鹼金屬鹵化物奈米薄膜在共價鍵半導體表面成長模式和電子結構

Abstract

本論主要研究主題有鹼金屬鹵化物在共價鍵半導體表面和雙原子鹵素分子在半導體表面的吸附。其中，鹼金屬鹵化物包含了氯化鈉和氯化鉀，而雙原子鹵素分子包含了氯化碘和氯化溴氣體。所涵蓋課題包括薄膜成長的機制、原子的結構、電子特性、在介面間電子的轉移。而我們利用分子束磊晶沉積技術和化學氣相沉積技術薄膜。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、核心層光電子激發術及第一原理計算。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。 不同表面能量和晶格係數差異的大小都影響了原子結構的排列和成長模式。氯化鈉薄膜和矽(100)-2×1的晶格係數差異約為5 %，而核心層光電子激發術的結果說明了，在氯化鈉的覆蓋率小於40 %時，部分的氯化鈉分子解離而另一部分的氯化鈉分子直接以分子的形式吸附在表面的矽原子上形成了矽－氯－鈉的結構。掃瞄穿遂顯微術的結果顯示，覆蓋率65 %時，在矽表面上觀察到c(2×2)、c(2×4)、p(2×2)等的排列結構圖形，而這些不同的結構，主要來自於解離的鈉原子、氯原子、矽原子懸鍵以及矽－氯－鈉的排列。當覆蓋率介於0.65~2.25個原子層時，在鈉、氯、氯化鈉所組成的層面上方有著島嶼狀的氯化鈉開始成長，這島嶼狀的氯化鈉是由單位晶格長度3.82埃的正方形晶格所組成的，且島嶼狀的氯化鈉高度為3.8埃。 氯化鉀薄膜和矽(100)-2×1的晶格係數差異為13 %，由核心層光電子激發術的結果說明了，在氯化鉀的覆蓋率小於一個原子層時，部分的氯化鉀分子解離。當覆蓋率超過一個原子層時，靠近費米面的價帶電子態密度有著類似氯化鉀塊材的特徵。掃瞄穿遂顯微術的結果顯示出，覆蓋率等於一個原子層時，表面出現了新穎的c(4×4)結構圖形。當覆蓋率大於一個原子層，會出現小部分群集的氯化鉀散落在c(4×4)的區域介面，並且有著線條般的鏈狀。覆蓋率超過兩個原子層時，氯化鉀是一層接著一層模式成長。結合第一原理的計算，我們可以知道這新穎的c(4×4)結構圖形是由金字塔幾何結構的原子所組成的。

This thesis studies the growth mechanism, atomic structure, electronic property, and the charge transfer at the interface between ionic solid films and covalent bonded semiconductor surface for several alkali halides and inter-halogen including NaCl and KCl, ICl, and IBr. Adsorption mechanisms, surface species, chemical reactions, atomic compositions and orderings, and film morphology during the growth using molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) at room temperature were thoroughly investigated. The measurements were carried out by synchrotron radiation core level photoemission and scanning tunneling microscopy. The combination of these complimentary techniques yields much new and detailed information and understanding on the growth mode, film characteristics, and interfacial property for ionic solids on the covalent semiconductor surfaces. Different surface energies and the dissimilar lattice mismatch lead to the different atomic structures and growth modes. For the smaller lattice mismatch (~5%) of the growth ultra-thin film of NaCl on Si(100)-2×1, the Si 2p, Na 2p, and Cl 2p core level spectra together indicate that adsorbed NaCl molecules at submonolayer coverage (θ < 0.4 ML) are partially dissociated and the rest adsorbed NaCl molecules bonded with Si to form Si-Cl-Na species. The STM results revealed that many dangling bonds are intact and form the c(2×4), c(2×2), and p(2×2) local ordered structures at the coverage of 0.65 ML. A simple model for the dangling bond ordering is presented: The different ordered structures were formed by the different arrangement of the Si-Cl-Na, dissociated Na and Cl, and dangling bonds. At 0.65 < θ < 2.25 ML, double layer NaCl islands begin to grow on top of the Na, Cl, and NaCl mixed ad-layer. The apparent height of a NaCl double layer island is 3.8 Å and shows well-defined protrusions which are arranged into a square lattice of 3.82 Å. The growth of KCl on the Si (100)-2×1 has a larger lattice mismatch (~13%). The Si 2p, K 3p, and Cl 2p core level spectra together indicate that adsorbed KCl molecules at submonolayer coverage also partially dissociate and that KCl nano-films above 1. 0 ML have similar features in the valance band density of states with those of the bulk KCl crystal. STM results reveal a novel c(4×4) structure at θ=1 ML. Over 1.0 ML, new clusters randomly disperse around the c(4×4) domain boundaries and often form linear chains. Further KCl deposition above 2 ML follows pseudo layer-by-layer growth. Ab initio calculations show that a model that comprises a periodic pyramidal geometry of the novel c(4×4) atomic structure is consistent with experimental results.