臺灣博碩士論文加值系統

近幾年由於自旋電子學快速的發展,吸引了世界上許多尖端的研究團隊投入大量的人力及資源，致力於將電子自旋與現今的半導體工業結合起來，而使得以半導體為基礎的自旋電子學逐漸成為目前最熱門的研究主題之一。在本篇論文中，將著重於半導體自旋電子學的新材料開發，以及其材料相關物理性質之研究，例如:微結構分析、居禮溫度、以及磁異向性等。本論文可以細分成三個主要的研究主題:第一個研究主題將著重於新型稀磁性半導體材料的研究與開發。在固態反應法所製作的鈷添加氧化鋅塊材中，顯現高於室溫的鐵磁特性，甚至高於350 K。除此之外，我們亦發現不同的淬火條件以及銅元素的共添加將會明顯影響磁性的表現，經由電子顯微鏡以及X光繞射儀鑑定其微結構特性，氧化鋅的晶格常數隨著添加鈷含量或銅含量的增加而分別有些微的增加與減少的趨勢。利用化學分析能譜儀以及次奈米級X射線能量散佈分析儀觀察鈷元素的化學環境及分布狀態，可發現鈷元素呈現均勻的散布於氧化鋅的基材中，並形成單一穩定的固溶體相，並無顯現任何的二次磁性相或鈷團簇。在磁性研究方面，我們認為鐵磁性特性跟材料中的載子濃度有著密切的關係，抑制n型載子的形成，將會有效的增強磁性的表現。在第二個研究主題方面，我們藉由平面霍爾效應的量測來觀察在(Ga,Mn)As磊晶薄膜以及錳delta添加異質結構中的磁異向特性。對於(Ga,Mn)As磊晶薄膜以及錳delta添加的異質結構而言，對應的平面霍爾電阻變化分別約為90及300歐姆，然而對一般的磁性金屬而言，其變化約只有幾個微歐姆左右。這個顯著的電阻變化提供我們一個穩定且精確的分析方式進而檢測稀磁性半導體的各項磁特性，例如:居禮溫度、磁異向性、以及磁化反轉情形。經由實驗結果發現，(Ga,Mn)As磊晶薄膜主要呈現一個<110>雙軸磁異向性﹔而在錳delta添加異質結構方面，則呈現一個[110]單軸磁異向性，此結果相當的有趣，因為此異質結構的基板為閃鋅礦結構的砷化鎵，亦即為等方向性的立方型結構。截至目前為止，此單軸磁異向性的機制雖未相當的清楚，但經由文獻的報導，我們認為在錳delta添加的原子平面上，由於局部化的高載子濃度及高含量的錳原子，而會使得原為<110>雙軸磁異向性逐漸轉變成為[110]單軸磁異向性。最後，在第三個研究主題方面，我們發現將IrMn/(Ga,Mn)As異質結構經由短時間的後熱退火處理，可得到具有高居里溫度的鐵磁性Mn(Ga)As奈米薄膜。即使退火溫度只有100 oC，此一Mn(Ga)As薄膜仍可自組裝於反鐵磁層IrMn與稀磁性半導體層(Ga,Mn)As之間，藉由電子顯微鏡分析，我們發現此Mn(Ga)As薄膜的厚度將隨著退火溫度的增加而變厚，且呈現相當平整且均勻的高品質介面。除此之外，我們亦觀察到經由場退火處理過後的IrMn/Mn(Ga)As/(Ga,Mn)As異質結構，在低溫時呈現明顯的交換異向性，對溫度10 K而言，其對應的矯頑場與交換異向場分別約為260 ± 10 G 及 90 ± 10 G，此現象亦說明了在反鐵磁層IrMn與奈米鐵磁層Mn(Ga)As中存在著相當強的交換作用力。

Seminconductor-base spintronics has attracted intense attentions over last decade for its potential applications. In this work, we have focused on the exploration of developing new spintronics materials, and the corresponging distinict physical properties, such as microstructure, Curie temperature (TC), and magnetic anisotropy. Here, three main topics have been involved in details in this dissertation. In the first topic, we have demonstrated the high Curie temperature (TC >350 K) ferromagnetic semiconductor (Zn,Co)O fabricated by standard solid-state reaction method. In addition, the different quenching conditions and co-doping with the additional element Cu show remarkable influence on ferromagnetic behavior of polycrystalline (Zn,Co)O samples. The microstructure of these polycrystalline samples were identified by XRD and HRTEM, in which, the lattice constant of ZnO wurtzite structure reveal slight increase and decrease with small amount of additional Co and Cu doping, respectively. Furthermore, the chemical environments analysis by ESCA, and the composition distributions characterized by nano-beam EDS spectra mapping confirm that Co element was uniform and homogeneously dispersed in the ZnO matix, and exhibit a single-phase solid solution property instead of magnetic secondary phase of Co clusters. The enhancement effect of ferromagnetic behavior by various queching temperature or small amounts of additional Cu doping seems to be ascribed to the reduction of electron carriers. In the second topic, we have developed the planar Hall effect measurement to examine the in-plane magnetic anisotropy of epitaxial (Ga,Mn)As layer and Mn delta (δ)-doped GaAs-based heterostructures. The planar Hall resistance in (Ga,Mn)As layer and Mn δ-doped GaAs heterostructures measured at liquid helium temperature (2.6 K) reveal very large RPH jumps of ~90 ohm and ~300 ohm, respectively, that is more than four order lager than those of metallic ferromagnets. This observed ”giant” Hall effect enables a very sensitive measurement of the field, and it is also robust to determine the magnetic anisotropy and associated magnetization switching behaviors. In particular, a specific in-plane <100> biaxial magnetic anisotropy was observed in (Ga,Mn)As epilayer. However, Mn δ-doped GaAs heterostructures shows the distinct in-plan uniaxial magnetic anisotropy along [110], which is not expected on the basis of the zinc-blend structure of GaAs. It has been suggested that the locally high Mn and holes concentrations may have led to the distinct in-plan uniaxial magnetic anisotropy along the [110] direction in the Mn δ�w-doped atomic plane. Finally, in the last topic, we have demonstrated a new formation method for obtaining nano-thickness ferromagnetic Mn(Ga)As layer by post field-annealing treatment of IrMn/(Ga,Mn)As hetero-structure. The Mn(Ga)As reaction layer can be self-organized between DMS (Ga,Mn)As and antiferromagnetic IrMn layers even though the annealing temperature is as low as 100 oC. In particular, TEM analyses clearly indicated the monotonical increase in thickness of Mn(Ga)As layer with increasing annealing temperature and high quality interface distribution through the whole sample. Furthermore, we have demonstrated the exchange bias effect in IrMn/Mn(Ga)As/(Ga,Mn)As hetero-structure at low temperature. The coercive field HC and exchanged field HE are estimated to be 260 ± 10 G and 90 ± 10 G at 10 K, respectively. These implication are reasonably attributed by the effective exchange coupling interaction between antiferromagnetic IrMn layer and ferromagnetic Mn(Ga)As layer.

Abstract (Chinese) I
Abstract II
Acknowledgement IV
Table of Contents V
Lost of Figures IX
List of Tables XVIII

Chapter 1 Motivation 1
1-1 Introduction to spin based electronics (Spintronics) 1
1-1-1 Metal-based spintronics 2
1-1-2 Semiconductor-based spintronics 8
1-1-3 Challenges in Semiconductor-based Spintronics 13
1-2 Purposes of study 19
1-2-1 Novel Diluted Magnetic Semiconductor with high Curie temperature 19
1-2-2 Particular characteristic of (III,Mn)V 20

Chapter 2 Literature Review 22
2-1 Curie Temperature TC in Diluted Magnetic Semiconductors 22
2-1-1 A synopsis of DMS theory 22
2-1-2 Experimental Results of III-Mn-V systems 31
2-1-3 Experimental Results of Zinc-Oxide (ZnO) systems 34
2-2 Properties of III-V based Diluted Magnetic Semiconductors 37
2-2-1 Lattice properties 37
2-2-2 Magnetic Anisotropy 41
2-2-3 Anomalous Hall Effect (AHE) & Planar Hall Effect (PHE) 45
2-2-4 Electronic structure, MCD study, Structure defects 49
2-3 Spin Functionality 53
2-3-1 Electric-field control of ferromagnetism 54
2-3-2 Large Tunneling Magnetoresistance in Ferromagnetic Semiconductor Tunnel Junctions 56
2-3-3 Current-Driven Magnetization Reversal 57
2-3-4 High spin-polarization in GaAs spin injection from a (GaMn)As Zener diode 59

Chapter 3 Experimental Procedures 62
3-1 Preparation of diluted magnetic semiconductors 62
3-1-1 Solid state reaction 62
3-1-2 Molecular-beam epitaxy method (MBE) 63
3-1-3 Fabrication of Hall-bars 67
3-2 Magnetic measurements 69
3-2-1 Superconducting Quantum Interference Device (SQUID) Magnetometer 69
3-2-2 Vibrating Sample Magnetometer (VSM) 71
3-2-3 Hall measurements 72
3-3 Others analysis methods: x-ray diffraction. TEM, XPS 74

Chapter 4 Ferromagnetism above Room Temperature in Bulk Co-doped ZnO 75
4-1 Introduction 75
4-2 Experimental Details 78
4-3 Basic properties of (Zn1-xCox)O 78
4-3-1 Structure analyses of (Zn1-xCox)O 78
4-3-2 Magnetic property of bulk (Zn1-xCox)O 81
4-3-3 XPS study 82
4-3-4 Discussion 82
4-4 Effect of quenching temperature 84
4-4-1 Experimental Details 84
4-4-2 Structure analyses 84
4-4-3 Magnetic properties 87
4-4-4 Discussion 89
4-5 Effect of additional Cu doping 90
4-5-1 Experimental Details 90
4-5-2 Structure analyses of Zn0.98-xCo0.02CuxO 91
4-5-3 Magnetic properties 93
4-5-4 TEM analyses 97
4-5-5 Photoluminescence (PL) analyses 100
4-5-6 XPS study 101
4-5-7 Discussion 103
4-5-8 Conclusions 105
4-6 Grand summary 106

Chapter 5 Planar Hall Effect and Magnetic Anisotropy of epitaxial (Ga,Mn)As and GaAs-Based Heterostructures with Mn Delta Doping 108
5-1 Introduction 108
5-1-1 Mn δ-doped GaAs/p-AlGaAs heterostructures 109
5-1-2 Planar Hall Effect 110
5-2 MBE Growth and Experimental Details 112
5-3 PHE and Magnetic Anisotropy in epitaxial Ga0.95Mn0.05As 114
5-3-1 Angular Dependent PHE 114
5-3-2 Planar Hall effect and Magnetic Anisotropy 118
5-3-3 Discussion 120
5-4 PHE and Magnetic Anisotropy in Mn δ-doped GaAs/p-AlGaAs heterostructure ………………………………………………………………………………121
5-4-1 Temperature Dependent PHE 121
5-4-2 Angular Dependent PHE 123
5-4-3 Planar Hall effect and Magnetic Anisotropy 128
5-4-4 Temperature dependent magnetic anisotropy 130
5-4-5 Dependence of PHE on the current direction 131
5-4-6 Low temperature annealing effect 133
5-4-7 Discussion 137
5-5 Grand summary 139
Chapter 6 Influence of annealing conditions on exchange bias of IrMn/GaMnAs heterostructures 140
6-1 Introduction 140
6-1-1 Purpose of study 140
6-1-2 Exchange Bias effect between antiferromagnetic-ferromagnetic systems 142
6-2 Experimental Details 145
6-3 Magnetic properties 146
6-3-1 Magnetization (M) versus Temperature (T) 147
6-3-2 Magnetization (M) versus Magnetic Field (H) 149
6-3-3 Annealing Effect 154
6-4 Microstructural Analyses 155
6-4-1 AFM analyses 155
6-4-2 TEM and EDS analyses 157
6-5 Discussion 166
6-6 Grand summary 167

Chapter 7 Concluding Remarks and Prospects 168
7-1 Conclusions of this study 168
7-2 Suggested future works 171

References 172
Appendix I 184
Appendix II 187

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