本研究中，我們利用離子束濺鍍系統成長CoFe(Co)/ZnO多層膜，並系統化研究其磁性與結構，以釐清過渡金屬摻雜氧化鋅的稀磁性半導體系統中之室溫鐵的鐵磁性來源。
首先，我們利用多層膜的成長方式嘗試製備鈷鐵摻雜氧化鋅的稀磁性半導體的樣品。我們研究[CoFe(x Å )/ZnO(20 Å)](x=1,2,5及9）多層膜樣品其磁性與結構的關聯性。 我們觀察到隨著鈷鐵厚度(x)的增加，樣品由良好的稀磁性半導體相(x=1)轉變為稀磁性半導體及金屬團簇的混相(x=2),到以金屬團簇為主的相 (x=5 及9) 。我們證實了多層膜成長技術可以有效的成長ZnO系統的稀磁性半導體。
另一方面，我們也發現在外加偏壓的阻抗頻譜量測中，可以觀察到在有金屬團簇析出的樣品，金屬團簇本身以及金屬團簇與氧化物間的介面會分別額外造成一電阻項及一電阻與電容並聯的等效電路項，使得其阻抗頻譜與純稀磁性半導體結構的樣品有明顯的不同。我們可以應用此技術檢驗在過渡金屬摻雜入氧化鋅的過程中，是否有金屬團簇的稀出。
此外，我們利用退火的方式去研究Co摻雜ZnO稀磁性半導體的磁性來源。我們研究不同退火溫度下(250oC, 500oC 及 750oC) ，對其結構、磁性及電性質的影響。我們的結果顯示剛沉積及退火250oC的樣品為良好的稀磁性半導體的結構。而退火500oC及750oC的樣品則由於Co或Co/CoO團簇的形成，呈現一個混相的結構。 因此我們也發現利用低溫(250oC)退火在不同氣氛下(air, Ar及Ar/H2)可在不影響Co的結構下改變其磁性。 Ar/H2退火的樣品具有最大的飽和磁化強度，而其在Zn K-edge的吸收光譜在吸收前緣也顯示出了一個最明顯的額外峰值。經由Zn K-edge的理論計算，此額外前緣峰值的出現是跟ZnO內氧空缺的形成有關，其強度可用以反應氧空缺數目的多寡。而Ar/H2退火的樣品其磁性增強的現象可被視為是氧空缺影響Co摻雜ZnO稀磁性半導體磁性的證據。
我們亦利用阻抗頻譜的量測，研究低溫(250oC)退火在不同氣氛下(Ar及Ar/H2)的Co摻雜ZnO稀磁性半導體其微結構的變化。我們發現具有最高飽和磁化強度的Ar/H2退火的樣品其在晶粒以及晶界中皆製造了較多的結構缺陷。此結果亦顯示晶粒及晶界中的微結構缺陷(如氧空缺)與Co摻雜ZnO稀磁性半導體的磁性有很強的關聯性。
最後,我們研究一系列[Co(1Å)/ZnO(dZnO)]25多層膜樣品(dZnO =10, 15, 20, 25, 30, 40, 及60 Å)的磁性變化。藉由改變ZnO的厚度(dZnO) ，我們可控制Co摻雜層彼此間的距離。隨著的dZnO增加，其磁性質漸漸的由三維耦合的束縛磁極化子轉換成準二維耦合的束縛磁極化子。此結果進一步的支持在Co摻雜ZnO稀磁性半導體系統中的磁性是來自於氧空缺所形成的束縛磁極化子。也使得我們對過渡金屬摻雜氧化鋅所形成的稀磁性半導體系統的磁性機制，有更深入的了解。

In this study, we have systematically investigated the magnetic properties and structures of CoFe(Co)/ZnO multilayers fabricated by ion beam sputtering to clarify the origin of the room-temperature (RT) ferromagnetism in transition metal (TM) doped ZnO.
In the first section, we focus on the fabrication of CoFe-doped ZnO DMS system by multilayer growth technique. The correlation of structure and magnetism of [ZnO(20Å)/Co0.7Fe0.3(x Å)]25 multilayers (MLs) with nominal thickness x=1, 2, 5 and 9 have been studied. With increasing x, the transformation from a good DMS dominated phase (x=1 sample) to a DMS-cluster mixed phase (x=2 sample) and to clustering dominated phase (x=5 and 9 samples) have been observed. Multilayer growth technique has been demonstrated as a good method to stabilize DMS phase in ZnO system.
On the other hand, samples with DMS structure and with clustering structure can be clearly characterized by bias-dependent impedance spectroscopy. For impedance spectra of clustering structure, the presence of metal nanoclusters and metal-oxide interface will cause a single resistance element together with an additional RC component; thus, make it differ from of DMS structure.
Besides, annealing treatments have been used to study the origin of intrinsic RT ferromagnetism in Co doped ZnO. We study the structural, electrical and magnetic properties of thermally annealed ZnO/Co DMS multilayer at 250oC, 500oC and 750oC, respectively. Our results reveal that the as-deposited and 250 oC-annealed samples show DMS dominated structures, while 500oC and 750oC-annealed samples show a mixed structure due to the formation of Co or Co/CoO clusters. Therefore, we observed that annealing at relatively low temperature 250oC under air, Ar and Ar/H2 atmosphere can change the magnetic properties without the variation of Co structures. The Ar/H2–annealed sample with the largest magnetization (Ms) shows the most pronounced pre-edge feature at Zn K-edge. From the theoretic calculation at Zn K-edge, additional pre-edge peak appears in correlation with the oxygen vacancies and can be applied to reflect the densities of oxygen vacancies in ZnO system. The enhancement of Ms after Ar/H2–annealing could be considered as evidence of oxygen vacancy enhanced RT ferromagnetism in Co doped ZnO.
On the other hand, the change in microstructure of Co-doped ZnO films under under air, Ar and Ar/H2 atmosphere at 250oC can be identified by impedance spectra measurement. Ar/H2-anneal process creates not only the grain boundaries defects but also the grain defects results in strong enhancement of Ms. The results show that microstructural defects (such as oxygen vacancies) among grain boundaries and grain in ZnO is strongly correlated with the RT ferromagnetism.
We also study a series of ion-beam sputtered [Co(1Å)/ZnO(dZnO)]25 MLs with dZnO =10, 15, 20, 25, 30, 40, and 60 Å. By varying the thickness (dZnO) of ZnO layers, the average spacing between adjacent Co sub-monolayer(1Å) can be controlled. The dependence of magnetic properties with increasing dZnO is consistent with the transformation from three-dimensional connected bound magnetic polarons (BMPs) percolated to quasi two-dimensional connectd BMPs. These results further support a oxygen vacancies constituted-BMP percolated model and give a deeper insight for understanding TM-doped ZnO DMS.

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