二氧化鋯鉿材料之金屬-鐵電層-金屬結構鐵電穿隧接面之電性分析與二氧化鋯鉿晶體結構分析

The Characteristic Analysis of HZO based/MFM structured Ferroelectric Tunneling Junctions and the Analysis of HZO Crystal Structure

周頎勛(Chi-Hsun Chou)

指導教授 ： 劉致為

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2022年)

https://doi.org/10.6342/NTU202200716

摘要

使用熟悉的材料達成新的應用，是半導體創新的動力之一，二氧化鋯在過去利用其高介電常數的特性，可廣泛應用於隨機存取記憶體或是互補式金屬氧化物半導體的介電質。而在利用原子層沉積技術，控制二氧化鉿參雜的方式，搭配適合的退火溫度，能有效改變二氧化鋯鉿材料中結晶的結構，其中有兩種晶體結構有其獨特的特性，這兩種晶體結構分別是斜方晶系與四方晶系，前著之鐵電特性與後者之反鐵電特性在鐵電穿隧介面上都有其應用的潛力，而四方晶系有高介電常數的特性適合作為介電質。
依照上述二氧化鋯鉿材料的特性，材料的應用方向或是領域可以說是取決於晶體的結構，所以此研究的首先使用低掠角X射線分析儀，獲取以二氧化鋯鉿為材料之金屬–鐵電層–金屬結構鐵電穿隧介面的繞射圖，並嘗試使用Rietveld Refinement之分析方法，試圖對斜方晶系與四方晶系坐定量分析，雖然這個分析方式理論上可行，但本研究也會討論在實際操作上的困難，而最後改用電子繞射圖的分析，則發現可以分辨這兩種晶體的方法。
即便在材料分析上沒有得到直接的答案，我們能間接的利用電性來定性上的分析我們元件是斜方晶系還是四方晶系，在量測極化對電壓的關係並確認是斜方晶系後，我們使用脈衝量測對此鐵電穿隧介面分析，而最重要的目的在於找到最佳的讀寫條件，使得元件有最大的開關電流比值。除此之外因為在二氧化鋯鉿有相對其他材料高的剩餘極化下，其鐵電電流也會相對更高，因此會依據穿隧電流與鐵電電流的特性設計量測實驗，並了解在不同的讀寫條件下，其各自的比例與變化是如何？
最後了解了鐵電電流與穿隧電流的特性後，設計了一套量測方式可以在不量測到鐵電電流的前提下，量測到穿隧電流，而設計此量測方法的原因在於其不隨時變的特性在應用上能更好掌握，這套量測方式也更有助於在未來對應不同的元件時，針對不同的寫入條件，能定義出有效的讀取範圍，讀取不被鐵電電流影響的穿隧電流。

關鍵字

鐵電穿隧介面 ； 二氧化鋯鉿 ； 斜方晶系 ； 四方晶系 ； 低掠角X射線分析 ； 電子繞射 ； 鐵電電流 ； 穿隧電流

並列摘要

One of the motivations for innovation for semi-conductor is to achieve a new application with existing, familiar material. In the past, HfO2 was widely used in DRAM or as the dielectric of CMOS for its high permittivity. As for the atomic layer deposition (ALD), by controlling the doping method of ZrO2 and with a matching annealing temperature, we could effectively change the crystal structure of HZO material; among which two types of crystal structure have their unique characteristics. Those two types are of orthorhombic phase and tetragonal phase separately. The ferroelectric of the former and the antiferroelectric of the fourth have both the potential to be applied on ferroelectric tunneling junction (FTJ); while the high permittivity of the tetragonal phase is suitable for being used as a dielectric.
According to the characteristics of HZO material, the applying method or its domain is decided by the crystal structure. Hence, this study would first use GIXRD to get the diffraction pattern of MFM structured metal who is based on HZO material. We would also try to use the Rietveld Refinement analyzing method, forming a quantitative analysis of both the orthorhombic phase and tetragonal phase. Although this analyzing method is theoretically feasible, this study would discuss the difficulties of its execution in the field as well. At last, when performing the analysis with an electron diffraction pattern, the method of how we could distinguish these two crystals is discovered.
While there's no direct answer from the material analysis, we could use the electrical characteristic to conduct qualitative analysis to see whether our device is in the orthorhombic phase or tetragonal phase indirectly. After having measured the polarization-voltage curve and from its hysteresis loop, we could confirm that it belongs to the orthorhombic phase. We then analyze the ferroelectric tunneling junction (FTJ) with pulse measurement, and the most crucial is to find the best reading condition, letting the device have the highest on/off current ratio. In addition, since HZO has higher remnant polarization compared to other materials, its FE current is relatively higher. As result, the experiments would be designed according to the characteristics of its’ tunneling current and FE current; with which we could comprehend under different reading conditions, what the ratio and the variation of each currency would be.
After having understood the characteristics of tunneling current and FE current, a measuring method was designed: with the precondition of excluding FE current, the tunneling current could be measured. The reason why we design this measuring method lies in its characteristic of being non-time-dependent, which is easily controlled in the application. This measuring method is also helpful when dealing with different devices in the future: targeting different writing conditions, the valid reading area would be defined to read tunneling current without the influence of FE current.

並列關鍵字

ferroelectric tunneling junction ； HZO ； orthorhombic phase ； tetragonal phase ； GIXRD ； electron diffraction ； FE current ； tunneling current

參考文獻

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