臺灣博碩士論文加值系統

由於現今自旋電子具前瞻性的運用，因而稀磁性半導體被廣泛的研究探討，特別是氧化物磁性半導體同時具有寬能帶半導體特性及鐵磁特性，許多的實驗驗證氧化鋅和氧化鈦在攙雜錳、鈷、鎳等磁性元素，其居禮溫度可達室溫。最近研究發現Co:TiO2之磁性機制主要並非來自於自由載子所造成之鐵磁性質，而且存在稀磁與介電特性。因此我們對於氧化鋅攙鈷此氧化物磁性半導體系統，亦針對介電特性及其鐵磁性質進一步深入研究與探討。
此研究對於將氧化鈷與氧化鋅混合於真空10-3torr做400-500℃固態燒結反應所製成之氧化鋅攙鈷材料，進行高頻阻抗與介電特性之探討，比較控氧下不同燒結對於晶體結構和磁性之影響，由XRD 、Raman、 HRTEM、EDXS 和 Co-XAS光譜量測可知結構上大致良好，證明無鈷金屬團簇存在，但可能出現氧空缺行為，而由VSM 量測亦發現在真空燒結下會有室溫鐵磁特性。藉由阻抗頻譜分析儀對於氧化鋅攙鈷做高頻磁電傳輸之機制探討，具有磁性半導體之高絕緣特性，並驗證此為真空燒結下增加本質鐵磁特性並存在介電行為，磁性主要來源並非自由載子所感應的鐵磁性，而偏向於局域磁偶極矩之行為，缺陷在磁性機制上扮演重要角色。

Dilute magnetic semiconductors (DMS) have been extensively studied in recent years due to promising appliacation for spintronics. Oxide-diluted magnetic semiconductors (O-DMS) are particularly interesting to the coexistence of wide bandgap semiconducting and ferromagnetic properties. Many reports have elaborated on several experimental studies about room temperature ferromagnetism in Mn/Co/Ni-doped ZnO- and TiO2-based systems. Recent studies of the mechanism of ferromagnetism in Co:TiO2 have already come to the conclusion that the presence of free carriers is not required for this material to be ferromagnetic. Furthermore, the Co-doped TiO2 system was referred to as a “dilute magnetic dielectric” rather than DMS before. It is now an interesting research that we could extend Co-doped ZnO to other O-DMS systems which embrace the ferromagnetism and the dielectric state, and explore mechanism underlying for O-DMS.
This thesis displays the study of high frequency impedance and dielectric properties of Zn1-xCoxO samples produced by solid-state reaction of mixing CoO with ZnO nano-powders. Moreover, the samples are annealed at 400-500℃ in 10-3torr vacuum. Structural and magnetic measurements of samples come out both before and after annealing. The crystalline structure quality is characterized by x-ray diffraction, Raman, HRTEM, EDXS, and Co-XAS spectra measurements. Also, there is no detectable evidence for the presence of Co metal or Co-rich clusters within the samples. The vibration sample measurement has shown the room temperature ferromagnetism of these samples under the circumstances of the vacuum annealing. The high frequency impedance spectra and dielectric properties, measured by impedance analyzer (HP4284), vary systematically with different doping of Co. Higher resistance and dielectrics of the samples are based on the vacuum annealing which enhances ferromagnetism. Our research provides the experimental evidence of the intrinsic ferromagnetism coexisting with the dielectric state in the Co:ZnO systems. The result suggests that the theoretical model of free charge carriers are not required for magnetic ordering in the O-DMS systems. Co-doped ZnO has the dilute magnetic dielectric behavior.

Contents

List of Figure……………………………………………………………I
List of Tables…………………………………………………………VII

Chapter 1 Introduction……………………………………………1
1.1 Semiconductor spintronics……………………………………2
1.1.1 Ferromagnetic Ⅲ-Ⅴ Semiconductors……………………2
1.1.2 Oxide-diluted magnetic semiconductors………………6
1.2 Motivation and thesis outline……………………………10

Chapter 2 Fundamental of Dilute Ferromagnetic Semiconductor and Related Impedance Theory…………………………………………13
2.1 Origins of ferromagnetism in DMS………………………13
2.1.1 Exchange in insulators: direct and superexchange………14
2.1.2 Carrier-mediated exchange………………………………15
2.1.3 Bound magnetic polarons…………………………………17
2.2 Soft X-ray absorption spectrum and MCD analysis …………19
2.3 Impedance spectrum……………………………………………21
2.3.1 Impedance-related Function…………………………21
2.3.2 S-parameter method……………………………………23
2.4 Frequency response in materials…………………………25
2.4.1 Dielectric relaxation……………………………………26
2.4.2 Electric modules analysis………………………………28
2.4.3 Magnetic after……………………………………30
2.4.4 Impedance in loss media………………………31
2.5 Defect…………………………………………………………32
2.6 AC conductivity……………………………………………33

Chapter 3 Sample Preparation and Measurements………39
3.1 Sample preparation……………………………………………39
3.2 Experimental instruments……………………………………41
3.2.1 Raman spectrum………………………………………………41
3.2.2 X-Ray absorption spectrum…………………………………44
3.2.3 Vibrating sample magnetometer …………………………45
3.2.4 Impedance analyzer…………………………………………47

Chapter 4 Measurement Result and Theoretical Analysis of Oxide Dilute Magnetic Semiconductor………………………………………50
4.1 Magnetic properties of Co-doped ZnO………………………50
4.1.1 Magnetic characteristics……………………………………50
4.1.2 Magnetic permeability of O-DMS……………………………54
4.2 Optical analysis……………………………………………………57
4.2.1 X-ray Diffraction pattern……………………………………57
4.2.2 Raman studies on DMS materials……………………………58
4.2.3 HRTEM and EDS analysis…………………………………………60
4.2.4 XAS and XMCD discussion………………………………………64
4.3 Impedance analysis……………………………………………70
4.4 Theoretical analysis of dielectric spectrum……………82
4.5 AC conductivity for Co-doped ZnO electric conduction mechanism…………………………………………………………………………93
4.6 Electromagnetic theoretical analysis for Co-doped ZnO magneto-transport properties…………………………………………107
Chapter 5 Conclusions …………………………………………………112

Bibliography…………………………………………………116

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