氧化鋁磁穿隧元件與摻錳或鈷鋁氧化鋅稀磁性半導體之交流阻抗特性

Magneto-impedance Properties of AlOx-based Magnetic Tunnel Junctions and Mn- or Co/Al- doped ZnO Diluted Magnetic Semiconductors

郭明峯(Ming-Feng Kuo)

指導教授 ： 張家歐

共同指導教授 ： 傅昭銘

國立臺灣大學/工學院/應用力學研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.00873

摘要

自旋相關材料由於其較傳統電子元件多了一個可操控的自由度-自旋，因而成為近年來諸多研究聚光的焦點之一。目前應用如磁阻式隨機存取記憶體(Magnetoresistive Random Access Memory, MRAM)、硬碟驅動讀寫頭及自旋電晶體(Spin-FET)等，在實際發展上更希望能朝向高速、高頻的方向邁進。基於此一趨勢，了解其頻率相關電傳特性實有其迫切之必要性。因而在此論文中我們利用交流阻抗分析的方法分別對磁穿隧元件(Magnetic Tunnel Junctions)與錳摻雜氧化鋅薄膜及摻鋁氧化鋅鈷薄膜三種自旋相關材料之高頻電傳特性做一深入之探討。
在本論文中第一部分我們研究了鈷鐵硼/氧化鋁/鈷鐵硼為主要結構之磁穿隧元件的磁阻抗特性。除了基本的三層結構之外，此研究中亦做了一僅有下層釘紮鐵磁層而無上層自由鐵磁層之對照樣品。外加磁場下所量測之阻抗頻譜分析結果發現磁電容效應僅出現於標準結構樣品中。進一步在直流偏壓下所量測得之阻抗頻譜可觀察出兩樣品有不同程度磁電阻與磁電容之變化。在鐵磁絕緣接面間與遮蔽效應有關之介面電容隨著外加偏壓的增加而增加，也使得弛豫頻率往更高頻方向所移動。
論文中第二部分分析了摻錳氧化鋅薄膜在不同工作氣體下對其高頻電傳特性之影響。實驗中提供氬氣、氬氣加氮氣及氬氣加氧氣三種不同製程條件，在阻抗量測的結果中可發現加入氧氣的樣品其具有較低的弛豫頻率與較高的電阻值。經由Brick Layer模型的引入可分析出樣品在不同製程條件下其晶粒、晶界等微觀結構的變化與對應於阻抗頻率響應之貢獻。在氬氣加氧氣下所成長的摻錳氧化鋅薄膜其高阻抗、低弛豫頻率之交流阻抗特性乃源於在此製程條件下其晶粒較其他兩樣品減小，且更多晶界之形成所致。此一微觀結構的變化在掃描式電子顯微鏡分析下更可得到一良好的對應關係。
論文的最後一部分我們量測了不同濃度鋁摻雜之氧化鋅鈷(Zn1-x-yAlxCoyO, x = 0 – 10.65 at.%)稀磁性半導體薄膜之阻抗頻譜。藉由阻抗頻譜分析來研究鋁摻雜濃度對樣品磁電特性的影響。在阻抗頻譜所得到的結果顯示，在鋁濃度為6.03 at.%樣品中具有較高的弛豫頻率與導電率。為更深入探討磁電頻率響應機制，進一步利用了Cole-Cole理論模型來分析實驗所得之阻抗頻譜，結果可發現未摻雜鋁樣品之阻抗頻率響為一單一弛豫機制，而摻鋁後之樣品阻抗轉而為多重弛豫貢獻所得。
藉由分析以上自旋相關材料之頻率相關電傳特性，相信更可為未來實際應用奠定良好之基石。

關鍵字

阻抗分析 ； 磁穿隧元件 ； 氧化鋅 ； 稀磁性半導體

並列摘要

Spin-dependent materials, due to the charge and spin degrees of freedom accommodated into single matter, are promising candidates for a wide range of spintronic applications. Recently, the study of frequency-dependent transport properties of these materials has received considerable interest because most of their applications require high speed functionality. The transport in these kinds of materials depends sensitively on the details of the growth conditions and geometric structures. The essential underlying question is how these factors affect the frequency-dependent transport properties. Therefore, in this dissertation we study this fundamental problem using impedance spectroscopy methodology.
In the first part of this dissertation, we study the bias voltage dependence of tunnel magneto-impedance in two types of AlOx-based magnetic tunnel junctions (i.e., CoFeB/AlOx/CoFeB and CoFeB/AlOx). The equivalent circuit model is applied to characterize the transport properties and barrier/interface behavior of the tunnel junctions. The bias voltage dependence of the impedance spectra shows different behaviors for each type of junction, thus contributing different physical parameters to the equivalent circuit model. By analysis of the physical parameters we discuss the effects of the barrier high and spin-dependent screening on the frequency-dependent transport properties. The results indicate that the decrease in magneto-impedance is the results of decreased effective barrier high and increased inverse screening length. Impedance spectroscopy can be also used as a tool for studying the microstructure-related transport properties. The second part in this dissertation we have carried out a systematic study of the dependence of magneto-electrical properties of Mn-doped ZnO thin films deposited in various gas (Ar, Ar + N2, and Ar + O2) ambiences. The magneto-impedance spectra of the Mn-doped ZnO thin films have been analyzed using brick layer equivalent model. Different contributions are identified, and the results show that both electrical conductivity and dielectric relaxation of grains and grain boundaries contribute to the magneto-dynamics of the polycrystalline film. The grain boundary is found to make a larger contribution in sample grown in Ar + O2 during the sputtering process, resulting in larger resistance and lower relaxation frequency. Also, the results of the impedance spectroscopy are found to agree well with the SEM inspection.
Finally, the Al doping effects on high-frequency magneto-electric properties of Zn1-x-yAlxCoyO (x = 0 – 10.65 at.%) thin films are systematically studied in this dissertation. The Zn1-x-yAlxCoyO thin films have been deposited by magnetron co-sputtering onto quartz substrates. The magneto-impedance spectra of the thin films are measured by an impedance analyzer. Among all the doped films studied, the thin film with 6.03 at.% Al-doping shows the highest ac conductivity and relaxation frequency. To characterize the relaxation mechanism underlying the magneto-electric properties, a Cole-Cole impedance model is applied to analyze the impedance spectra. The analyzed result shows that the magneto-impedance of the Zn1-x-yAlxCoyO is contributed by multiple processes of magnetization dynamics and dielectric relaxation.

並列關鍵字

Impedance Spectroscopy ； Magnetic Tunnel Junctions ； Mn-doped ZnO ； Co/Al-doped ZnO ； DMS

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