研究生: |
朱彥璋 Yen-Chang Chu |
---|---|
論文名稱: |
The growth kinetics of the elongated NiSi2 clusters and the shape transition between symmetric and asymmetric structures in epitaxial three-dimensional strained islands 長形NiSi2奈米磊晶結構的成長機制及三維奈米磊晶結構對稱及非對稱形狀間的轉變 |
指導教授: |
蔡哲正
Cho-Jen Tsai |
口試委員: | |
學位類別: |
博士 Doctor |
系所名稱: |
工學院 - 材料科學工程學系 Materials Science and Engineering |
論文出版年: | 2008 |
畢業學年度: | 96 |
語文別: | 英文 |
論文頁數: | 166 |
中文關鍵詞: | 三維磊晶結構 |
外文關鍵詞: | 3-D epitaxial structure |
相關次數: | 點閱:2 下載:0 |
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近年來,自發性生成的奈米磊晶金屬矽化物結構不論是在實驗上或是理論上都受到廣泛的矚目。因為具有微小的尺寸及良好的結晶性,這些結構有機會被用來製作自組裝的量子點或量子線。奈米磊晶結構的形狀及尺寸對結構本身的光性及電性都會有影響,因此瞭解奈米結構的成長機制以對奈米結構的形貌進行精確的控制是很重要的。
過鍍金屬奈米磊晶矽化物的成長機制一般都是引用自其它磊晶系統的成長機制。然而,這些機制是否真的適用於這些矽化物卻未經過詳細地確認。為了確認奈米磊晶矽化物的成長機制,此實驗對NiSi2奈米結構的結構、能量及形狀分佈做了詳盡的探討。
數種不同的NiSi2奈米結構在此實驗中被觀察到,各種結構和矽基板間的磊晶關係都被分析。實驗中發現,NiSi2的結構受到鍍膜速率的影響。藉由控制鍍膜速率在適當的範圍內,長方形NiSi2磊晶結構可單獨存在。這方法將有助於控制NiSi2磊晶的結構。
NiSi2磊晶結構的應變能在此實驗中以有限元素方法模擬而得。在和表面能相加後,我們可以推測NiSi2磊晶結構最穩定的形狀。因為實驗中觀察到的形狀分佈和理論計算的穩定形狀不合,我們認為長方形NiSi2磊晶結構之所以會沿一軸伸長的原因主要是由成長動力學的因素所引起。成長動力學的機制可成功用來解釋實驗中觀察到的NiSi2磊晶結構的形狀分佈。
有限元素方法在此實驗也被用來模擬應變能在不同磊晶結構中隨結構長寬比變化的情形。我們發現磊晶結構應變能隨長寬比變化的方式將受到結構的高度、磊晶薄膜和基板的彈性係數、及晶格的對稱性所影響。如果磊晶結構的高度相對於長度及寬度很小、磊晶薄膜的彈性系數相對於基板的彈性系數很小、或是薄膜及基板間具有非對稱的晶格匹配,則非對稱結構將具有較低的應變能。對稱結構具有較低的應變能在相對的條件下。藉由模擬所得的應變能結合磊晶結構可能的表面能,我們可以預測在不同成長條件下磊晶結構在成長過程中形狀變化的過程。
The spontaneous formation of epitaxial three-dimensional transition-metal-silicide nanostructures is an active subject in experimental and theoretical research in recent years. The small size and the crystalline perfection of these nanostructures imply the possibility of using them as self-assembled metallic quantum dots or quantum wires. Well-defined shapes and uniform sizes are the most critical issues concerning both optical and electronics applications of nanostructures. To control the morphology, we must truly understand the growth mechanisms of these nanostructures.
Most of the growth mechanisms used to explain the growth process of the transition-metal-silicide nanostructures are quoted from the growth mechanisms of other epitaxial systems, however, there is no confirmation that these theories are suitable for the epitaxial silicide nanostructures. To decide the growth mechanism of the epitaxial NiSi2 nanostructures, the structures, the energy, and the shape distribution of the epitaxial NiSi2 nanostructures are investigated exhaustively in this thesis.
Several kinds of epitaxial NiSi2 nanostructures were found in this experiment, and the epitaxial relationship with the Si substrates were analyzed for each structure. The structures of the NiSi2 nanostructures were found affected by the deposition rate; by controlling the deposition rate, the rectangular hut structures could exist alone. This method is helpful in controlling the structure of the epitaxial NiSi2 nanostructures.
Finite-element method was used to simulate the strain energy of the epitaxial NiSi2 nanostructures. After combining with the surface energy, the stable shape of the NiSi2 hut structures was determined. Because the shape distribution of the NiSi2 clusters is not consistent with the stable shape of the NiSi2 nanostructures calculated here, we infer that the elongation process of the NiSi2 clusters should be mainly governed by the growth kinetics. A growth process based on the kinetic model was used to explain the existence of the elongated NiSi2 clusters and the shape distribution of the clusters observed in this experiment.
Finite-element method was also used to simulate the strain energy of three-dimensional epitaxial structures which grow under different conditions. The evolution of strain energy with the aspect ratio (length/width) of the epitaxial structures was found to be affected by the height, the elastic constant of the substrates and the epilayers, and the symmetry of the lattice mismatch of these structures. Asymmetric structures will have smaller strain energy if the height of the epitaxial structures is fixed or growing very slowly, or the ratio of the elastic constant of the epilayer to that of the substrate is smaller, or the lattice mismatch between the substrates and the epilayers is asymmetric on the epitaxial surface. For different growth and surface energy conditions, the shape-transition process between symmetric and asymmetric structures is discussed.
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