奈米銀結構在鉛量子島上之自我有序成長與電性結構

Date
2005
Authors
邱雅萍
Ya-Ping Chiu
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Abstract
本篇論文主要是利用掃描穿隧顯微鏡(STM)探討鉛覆蓋在矽(111)表面上的系統之研究。由於鉛覆蓋在矽表面上時並不會和矽發生化學或互溶的反應,因此鉛覆蓋在矽表面的系統是一個研究金屬覆蓋在半導體上的典型系統。然而,鉛覆蓋在矽表面時, 隨著不同的溫度或是加鍍量的改變所呈現的相圖非常複雜。例如,當鉛的加鍍量大約是單一鉛原子層厚覆蓋在矽(111)表面上時,在室溫之下隨著鉛量的增加,鉛則會從(1x1)相轉變到不相稱相(incommensurate phase)。在本論文中 將近似單一鉛原子層厚覆蓋在矽(111)表面上的多重相相圖,將實驗上所表現出多重相的穩定度和鍍量及溫度的關係以所謂的階梯般相圖來表示。將同一區域漸漸增加鍍量,可以觀察到缺陷的存在將影響到相變化的過程。而這種因為缺陷的存在對這個系統相變化的影響也將在此探討及和理論的預測比較。 第二部分, 以矽(111)的(7x7)重構表面當成模板成長奈米大小的鉛島。鉛加鍍在矽(111)-(7x7)重構表面的成長過程將研究從起始成核到最後成長成鉛島的整個過程。然而, 同一區域隨著鍍量的增加的觀察研究, 將可更清楚地表示出三維的鉛原子團形態上如何轉變成以偏好的層厚存在的鉛島。 第三部分, 將探討鉛在另一表面, 不相稱相(incommensurate phase), 上的成長行為。 矽(111)的(7x7)重構表面和不相稱相(incommensurate phase)的最大差異即在於電子是否表現出穩定的量子阱態行為。這個差異將造成鉛在這兩種表面的成長行為有不同的表現。而這種的成長差異將在論文的第三部分討論。 將鉛加鍍在溫度約200K時, 同一層高的鉛島將呈現出同一週期但不同對比的影像。而這種對比的不同, 是源自於電子在因堆疊不同的介面上散射的結果。對於擁有較明顯影像對比的鉛島, 稱之為第一形態的鉛島, 而具有較模糊影像對比的鉛島, 則稱之為第二形態的鉛島。而這種因電子因素所造成的圖案可被用來當作模板成長不同材料的奈米結構。以銀為例, 加鍍在第一形態的鉛島時, 可形成高度有序性的二維陣列排列。而所受到的束縛能大小也被量化計算出。由計算結果得知, 銀加鍍在第一形態的鉛島和第二形態的鉛島時受到迥然不同的束縛能強度。 這個結果也被用來表現出鉛島上影像對比清晰程度具有和鉛島的兩倍層高相同的震盪週期特色。再者, 穿隧電流大小對所加的偏壓微分的量測也可表現出鉛島上影像以雙層鉛島高為震盪週期的特色。 在穿隧電流大小對所加的偏壓微分的量測中, 在能譜上表現出微小的震盪, 這個結果意味著有源自於橫向的電子束縛能態的貢獻。因此 在這本論文的最後一部分, 將對奈米銀結構的電子形態隨著奈米銀成核在不同位置、或奈米銀結構的大小及形狀之關係探討。另外, 對於鉛島對奈米銀結構的電子形態的影響, 也將研究及探討。
Self-organized growth and electronic structures of Ag nanopucks on Pb quantum islands Abstract Scanning tunneling microscopy (STM) is employed to study the system of Pb deposited on a Si(111) surface. The low reactivity and solubility of Pb in bulk Si makes it an ideal model of a metal/semiconductor system. However, the phase diagram of this system is very complex and varies with the coverage and temperature. At room temperature when the coverage of Pb is nearly one monolayer on the Si(111), a phase transition from the 11 phase to the incommensurate phase occurs as the coverage increases. In this thesis, a new phase diagram of a near-single monolayer of Pb deposited on Si(111), called the “devil’s staircase” phase diagram, experimentally reveals the stability of multiple phases as the function of coverage and temperature. In-situ experimental observations show that the presence of defects can critically affect the phase transition. The effect of defects on the phase transition of a system is also considered by comparing with theoretical and experimental phase diagrams. Second, the Si(111)77 surface acts as a template for the growth of Pb nano islands. The growth behavior of Pb deposited on the Si(111)77 surface is investigated from initial nucleation to growth. In-situ observations indicate the relationship between the morphological transition and the preferred thickness of the Pb islands on the Si(111)77 surface. Third, the growth behavior of Pb on another phase, the incommensurate phase of Pb/Si(111)11, is also studied. The primary difference between Pb islands grown on the Si(111)77 and those grown on the incommensurate phase of Pb/Si(111)11 surface is the appearance of the quantum-well states of electrons. This characteristic elucidates the great difference between the growth properties of Pb on these two surfaces. The third section discusses the growth characteristics of Pb on the incommensurate phase of the Pb/Si(111)11 surface. When Pb islands grow on the incommensurate phase of Pb/Si(111)11 at low temperature, they form patterns of different contrasts. These patterns are resulted from electron scattering, which is related to the variation in the potential of the two different stackings at the Pb-Si interface. The Pb islands with a strong contrast pattern are thus called Type I islands, and those with a weakly contrasting pattern are called Type II islands. The patterns with an electronic origin can be used as a template for growing various nanoclusters (or called nanopucks). In the case of Ag, Ag nanoclusters can be grown in an ordered two-dimensional array on Type I islands. The trapping power is also measured and quantified. The large difference between the binding energies of Ag nanopucks grown on Type I and Type II islands further demonstrates the bi-layer oscillatory behavior of Pb islands. The dI/dV measurements also indicate the oscillatory behavior of the bi-layer . The dI/dV measurements reveal small undulations in each spectrum, suggesting the lateral electron confinement in Ag nanopucks is associated with the electronic structures. Therefore, the final section of this thesis explores the electronic properties as manifested in site-, size- and shape-dependent characteristics. The effect of substrate Pb islands on electronic structures, as the metal nanoclusters are grown on the metal surface, is examined and discussed.
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掃描穿隧顯微鏡, 奈米結構, 鉛量子島, 自我有序成長, 電性結構
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