超薄膜氧化鎳在銀(100)表面的成長與電子結構

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研究生：	王清鍾 Ching-Chung Wang
論文名稱：	超薄膜氧化鎳在銀(100)表面的成長與電子結構 The growth and electronic structure of ultrathin NiO films on Ag(100) surfaces
指導教授：	崔古鼎 Ku-Ding Tsuei
口試委員:
學位類別：	碩士 Master
系所名稱：	理學院 - 物理學系 Department of Physics
論文出版年：	2006
畢業學年度：	94
語文別：	英文
論文頁數：	78
中文關鍵詞：	氧化鎳、銀(100) 、光電子能譜、低能電子繞射、反射式高能電子繞射、角解析光電子能譜、費米能級、布里淵區
外文關鍵詞：	NiO, Ag(100), photoemission spectroscopy (PES), low energy electron diffraction (LEED), reflected high energy electron diffraction (RHEED), angle-resolved photoemission spectroscopy (ARPES), Fermi level, Brillouin zone
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本論文主要以高解析光電子能譜(HRPES)探討超薄膜氧化鎳(NiO)在銀(Ag)(100)表面的成長與電子結構。氧化鎳薄膜是於超高真空(UHV)中藉在氧(O2)的環境下蒸鍍金屬鎳(Ni)於Ag(100)來獲得。成長時基材溫度及氧壓大小對單層(monolayer, ML)氧化鎳薄膜的組成和結構影響甚巨。高溫成長時(約180℃)，金屬鎳沉入Ag(100)次表面的速度比與O2反應形成氧化鎳的速度還快；光電子能譜顯示於高溫成長時在單層範圍內有大量金屬鎳存在。於室溫成長時，低能電子繞射(LEED)顯示1.2 ML氧化鎳薄膜在氧壓5×10-7~8×10-7 Torr範圍內具有(2×1) pattern。但是當氧壓高於8×10-7 Torr或隨著薄膜厚度的增加，此(2×1) pattern則變成(1×1) pattern。我們發現當氧壓於7.5×10-7 Torr，0.9 ML氧化鎳薄膜在氧的環境下post-annealing至180℃具有最佳(2×1) LEED pattern。此外，此(2×1) LEED pattern在annel至350℃維持15分鐘後會轉變為(1×1) pattern。

光電子能譜指出當成長於Ag(100)的氧化鎳薄膜其厚度在3.2 ML以上時，已成為絕緣體。然而0.9 ML (2×1) NiO/Ag(100)及2 ML (1×1) NiO(100)/Ag(100)在費米能級(Fermi level)卻有finite DOS。這表示其具有金屬性質，其中0.9 ML (2×1) NiO/Ag(100)的金屬性質比2 ML (1×1) NiO(100)/Ag(100)的更為強烈。另一方面我們用角解析光電子能譜(ARPES)來研究0.9 ML (2×1) NiO/Ag(100)、2 ML (1×1) NiO(100)/Ag(100)和10 ML NiO(100)/Ag(100)的能帶色散(band dispersion)。我們發現10 ML NiO(100)/Ag(100)的能帶色散與文獻上bulk NiO(100)的相似。對於0.9 ML (2×1) NiO/Ag(100)而言，其valence band只有微弱的色散，同時與2 ML (1×1) NiO(100)/Ag(100) 的有明顯的不同。以上所敘述以及我們實驗所觀測到的現象，將於論文內有詳細的討論。

We have performed a high resolution photoemission study on ultrathin NiO films on Ag(100) surfaces. The ultrathin NiO films were prepared under UHV conditions on Ag(100) substrate by evaporation of metallic Ni in an O2 atmosphere. The Ni deposition rate was measured by a quartz crystal thickness monitor (QCTM) and the thickness of NiO films on Ag(100) was determined by reflection high energy electron diffraction (RHEED). We found that the substrate temperature and the O2 pressure affect greatly the composition and structure of NiO films on Ag(100) for monolayer (ML) coverage. For the high temperature (~ 180℃) growth, the incorporation rate of Ni into the silver subsurface region is faster than the reaction rate of Ni with oxygen and PES data show substantial metallic Ni for submonolayer coverages. For the room temperature (RT) growth, the low energy electron diffraction shows a (2×1) pattern (in two orthogonal domains) for 1.2 ML NiO films on Ag(100) at O2 pressures in the 5×10-7 ~ 8×10-7 Torr range; while the (2×1) pattern evolves to a (1×1) pattern for O2 pressure above 8×10-7 Torr or with the increase of film thickness. The best (2×1) pattern was observed for 0.9 ML NiO films on Ag(100) at O2 pressure of 7.5×10-7 Torr with post-annealing at 180℃ in an O2 atmosphere. The (2×1) LEED pattern is transformed into a (1×1) pattern upon annealing at 350℃ for 15 min.

PES results indicate that the ultrathin NiO films on Ag(100) becomes an insulator at coverage above ~ 3.2 ML. However, there are finite density of states at Fermi level for 0.9 ML (2×1) NiO/Ag(100) and 2 ML (1×1) NiO(100)/Ag(100). This means that they have metallic character, and the metallic character of 0.9 ML (2×1) NiO/Ag(100) is stronger than that of 2 ML (1×1) NiO(100)/Ag(100). Angle-resolved photoemission spectroscopy has been used to study the band dispersion of 0.9 ML (2×1) NiO/Ag(100), 2 ML (1×1) NiO(100)/Ag(100) as well as 10 ML NiO(100)/Ag(100). The band dispersion of 10 ML NiO(100)/Ag(100) was similar to that of bulk NiO(100) crystal. Weak dispersion of valence bands was observed on 0.9 ML (2×1) NiO/Ag(100), which is very different from that on 2 ML (1×1) NiO(100)/Ag(100). The details of the above observation are discussed in this thesis.

Contents

Chapter 1 Introduction ……………………………………………1
References ……………………………………………………………3

Chapter 2 Theories and Principles ……………………………4
2.1 Photoelectron Spectroscopy …………………………………4
  2.1.1 Introduction ………………………………………………4
  2.1.2 The Light Source …………………………………………7
  2.1.3 Theory of Photoemission: The Three-Step Model ……9
  2.1.4 Band Mapping ………………………………………………14
2.2 Low Energy Electron Diffraction (LEED) …………………18
2.3 Reflection High Energy Electron Diffraction (RHEED) …21
References ……………………………………………………………24

Chapter 3 Sample Preparation and Experiments ………………25
3.1 Introduction ……………………………………………………25
3.2 Sample Preparation ……………………………………………26
  3.2.1 The determination of the thickness of the NiO
        films on Ag(100) …………………………………………26
  3.2.2 The NiO(100)/MgO(100) case ……………………………29
  3.2.3 The NiO(100)/Ag(100) case ………………………………33
3.3 Inspection of the growth conditions for NiO/Ag(100)
    with photoemission spectroscopy ……………………………37
  3.3.1 Apparatus ……………………………………………………37
  3.3.2 Experimental Details ……………………………………37
    3.3.2.1 Growth of NiO at substrate temperature of 180℃
            and RT …………………………………………………41
    3.3.2.2 The effect of O2 pressure on the (2×1) NiO
            structure ……………………………………………44
    3.3.2.3 Photoemission spectra of different NiO films
            thickness ……………………………………………47
References ……………………………………………………………49

Chapter 4 Results and Discussion ………………………………50
4.1 Angle-Integrated Valence band photoemission of NiO thin
    films on Ag(100) ………………………………………………50
4.2 The structure of (2×1) and (1×1) NiO on Ag(100) ……58
4.3 Angle-Resolved Photoemission Spectroscopy (ARPES) of
    NiO ultrathin films on Ag(100) ……………………………60
  4.3.1 The effect of the (2×1) structure in two
        orthogonal domains on ARPES measurement …………60
  4.3.2 Relative geometry of the light source and the
        Scienta SES-200 analyzer ……………………………61
  4.3.3 ARPES for 10ML NiO(100) on Ag(100) …………………63
  4.3.4 ARPES for 0.9ML (2×1) NiO and 2ML (1×1) NiO(100)
        on Ag(100) ………………………………………………68
References ……………………………………………………………77

Chapter 5 Conclusion ………………………………………………78

Appendix

A. Operation of Quartz Crystal Thickness Monitor …………i
B. Features of Luxel Radak-I Furnace …………………………vii
C. Features of Thermal Gas Cracker TC-50 ……………………xi

                                

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