在低溫下研究Pb/Si(111)表面Si原子沉積的早期階段:吸附，動態過程，擴散和操作

Study of early stages of deposition of Si atoms at Pb/Si(111) surface at low temperature: Adsorption, dynamic processes, diffusion, and manipulation

庫瑪(Kumar, Rakesh)

指導教授 ： 黃英碩 ； 李志浩

國立清華大學/原子科學院/工程與系統科學系/博士(2017年)

摘要

我們使用了可變溫掃描穿隧電子顯微鏡(VT-STM)研究矽原子的吸附行為、矽原子 的動態特性以及在鉛/矽(111)表面上的矽原子之操控。本論文共分為三個部分。在 第一部分，探討了單個矽原子於低溫表面上的吸附狀態與位置。第二部分則討論了矽 原子的動態特性和穩定性。在第三部分，通過使用掃描穿隧電子顯微鏡之針尖，操作 吸附的矽原子沿著或跨越三聚體之直列來運動。 此外，矽二聚體的性質和動力學於此 論文中也有所探討。
在第一部分，研究了在低溫(〜125K)下鉛/矽(111)表面上單個矽原子的吸附。眾 所周知，在室溫下，矽(111)表面上的單層鉛表現出(1×1)結構。這種表面結構在 轉變溫度為270 K時會可逆地轉變為低對稱的(√7×√3)-鉛結構。在125 K時，我們 發現了吸附的矽原子是不會擴散，因此可以詳細檢查了其吸附位置。單個矽原子傾向 於出現在矽(111)基板的頂部位置(T1位點)附近。吸附的矽原子在高穿隧電壓差 條件下的電子空態和滿態的掃描穿隧電子顯微鏡圖像具有不同的外觀;然而，它們在 低穿隧電壓差下具有相同的外觀。原子解析的掃描穿隧電子顯微鏡圖像顯示吸附的矽 原子優先佔據T1A或T1B的位置處。表面上單個矽原子的吸附行為為(√7×√3)-鉛結 構中的鏡像對稱性破壞提供了有力的證據。
在第二部分，我們研究了升溫時矽原子的穩定性和動力學。我們發現了矽原子在高於 150K的溫度下開始在鉛三聚體內的T1A和T1B位置之間轉變。當溫度升至約160K以上 時，吸附的矽原子可以沿相同的三聚體直列躍遷到其它三聚體。低於〜170K，主導的 是相鄰三聚體間的短跳，但是溫度高於〜170K時，長跳占了主要部分。在160〜174K 的溫度下連續時間成像的分析，我們得出的矽原子擴散的活化能和前因子。此外，在
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高於〜170K的溫度下，單個矽原子到矽簇的不可逆聚集開始發生在相邊界或缺陷的部 位。在高於〜180K的溫度下，幾乎所有矽原子在表面上消失並聚集成簇，這可能對鉛 / 矽(111)表面上矽原子的外延生長機制有重要的意義。
本論文的最後部分中，我們使用掃描穿隧電子顯微鏡的針尖在〜125K處橫向操控了單 個矽原子。在這個樣品溫度下，矽吸附的原子不會擴散。使用掃描穿隧電子顯微鏡針 尖並通過採用單點I-Z能譜，我們可以操控矽原子使其沿著表面三聚體直列或跨過表面 三聚體直列移動。在同一個三聚體直列上，一個矽原子被帶往更接近另一個矽吸附原 子的位置。當形成矽二聚體時，其將在鉛覆蓋的表面上快速擴散，直到其被邊界或缺 陷捕獲。這項工作展示了可以使用原子操控方法來揭示一事實:僅用掃描穿隧電子顯 微鏡幾乎是不能觀察到最初期的原子表面動態過程。此外，原子操控揭示了矽吸附原 子在樣品表面上的真實原子位置，這解決了掃描穿隧電子顯微鏡圖像可能不能反映吸 附原子的真實位置的常見問題。

關鍵字

吸附行為 ； 動力學屬性 ； 跳躍能量 ； 不可逆的聚集 ； 原子操縱 ； 熱行為

並列摘要

Si atoms adsorption behavior, Si atoms dynamic properties, and the manipulation of Si atoms on the Pb/Si(111) surface have been studied with variable-temperature scanning tunneling microscopy (VT-STM). This thesis has been divided into three parts. In the first part, the adsorption of single Si atoms and the adsorption sites on the surface at low temperature (LT) have been reported. The dynamic properties and stability of the Si atoms have been discussed in the second part of this thesis. In the third part, Si adatoms were manipulated either along or across the trimer row by using STM tip. Additionally, the nature and dynamics of Si dimer have been explored.
In the first part, the adsorption of single Si atoms on the Pb/Si(111) surface at low temperature (~125 K) has been studied. It is well known that at room temperature a monolayer Pb on Si(111) surface exhibits (1×1) structure. This surface structure transformed reversibly into a low- symmetry row-like (√7×√3)-Pb structure at a transition temperature ~270 K. At 125 K, the adsorbed Si atoms are found to be immobile and their adsorption sites were examined in details. Single Si atoms tend to appear near the on-top site (T1 site) of the Si(111) substrate. The adsorbed Si atoms have different appearance at empty- and filled-state STM images in high tunneling conditions; however, they have the same appearance at low tunneling conditions. The atomic-resolved STM images revealed that adsorbed Si atoms prefer to occupy either at T1A or T1B sites. The adsorption behavior of single Si atoms on the surface provided a strong evidence of breaking of mirror symmetry in the (√7×√3)-Pb structure.
In the second part, the stability and dynamics of Si atoms have been studied with increasing sample temperature. We have found that Si atoms started to switch between T1A and T1B sites inside a Pb trimer at the temperature higher than 150 K. When the temperature raised above
~160 K, the adsorbed Si atoms could hop to other trimers along the same trimer row. Below
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~170 K, short hops to adjacent trimers dominated, but long hops dominated at temperature above ~170 K. The activation energy and prefactor for the Si atoms diffusion were derived through analysis of continuous-time imaging at temperature from 160 to 174 K. In addition, irreversible aggregation of single Si atoms into Si clusters started to occur at the phase boundaries or defective sites at temperature above ~170 K. At temperature above ~180 K, nearly all Si atoms disappeared on the surface and aggregated into clusters, which may have important implications on the atomic mechanism of epitaxial growth of Si on the Pb/Si(111) surface.
The lateral manipulation of single Si atoms was demonstrated at ~125 K using the tips of an STM and this study has been included in the last part of this thesis. At this sample temperature, Si adatoms are not mobile. STM tips were used to move Si adatoms either along or across the surface trimer rows by employing single-point I-Z spectroscopy. One Si adatom was brought closer to another Si adatom on the same trimer row. When a Si dimer was formed, it diffused rapidly on the Pb-covered surface until it was trapped by the domain boundaries or defect sites. This work demonstrates the prospective to use atomic manipulation methods to reveal the surface dynamic processes that hardly be observed with STM alone. In addition, the manipulation revealed the true atomic positions of Si adatoms on the sample surface, which solves a common problem that STM images may not reflect the real positions of adsorbed adatoms.

並列關鍵字

Adsorption behavior ； Dynamics properties ； Hopping energy ； Irreversible aggregation ； Atom manipulation ； Thermal behavior ； VT-STM

參考文獻

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