉換機制，使元件變得更加穩定。

For the development of high density memory arrays of resistive random access memory (RRAM), nanowires provide the potential to reduce the device size to overcome the limitation of conventional lithography. In this work, copper oxide nanowires(NWs) were synthesized and served as the building block for 1-D RRAM nanodevice. The phase of Copper oxide was identified to be CuO by selected area electron diffraction (SAED) patterns and energy dispersive spectrometer (EDS). In addition, we fabricated the Au/Ti/CuO NW/Ti/Au RRAM nanodevices by electron-beam lithography system. The unipolar measurement demonstrated that the RESET process was resulted from Joule-heating effect. Also, the fitting result showed the conducting mechanism of LRS is space charge limited current(SCLC). From the HRTEM observation, we found the Cu2O phase generated from cathode to anode around the surface of nanowire, which result from the higher concentration of oxygen vacancies segregated at surface. The oxygen vacancies path contributed to the resistive switching behavior, which was confirmed by Electron Energy Loss Spectrum (EELS). Furthermore, we replaced the top electrode by silver which further enhanced the resistive switching properties. In this study, the switching mechanism was be confirmed by I-V measurement, HRTEM, and EELS. The result would be beneficial to improve the switching properties of nonvolatile memory device at nanoscale.