以X光吸收光譜研究鈷摻雜氧化鋅稀磁性半導體之磁性來源

X-ray absorption spectroscopy study the origin of ferromagnetism in the Co doped ZnO diluted magnetic semiconductor

廖彥發(Yen-Fa Liao)

指導教授 ： 李志浩

國立清華大學/原子科學院/工程與系統科學系/博士(2009年)

https://doi.org/10.6843/NTHU.2009.00760

摘要

本研究中，我們利用同步輻射吸收光譜技術研究影響鈷摻雜氧化鋅稀磁性半導體磁性來源，並且藉由吸收光譜研究樣品之電子結構以釐清束縛極化子模型與本質性氧化物稀磁半導體之室溫鐵磁性之關連性。
首先，我們利用熱解法製備鈷摻雜氧化鋅奈米顆粒，同時發現奈米顆粒具有室溫鐵磁性質。我們同時觀察到磁性的大小隨著鈷離子摻雜的濃度增加而減少，同時在電子結構上面亦同樣觀察到有新的摻雜能帶的產生。我們可以成功的利用熱解法成長本質性稀磁性半導體奈米顆粒，並且排除鐵磁性是來自鈷原子金屬團簇的影響。
此外，我們利用退火改變奈米顆粒的氧缺陷，觀察氧缺陷在稀磁性半導體上面對於磁性強弱的影響。由吸收光譜和螢光光譜上發現氧缺陷數量隨著退火溫度增加而減少，同時樣品磁性也隨著退火溫度的增加而消失。證實了氧空缺是影響稀磁性半導體磁性的主要因素之一。
此外我們研究載子對於稀磁性半導體磁性的影響。我們首先使用離子束濺鍍高品質的氧化鋅磊晶薄膜，利用離子佈值方式控制氧化鋅半導體載子濃度和鈷摻雜離子的濃度，發現在不同鈷離子濃度下，室溫鐵磁性來源主要都是來自鈷原子金屬團簇。隨著鈷離子濃度增加，鈷原子金屬團簇的影響就越明顯。而高摻雜鈷原子的樣品磁特性表現越接近金屬鈷。
我們同樣利用熱退火方式改變鈷離子佈值在氧化鋅薄膜的磁性機制。在不同的退火溫度之下（600、700和800度C），研究其磁性與結構的關連性。發現鐵磁性來源有兩種機制：鈷原子金屬團簇和鈷格隙所引發的本質性磁性。在未退火的樣品中，磁性來源是由鈷原子金屬團簇而來的， 600度C退火的樣品則由兩種的混和相存在。而700度C退火的樣品磁性主要是由於鈷格隙所引發的本質性磁性。我們同時在軟X光吸收光譜中發現到新的摻雜能階，藉由研究700度C退火的樣品，我們發現磁性可能來自於鋅原子電子藉由氧空缺交互耦合轉移到鈷電子軌域，進一步證實氧空缺模型可合理的解釋氧化物稀磁性半導體磁性來源。

關鍵字

稀磁性半導體 ； X光吸收光譜 ； 同步輻射 ； 氧化鋅 ； 離子佈值 ； 磁圓偏振二向性能譜學

並列摘要

The purpose of this thesis is to study the origin of ferromagnetism of the Co doped ZnO diluted magnetic semiconductor (DMS) by synchrotron x-ray absorption spectroscopy. The origin of ferromagnetism in oxide-based magnetic semiconductor is closely related to the BMP model, which can be understood clearly by the formation of the electron structure in this system.
Firstly, we prepared the Co doped ZnO nanoparticles by thermal hydrolysis. The room temperature ferromagnetism was observed in these nanoparticles. The magnetization of the Co doped ZnO nanoparticles was decreased as the Co concentration increased. An impurity band structure caused by hybridization between the Co 3d states and Zn 4s states was observed by the soft x-ray absorption spectroscopy. The mechanism of the origin ferromagnetism that caused by the cobalt cluster effect was excluded in our system.
The annealing process has been used to change in the number of oxygen vacancies in the nanoparticles. The effect of oxygen vacancies leads to the magnetization in the Co doped ZnO nanoparticles. Decrease in the number of oxygen vacancies which are as follows the annealed temperature increased was determined by the x-ray absorption spectroscopy and photoluminescence spectroscopy. The magnetization of the sample was absence after annealing treatment. We consider that the magnetization of the samples are attributable to the effect of the oxygen vacancies in the Co doped ZnO nanoparticles.
Furthermore, we had studied the influence of the magnetization related to the carrier effect in DMS. High quality ZnO epitaxial thin film was deposited by ion beam sputtering. The concentration of the carrier and the cobalt dopants in ZnO host matrix are controlled by ion implantation method. With the observation of various cobalt doses, the origin of the room temperature ferromagnetism was caused by the metallic cobalt cluster in the Co implanted ZnO thin film. The effect of the cobalt cluster was observed positively as the implanted cobalt dose increased. The effect of the cobalt clusters leads to magnetic properties in high dose samples.
The annealing treatment has been used to study the origin of ferromagnetism in the Co implanted ZnO thin film. Intrinsic DMSs phase in the Co implanted ZnO thin film was observed after annealing process. Intrinsic DMSs is defined as the characteristic that the structure of the Co substituted on Zn sites results in the ferromagnetic coupling. We study the structural and magnetic properties of thermally annealed Co implanted ZnO thin film at 600℃, 700℃ and 800℃, respectively. Two mechanisms of ferromagnetism in DMSs are present in the following examples: the cobalt cluster effect and the intrinsic DMSs. We observed the formation of the cobalt cluster before annealing treatment. The magnetic properties of the 600℃ annealing sample show two ferromagnetic phases in these systems. The effect on the Co substituted in ZnO host matrix results in the magnetization in the 700℃ annealing sample. An impurity band observed at Zn L edge was measured by soft x-ray absorption spectroscopy. The origin of ferromagnetism in DMS might be due to the charge transfer that the Co substituted on Zn site leads to the interaction between the Co electron and Zn electron through oxygen vacancies in DMS. Hence, we believed that the oxygen vacancy model could explain to the origin of the ferromagnetism in oxide-based DMS.

並列關鍵字

Diluted magnetic semiconductor ； x-ray absorption ； synchrotron ； ZnO ； ion implantation ； XMCD

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