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研究生：	陳淑芳 Chen, Shu-Fang
論文名稱：	磁性奈米結構及稀磁性半導體的成長與分析 Growth and Characterization of Magnetic Nanostructures and Diluted Magnetic Semiconductors
指導教授：	劉全璞 Liu, Chuan-Pu
學位類別：	博士 Doctor
系所名稱：	工學院 - 材料科學及工程學系 Department of Materials Science and Engineering
論文出版年：	2010
畢業學年度：	98
語文別：	英文
論文頁數：	148
中文關鍵詞：	奈米結構、穿透式電子顯微鏡、磁性、稀磁性半導體、磁阻
外文關鍵詞：	nanostructures, TEM, magnetic, DMS, magnetoresistance
相關次數：	點閱：77 下載：0
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本研究的目的是了解微結構對於奈米結構及稀磁性半導體磁性質及磁傳輸性質的影響。吾人利用穿透式電子顯微鏡來分析來研究不同維度的磁性奈米結構，本研究的材料系統包含鐵、矽化鐵奈米島嶼、鎳奈米線以及摻雜鈷的氧化鋅稀磁性半導體。
首先，自組裝鐵島嶼成功地以離子束濺鍍在矽基板上，成長溫度為室溫。尺寸介於25-100 nm且具有小尺寸分布的鐵島嶼，可以透過類似S-K成長模式的方式生長，亦即，鐵島嶼成長在矽化鐵薄層上。此外，鐵島嶼的垂直異向性會隨著尺寸增加而降低，三維島嶼形貌引起的表面效應是造成磁化(magnetization)偏轉的主要原因。
隨著成長溫度增加至200oC，矽原子與鐵原子間的劇烈反應會導致矽化鐵島嶼的成長，吾人觀察了奈米島嶼隨著溫度及濺鍍厚度的演化。吾人發現在某些濺鍍條件下，奈米島嶼會自發的團聚在矽化鐵模板(pattern)上。這個特別的團聚現象與溫度及濺鍍厚度有關。其成長機制可以利用應力釋放及原子重新分布來解釋。
關於一維的奈米結構，吾人利用陽極氧化鋁模板以電鍍的方法成長鎳奈米線並研究了他們的磁性及磁運輸性質。陽極氧化鋁模板的孔洞大小介於35-75 nm之間，而鎳奈米線的結晶性可以藉由施加不同的偏壓來控制。在本研究中，吾人製備了多晶以及沿著[100]和[110]方向成長的單晶奈米線並發現奈米線的磁性及電性行為會受到結晶結構的影響，結晶異向性和形狀異向性之間的競爭是決定其磁性行為的主因。
最後，吾人也研究了被廣泛應用在自旋電子元件的稀磁性半導體。吾人利用多層膜成長的方式來製作摻雜鈷的氧化鋅薄膜，利用改變氧化鋅層以及鈷金屬層的厚度比例，吾人可以控制鈷離子在氧化鋅薄膜中的分布以及價電子狀態。穿透式電子顯微鏡的影像指出摻雜鈷的氧化鋅薄膜是多晶結構但具有(0002)優選方向。其中，氧化鋅(1.5 nm)/鈷(0.1 nm)的樣品具有均勻的鈷離子分布，鈷離子是透過在氧化鋅中擴散的方式摻雜於薄膜中。然而，降低或增加氧化鋅層的厚度分別會造成鈷金屬的析出及形成Zn1-x(Co)xO多層膜。吾人發現當氧化鋅厚度大於1.5 nm時，鈷離子會取代鋅離子，為二價電子。此外，所有的樣品都具有室溫鐵磁性並且其磁性和鈷離子的分布有關。

The aim of this research is to understand how the microstructure influences the magnetism and magneto-transport properties of nanomaterials and diluted magnetic semiconductors (DMS). Microstructural investigation by TEM is performed to characterize magnetic nanostrucutres in various dimensions. The materials covered in this thesis include iron and iron silicide nanoislands, Ni nanowires and Co-doped ZnO films.
First, self-assembled Fe islands were successfully grown on Si(001) by ion-beam sputtering at room temperature. Nanometer-scale iron islands, ranging from 25 to 100 nm with narrow size distributions, can be achieved with a silicide interfacial layer analogous to the S-K growth mode. Furthermore, the perpendicular magnetic anisotropy is found to fall away with increasing island size. It is suggested that surface effects from the morphologies of 3D islands are mainly responsible for the spin reorientation.
With increasing growth temperature to 200oC, the reactive interaction between Si and Fe leads to the formation of FeSi islands, the evolution of the growth of FeSi nanoislands on Si(001) is investigated. Under proper growth conditions, nanoislands spontaneously cluster into groups on rectangular FeSi terraces, depending on both substrate temperature and deposition coverage. This study discussed the self-clustering mechanism in the context of strain relaxation and mass transportation between nanoislands and terraces.
As for Ni nanowires fabricated by electrodeposition on Anodic-aluminum-oxide (AAO) templates, their magnetic and magneto-transport properties have been investigated. The AAO pores have diameters ranging from 35 to 75 nm, while the crystallinity of Ni NW arrays could change from polycrystalline to single-crystalline with the [111] and [110] orientations based on electro-deposition potential. The crystalline orientation of Ni NW arrays significantly influenced the corresponding magnetic and magneto-transport properties. It s suggested that these magnetic behaviors are dominated by the interplay between magnetocrystalline and shape anisotropy.
Finally, DMS, which is most commonly used for spintronics has also been studied. Co-doped ZnO films were synthesized by ion beam sputtering using multilayer (ZnO/Co) growth. Both the distribution and the chemical states of Co in ZnO can be well controlled by varying the ratio of the nominal layer thickness of ZnO to Co. Transmission electron microscopy indicated that all of the as-deposited Zn1-x(Co)xO films were polycrystalline with a (0002) preferred orientation. In ZnO (1.5 nm)/Co (0.1 nm), homogeneous Co-doped ZnO was observed to have been formed through inter-diffusion. However, decreasing or increasing the thickness of ZnO leads to the formation of Co clusters in the ZnO matrix or Zn1-x(Co)xO multilayers, respectively. For ZnO thickness≧1.5 nm, Co is substituted for Zn, and its valence state is 2+. All Co-doped ZnO films show room-temperature ferromagnetic behavior, which appears to depend strongly on the Co distribution.

Contents
Acknowledgement………………………………………………………………………….I
摘要………………………………………………………………………………………...II
Abstract…………………………………………………………………………………...IV
Contents…………………………………………………………………………………..VI
List of Tables……………………………………………………………………………....X
List of Figures………………………………………………………………………….....XI
Chapter 1 Introduction to magnetic nanostructures and diluted magnetic semiconductors…………………………………………………………………………….1                                                                       
1	Overview of nanostructures……………………………………......................................2
1.1 Atomic scale effects………………………………………………………………4 
2	Historical development of Fe and FeSi nanoislands …………………………………...6
2.1	Fe nanoislands……………………………………………………………………6
2.2	Iron silicide nanoislands…………………………………………………...........15
3	Historical development of magnetic nanowires……………………………………….23 
3.1	Electrodeposited magnetic nanowires…………………………………………..24
3.2	Magnetic properties……………………………………………………………..25
3.3	Applications……………………………………………………………………..30
4	Diluted magnetic semiconductor (DMS).......................................................................31 
4.1	What is DMS?.....……………………………………………………………….31
4.2	Review of ZnO-based DMS…………………………………………………….33
5	 Motivation and outline of the thesis….………………………………………………36
Chapter 2 Growth methods and magnetic behaviors………………………………….45
1 Growth methods for nanostructures and DMS…………………………………...........45
1.1 Self-assembly nanoislands………………………………………………………45
1.2 Synthesis of nanowires…………………………………………….……………47
1.2.1 Electrodeposition of nanowires………………………………….………47
1.2.2 Preparation of anodized aluminum oxides (AAO)………………...........48
1.2.3 Process of electrodeposition…………………………………………….49
1.3 Fabrication of DMS……………………………………………………………..52
1.3.1 Introduction to Zn1−xMxO compounds (M = transition metal elements)…………………………………………………………...….52
1.3.2 The synthesis……………………………………………………………52
2 Magnetic behaviors……………………………………………………………………53
2.1 Magnetic anisotropy..……………………………………………………...........54
2.2 Magnetoresistance………………………………………………………………58
2.3 Origins of room temperature ferromagnetism in DMS…………………………63 
Chapter 3 Sample Preparation and Experimental Apparatus………………………...72
1 Instrument.......................................................................................................................72
1.1 Ion Beam Sputter……………………........…………………………….....…….72
2 Experimental details…………………………………………………………………...74
2.1 Growth of iron and iron silicide nanoislands……………………………………74
2.2 Synthesis of Ni nanowires………………………………………………………75
2.3 Fabrication of Zn(Co)O films…………………………………………………...76
3 Experimental Apparatus……………………………………………………………….76
3.1 Transmission electron microscopy (TEM)…………………….………………..77
3.1.1 Imaging and diffraction pattern………………………………………….77
3.1.2 Scanning transmission electron microscopy (STEM)……………....…...79
3.1.3 Electron energy loss spectroscopy (EELS)…..………………………….80
3.1.4 TEM sample preparation………………………………………………...83
3.2 X-ray photoelectron spectroscopy (XPS).............................................................83
3.3 Scanning electron microscopy (SEM)……………………..................................84
3.4 Atomic force microscopy (AFM).........................................................................85
3.5 Superconducting Quantum Interference Device magnetometer (SQUID)...........86
3.6 Magneto-transport measurement…………………………………………..........87
Chapter 4 Characterization and fabrication of magnetic nanostructures: nanoislands and nanowires…………………………………………………………………………….90
1 Growth and characterization of epitaxial Fe nanoislands on Si(001): size effects on ferromagnetic anisotropy…………………………………………………………………..90
2 Self-clustering of epitaxial FeSi nanoislands on Si (001)……………………………102
3 Effect of microstructure on magnetic and magneto-transport properties of electrodeposited Ni nanowire arrays……………………………………………………..113
Chapter 5 High angle annular dark field and electron energy loss spectroscopy study on the substitution and distribution of cobalt in ZnO by multilayer growth………………………………………………...…….…………………………...129
Chapter 6 Conclusion………………….………………………………………………..141
Chapter 7 Future work…………………………………………………………………144
Appendix………………………………………………………………………........…...146
A.1 Curriculum Vitae…………………………………………………………………….146
 

                                    

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