氧化鋅奈米線基本物理性質之研究

Fundamental Physics Properties of ZnO Nanowires; Optical, Electrical and Magnetic Properties

吳忠益(Zhong-Yi Wu)

指導教授 ： 開執中 ； 陳福榮

國立清華大學/原子科學院/工程與系統科學系/博士(2009年)

摘要

隨著科技不斷的進步，現今當紅的磁、光、電三大產業的發展也邁入了奈米尺度之維度。當材料的尺寸縮小至奈米(Nano-meter)的大小，其物理特性也隨之變化，因此近年來半導體奈米線的合成技術與物理性質分析是個相當熱門的研究主題，並且可提供未來在各式新穎的奈米電子元件上應用之依據。
由於製備容易、價格低廉、穩定安全、透明且具有優秀的光電特性等，使得ZnO成為熱門的光電半導體材料。在本論文中，首先利用高溫退火爐在不同條件下製備圓柱形與六角形兩種不同形貌的ZnO奈米線，並觀察兩種奈米線的形貌包括表面狀態與粗糙度，去討論奈米線成長與疊差形成之機制，最後把研究焦點放在觀察其光學及電學性質之測量上的差異。
結果發現六角型ZnO奈米線其線徑尺寸分佈較不均勻，具有筆直的外觀，頂端為 的平面與六個最低指數軸 非常平整側面，在結構中未觀察到疊差；而圓柱型ZnO奈米線其尺寸可由金催化劑大小所控制，尺寸分佈均勻，形貌上有些微彎曲的現象；並且觀察到奈米線之側面表面粗糙度與疊差的存在，與探討結構中疊差形成機制。從PL分析結果發現圓柱形ZnO奈米線整體具有較高的缺陷濃度導致綠光相對強度較六角形ZnO奈米線高，另外觀察到奈米線“表面狀態”應是造成UV光峰值異常藍移的主要原因。之後從四點電極與TEM-STM量測結果，發現結構中載子濃度的差異造成圓柱型奈米線的電阻率較六角型奈米線為低，與光學測量結果一致。另外針對在量測過程中可能造成兩種奈米線接觸電阻差異的原因進行詳細的討論，結果發現除了本身ZnO奈米線結構中載子濃度的差異，另外與探針所接觸局部表面狀態差異與電子束輻照過程等，均可能改變其接觸界面之蕭基能障高度進而造成接觸電阻的差異。這些研究結果對於將來利用ZnO奈米線作為材料選擇與製程元件上，提供相當重要的參考依據。
另外將成長之不同形貌與尺寸的ZnO奈米線利用離子佈植的方法摻雜Co離子以合成稀磁半導體合成Zn1-xCoxO奈米線，並進行結構分析與磁性質量測。
針對剛佈植後之ZnO奈米線進行結構與成分分析，指出Co含量與結構中疊差數目隨著佈植劑量而增加，經由磁性量測均顯示為順磁訊號並無鐵磁行為。進一步利用三種不同退火條件，包括Ar氣退火、真空退火與氧氣退火進行磁性質的比較。結果發現Ar氣退火後消除部分佈植所造成的缺陷，並且開始產生磁滯行為，而真空退火後明顯的增強其鐵磁訊號且均高達室溫(300K)，接著氧氣退火卻導致鐵磁訊號的明顯的減弱。這部份重要結論主要是利用不同條件的退火效應，分別造成Zn1-xCoxO奈米線本身結構修復與提高氧缺陷濃度，前者結構修復為奈米線起初鐵磁形成機制的關鍵，而後者提高氧缺陷濃度是造成室溫鐵磁的主因。
最後量測不同尺寸與形貌之Zn1-xCoxO奈米線的磁性質，其主要目的為探討尺寸效應對鐵磁機制的影響，而重要的結論為尺寸越小的Zn1-xCoxO奈米線，因為表面積比越大，經過真空退火後，奈米線中氧原子越容易擴散出表面形成氧缺陷，進而達到室溫鐵磁行為，上述結果較符合束縛磁極化子模型（Bound magnetic polaron model）理論，強調氧缺陷濃度提高導致束縛磁極化子數量增加，彼此極化子間磁區距離變近，是形成Zn1-xCoxO奈米線室溫鐵磁機制的重要過程。

關鍵字

氧化鋅奈米線 ； 基本物理性質 ； 稀磁半導體 ； 詞性質 ； 電性 ； 光學

並列摘要

We have fabricated the cylindrical and hexagonal-shaped ZnO nanowires by using thermal evaporation with different growth mechanism. Firstly, we will focus on the difference of crystal characterization, structure defects, and physic properties between the two types of ZnO nanowires, which were inspected by using scanning electron microscopy, x-ray diffraction, high-resolution transmission electron microscopy, photoluminescence, four-probe electric measurement and two-probe TEM-STM instrument. From the results of this study, the hexagonal ZnO nanowires yield better stoichiometry and optical property. The electrical measurement of TEM-STM technique indicates that the resistivity of hexagonal nanowires will be ten times higher than those in cylindrical nanowires. In contrast, the cylindrical ZnO nanowires display lower resistivity and higher density of green defect emission. The above findings are closely related to the structure defects, including surface roughness and point defects such as O-vacancies or Zn-interstitials, which results from the non-stoichiometric compounds. Additionally, I will discuss three kinds of mechanism of forming stacking faults in cylindrical ZnO nanowires which are closely related to its growth mechanism.
Furthermore, we prepared Zn1−xCoxO nanowires by using Co ion implantation, and study its magnetic properties. The bombardments by Co ions produced a good number of structural defects (stacking faults and orientational variations) in the nanowires. The as-implanted Zn1−xCoxO nanowires were paramagnetic. We performed two types of thermal annealing, one in 1 atm argon flow and the other in a high vacuum, at 600 ◦C, and studied the effects of annealing on the magnetic properties of these nanowires. Argon annealing removed structural defects in the nanowires and the nanowires then revealed ferromagnetic ordering. This result suggests that structure defects are harmful to the occurrence of ferromagnetism in the Co-implanted ZnO. Noticeably, the nanowires even displayed largely enhanced ferromagnetism after annealing in a high vacuum. A subsequent annealing in oxygen has also been performed on those vacuum-annealed nanowires to study the roles played by the O vacancies in determining the ferromagnetic properties of the nanowires. Our results indicate that both the improved structural quality and the increased number of O vacancies are key factors for the occurrence of ferromagnetic ordering in the Zn1−xCoxO nanowires. The furthermore investigation will discuss the magnetic properties of size effect on cylindrical Zn1−xCoxO nanowires. The important conclusion is high surface-to-volume ratio in thin Zn1−xCoxO nanowires also facilitated enhancement in ferromagnetism through the high-vacuum annealing process, which is closely corrected the O vacancies. The results provide a unified basis for better understanding mechanism of ferromagnetism in Zn1-xCoxO system.

並列關鍵字

ZnO nanowires ； surface effect ； Photoluminescence ； Diluted magnetic semiconductor ； Zn1−xCoxO nanowires ； Electric measurement ； O vacancies ； Mechanism of ferromagnetism

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