二氧化鈦雙極非線性選擇器元件電流傳導機制研究

Abstract

電阻式記憶體由於省能、速度快以及製程簡單等優點，為目前最具發展潛力的記憶體之一，近年來為了提高記憶體的密度，而採用交錯式陣列記憶體布局，然而記憶胞彼此間的串擾效應卻會造成資料判讀錯誤。為了解決串擾效應的影響，串接一個選擇器元件是必要的。實驗室團隊日前已經成功地製備了金屬-絕緣層-金屬結構的單向導通二極體以及雙向導通選擇器，可以分別應用於1D1R和1S1R的電路布局中。 選擇器的特性優良與否，可由電流非線性因子和電流密度這兩個參數來判斷。為了提高這兩個參數，了解金屬-絕緣層-金屬的電流傳導機制便成了第一要務。 本篇論文針對二氧化鈦此材料，搭配不同金屬上電極，量測了一系列的二極體電壓-電流特性，並與各種傳導機制公式進行比對，整理出幾種可能的電流傳導機制。為了進一步確認先前的假設，我們在原本上電極與絕緣層的接面位置進行缺陷濃度的改變，並發現電流特性明顯的變化，除了可以證明此假設的正確性，也讓我們獲得一個重要的結論:未來若需提高電流非線性因子以及電流密度，改變接面缺陷的濃度是一個可行的作法。 此外我們針對不同元件進行了脈衝式電壓-電流量測，發現隨著脈衝頻率增加，電流會有下降的趨勢，這個現象與介面氧缺活化的速度有關，進一步的脈衝電壓電流量測，我們固定脈衝基底(pulse base)的時間長度並改變脈衝寬度(pulse width)，進行變溫量測，定性上說明了氧缺濃度對於脈衝寬度的反應差異。

Resistive random access memory (RRAM) has become one of the most promising emerging memories due to its low power consumption, fast speed and simple structure. In recent years, the crossbar array is adopted for ultra-high density memories. However, the crosstalk effect would cause read interference of the memory state. To solve this problem, RRAM should be connected with a selection device. Previously our group has successfully demonstrated metal-insulator-metal (MIM) devices, including the unipolar diode and bipolar selector, applicable for the one diode-one resistor (1D1R) and one selector-one resistor (1S1R) arrays, respectively. The performance of the selection devices is judged by nonlinearity factor and maximum current density. To enhance these two factors, thorough understanding of the current conduction mechanism in the MIM structures is essential. This research focuses on the TiO2-based selection devices using different metal top electrodes. We measured their current-voltage curves and then fitted to various current conduction mechanisms. Furthermore, we varied the defect concentration at the top electrode/TiO2 interface using additional oxygen gettering layers and discovered significant changes in IV characteristics. This result supported that modulating the interface defect concentration is a feasible way to enhance nonlinearity and current density of nonlinear MIM selectors. Besides, we also measured various devices using a pulse IV setup and discovered that the current density is lower using higher frequency pulse. This result is related to the ionization rate of oxygen vacancies. Furthermore, we conducted temperature-dependent pulse IV measurement using fixed pulse base time and various pulse widths. Finally, a simple physical model is proposed to explain the results qualitatively.