氫與氯在矽晶面上的交互作用與競爭

Abstract

氫本論文在研究氫原子與氯原子在矽(100)表面所發生的各種交互反應，涵蓋課題包括氣體-表面交互作用、吸附原子的擴散以及雙原子分子的化學吸附。實驗方法主要利用變溫掃瞄穿遂顯微術 (variable-temperature scanning tunneling microscopy, VT-STM) 、核心層光電子激發術 (core-level photoemission spectroscopy) 及電腦模擬。STM提供原子解析影像，可分辨表面上不同的吸附原子，也可看出表面反應發生的位置。核心層光電子激發術則利用核心層電子束縛能的位移，來判斷表面原子鍵結型態以及吸附原子的種類與比例。結合此兩種顯微術及光譜學的技術，便可更進一步解析表面的各種反應。 本論文大致編排如下：第一章簡介研究動機與文獻，概述矽(100)重構後之表面結構與單純的氫或氯飽和吸附後的矽(100)表面，並介紹關聯性函數 (correlation function) 的分析方法。第二章則簡介實驗儀器與操作原理，以及樣品與探針的製備方法。第三章到第五章則分別針對三個主要專題進行研究、分析與討論。 第三章主要探討氣體-表面交互作用：當表面已經飽和吸附原子時，入射的氣體原子會否與表面的吸附原子反應？我們利用氫原子碰撞飽和吸附氯原子後的矽(100)表面，發現當入射的氫原子碰撞到表面的氯原子，會與氯原子形成氯化氫分子脫離矽表面。從核心層光電子光譜可看出，這種反應除了會抽離吸附的氯原子外，尚有其他的反應發生，且氫原子最後會取代氯吸附於矽表面。由關連性函數來分析STM影像，可發現氯原子被抽離的反應位置並非隨機分佈，隨著氯抽離量越多而反應位置明顯有聚集現象，故此反應並非直接、而是有選擇性發生。此結果可驗證氫原子是以熱原子 (hot atom) 狀態在表面游移，直到適當位置才會與氯原子結合、形成氯化氫分子從矽(100)表面脫附。 第四章討論吸附原子的擴散。當表面飽和吸附兩種原子時，吸附原子會是怎樣的擴散行為？我們經由變溫STM觀測不同溫度的矽(100)表面，發現當加熱樣品至一定溫度時，氫原子會與鄰近的氯原子會互換吸附位置。由連續的STM影像可看出，氫原子像是在雙原子單體排內進行布朗運動。氫原子會與同一雙原子單體 (intra-dimer) 內之氯原子直接交換、或是與同一雙原子單體排中最鄰近任兩個雙原子單體 (intra-row) 其中之一的氯原子互換。經由實驗結果所計算得的原子交換能量比理論計算來得低，這是因為在直接交換擴散的過程中，可能存在有短暫的氯化氫分子中間態使然。 第五章在探討雙原子分子吸附行為。雙原子分子是以解離吸附或是抽離吸附表面？我們觀察氯化氫分子在不同溫度 (110、300及450 K) 的矽(100)表面上所進行的化學吸附。由STM實驗結果發現矽(100)表面會完全飽和吸附氯原子與氫原子，且氫原子的吸附覆蓋率比氯原子要多出百分之十。此結果表示氯化氫分子除了解離吸附外，仍會有許多抽離吸附發生，且氫要比氯容易吸附於表面。由STM影像顯示氯原子的吸附位置在低溫110 K下呈現區域的2×2結構，而其有序排列的程度會隨著溫度增高而減弱。透過電腦模擬雙原子分子吸附的結果，驗證了當雙原子分子隨機吸附於表面時，分子解離後的原子與已吸附之原子是存在有交互作用的。 第六章則總結第三章至第五章的實驗結果。本論文之研究應用真實空間及原子解析度的STM，結合核心層光電子激發術，詳盡地解析矽(100)表面上氫與氯原子的各種化學反應，根據研究提供的許多新數據，本人提出的一些表面科學現象的新見解與新發現。本文之外，附錄收錄氧分子在銀(100)金屬表面之解離吸附的研究報告，此為作者在博士班期間參加2007年德國三明治計畫，於德國Leibniz University of Hannover參與Karina Morgenstern教授的低溫STM團隊，進行研究所得到的實驗成果。

This study investigates interactions and competition of hydrogen (H) and chlorine (Cl) atoms on the Si(100) surface. Several fundamental issues in the field of surface science are examined experimentally, including the gas-surface reaction, the diffusion mechanism of adsorbates and the detailed adsorption processes of diatomic molecules. The measurements were carried out by utilizing a variable-temperature scanning tunneling microscopy (VT-STM), synchrotron radiation core-level photoemission spectroscopy and Monte Carlo simulation. STM images provide images of the surface with atomic-scale resolution, allowing direct viewing of adsorbates species and the reaction sites after interactions. The core-level spectra are used to distinguish atoms in different chemical bonding configurations by the chemical shift of binding energies. The combination of these complimentary techniques yields much new and exceptional detailed information and understanding of the interactions of adsorbates on the surface. This dissertation is organized into six chapters. In Chapter 1, the background and motivations of this research and a review of literatures are introduced. The sample systems and the concept of correlation function are also presented. Chapter 2 describes the sample preparation procedures and the principles and operations of the experimental apparatus. The following three chapters present the three major experiments with detailed results and discussions. In Chapter 3, I discuss the issue of gas-surface reactions. The main question I ask is that whether the gaseous atoms react with the adsorbate on the surface randomly or not. Specifically, I use an H atomic beam to bombard the Cl-saturated Si(100)-2×1 surface and examine if any correlation exists between the reaction sites. The results show that the incident H atoms collide on Cl adatoms and form HCl molecules, which are desorbed from the silicon surface. Core-level measurements indicate that some additional reactions occur besides the removal of Cl and that H atoms eventually terminate the Si(100) surface. The correlation function calculated from STM images show that the Cl-extracted sites disperse randomly in the initial phase of the reaction, but form small clusters as more Cl is removed, indicating a correlation between Cl-extracted sites. These results suggest that the hot-atom process may occur during the atom-adatom collision. Chapter 4 describes a newly-found mechanism of surface diffusion. Specifically, the diffusion behavior of H substitutional sites on the Cl-terminated Si(100) surface was investigated at variant temperatures. STM movies show that each H atom undergoes Brownian motion within a monochloride dimer row. The position of an H substitutional site is exchanged directly with that of an immediate neighboring Cl atom in either the same dimer or in one of the two adjacent dimers in the same row. Accordingly, conceptual direct exchange diffusion in a two-dimensional lattice was experimentally observed. Analysis of STM movies at various temperatures yielded rather low attempt frequencies and energy barriers, leading to the suggestion that the diffusion mechanism involves an intermediate low-energy molecular state. In Chapter 5, I examine the atomic process of the chemisorption of diatomic molecule. Are diatomic molecules chemisorbed on the surface dissociatively or through an abstractive reaction? To answer this question, the Si(100) surface was exposed to gas-phase HCl molecules at various substrate temperatures. Experimental results show that saturation exposure to HCl causes all surface dangling bonds to be terminated by the two fragment H and Cl atoms and that the number of H-termination sites exceeds that of Cl-termination ones by >10 %. This finding suggests that, in addition to the dominant dissociative chemisorption, many abstraction reactions occur. STM images reveal that Cl-termination sites form local 2×2 structure at 110 K and that the degree of ordering is reduced as the substrate temperature increases. Simulation results demonstrate the importance of including dissociative fragment-adsorbates interactions during the random adsorption of diatomic molecules. Finally, in Chapter 6, research results are summarized and the conclusions are made. The experimental works in this thesis produce much atomic-scale information of several surface reactions and processes during the coadsorption of two mixed adsorbates on the Si(100) surface. Appended at the end of this dissertation is the study of the dissociative adsorption of oxygen on the Ag(100) surface. This research was conducted when the author was supported by the Sandwich Program supported by Germany and Taiwan in 2007. The experiments were carried out in the low-temperature STM group of Professor Karina Morgenstern in Leibniz University of Hannover.