Abstract

The atomic structure of several Si{111}rt3*rt3-30-metal metal surfaces has been investigated using dynamical low-energy electron diffraction. A LEED intensity analysis of the Si{111}rt3*rt3-30-B surface has determined that boron atoms replace second-layer Si atoms and a Si adatom is located above each boron atom, the B_5 site. The ideal coverage of this structure is 1/3 of a monolayer of boron atoms and the surface is relaxed. The Si{111}rt3*rt3-30-Mg surface could not be formed; the Si{111}(2/3)rt3*(2/3)rt3-30-Mg surface could, however, be created and is most likely that of a reacted silicide. The formation of a Si{111}3*1-Mg surface is reported with LEED I(V) spectra essentially identical to those of Si{111}3*1-Li, -Na, and -Ag. Therefore, these metal atoms induce the formation of the Si{111}3*1 surface structure and are not ordered in the unit cell. The surface structure of the Si{111}rt3*rt3-30-Au surface has been determined to involve a coverage of one monolayer of Au. The Au atoms are chemisorbed on top of the substrate surface in the form of trimers, with the trimer centers located above the position of fourth-layer Si atoms, and the Au atoms are on off-first-layer Si sites. The first layer of Si atoms is missing and the second-layer Si atoms are displaced 0.5 Ang. away from the center of symmetry. Si{111}rt3*rt3-30-Ce and 2*1-Ce surfaces have been formed and are most likely those of a silicide.

Dedication

This thesis is dedicated to Mr. Phillip Stitt, who passed away. Phil was a kind and gentle man. His assistance to and training of Engineering students will never be forgotten. His skill as a machinist will never be replaced. Quality of design, creation, execution, and life were essential to Phil. I can only hope to aspire to the level of quality that he has set.

Contents

List of Tables

List of Figures

Glossary A. Technique Acronyms

Glossary B. Structure Acronyms

Acknowledgements

• 1 Introduction
  • 1.1 Surfaces
  • 1.2 Silicon Surfaces
  • 1.3 Si{111}rt3*rt3-30-X Surfaces

• 2 Experiment
  • 2.1 Low-Energy Electron Diffraction
    • 2.1.1 The Basics
    • 2.1.2 Data Collection
    • 2.1.3 Analysis
  • 2.2 Auger Electron Spectroscopy
  • 2.3 Sample Preparation

• 3 B/Si{111}
  • 3.1 Introduction
  • 3.2 Experiment
    • 3.2.1 Si{111}rt3*rt3-30-B
    • 3.2.2 Si/Si{111}rt3*rt3-30-B
  • 3.3 LEED Intensity Analysis
  • 3.4 Keating Energy Analysis
  • 3.5 Discussion
    • 3.5.1 Si{111}rt3*rt3-30-B
    • 3.5.2 Si/Si{111}rt3*rt3-30-B

• 4 Mg/Si{111}
  • 4.1 Introduction
  • 4.2 Experiment
  • 4.3 Discussion
    • 4.3.1 Si{111}(2/3)rt3*(2/3)rt3-30-Mg
    • 4.3.2 Si{111}3*1-Mg

• 5 Au/Si{111}
  • 5.1 Introduction
  • 5.2 Experiment
  • 5.3 Keating Energy Analysis
  • 5.4 LEED Intensity Analysis
  • 5.5 Discussion

• 6 Ce/Si{111}
  • 6.1 Introduction
  • 6.2 Experiment
  • 6.3 Discussion
    • Si{111}2*1-Ce
    • Si{111}rt3*rt3-30-Ce

• 7 Conclusion

  Appendix A. Al, Ga, In, and Sn/Si{111}

  Appendix B. Pb/Si{111}

  Appendix C. Sb and Bi/Si{111}

  Appendix D. Ag/Si{111}

  References

  Tables

  Figures

List of Tables

• 2.1: The Auger peaks of the elements studied.
• 3.1: The structural parameters for the relaxed B_5-model surface of Si{111}rt3*rt3-30-B, as determined by LEED, SXD, TEC, and Keating analyses.
• 3.2: The bond lengths and the Keating energies for the relaxed B_5-model surface of Si{111}rt3*rt3-30-B, as determined by LEED, SXD, TEC, and Keating analyses.
• 3.3: Three reliability factors ( (1/2)R_p, r_ZJ, and R_VHT ) for the optimized B_5-model Si{111}rt3*rt3-30-B structure.
• 3.4: The structural parameters for the relaxed H_3-, T_4-, and B_5-model surfaces of Si{11}rt3*rt3-30-Al and -B, as determined by a Keating analysis.
• 3.5: The bond lengths and the Keating energies for the H_3-, T_4-, and B_5-model relaxed surfaces of Si{11}rt3*rt3-30-Al and -B, as determined by a Keating analysis.
• 3.6: The structural parameters for the relaxed T_4-model surfaces of Si{111}rt3*rt3-30-X, as determined by a Keating analysis.
• 3.7: The structural parameters and Keating energies for the relaxed T_4-model surfaces of Si{111}rt3*rt3-30-X, as determined by a Keating analysis.
• 3.8: The structural parameters for the H_3-, T_4-, and B_5-model surfaces of Ge{11}rt3*rt3-30-Al and -B, as determined by a Keating analysis.
• 3.9: The bond lengths and the Keating energies for the relaxed H_3-, T_4-, and B_5-model relaxed surfaces of Ge{11}rt3*rt3-30-Al and -B, as determined by a Keating analysis.
• 3.10: The structural parameters for the relaxed T_4-model surfaces of Ge{111}rt3*rt3-30-X, as determined by a Keating analysis.
• 3.11: The bond lengths and the Keating energies for the relaxed T_4-model surfaces of Ge{111}rt3*rt3-30-X, as determined by a Keating analysis.
• 5.1: The structural parameters for Si{111}rt3*rt3-30-Au, as determined by a Keating analysis, using several of the models described in the text.
• 5.2: The structural parameters for Si{111}rt3*rt3-30-Au, as determined by a Keating analysis, using several of the models described in the text.
• 5.3: The structural parameters for Si{111}rt3*rt3-30-Au, as determined by a Keating analysis, using several of the models described in the text.
• 5.4: Three reliability factors ( (1/2)R_p, r_ZJ, and R_VHT ) for the optimized MTL-H_3-T_1-model Si{111}rt3*rt3-30-Au structure.
• C.1: The model parameters for the Si{111}rt3*rt3-30-Sb surface, as determined by a Keating analysis.
• C.2: The model parameters for the Si{111}rt3*rt3-30-Bi surface, as determined by a Keating analysis.
• C.3: The model parameters for the Ge{111}rt3*rt3-30-Sb surface, as determined by a Keating analysis.
• C.4: The model parameters for the Ge{111}rt3*rt3-30-Bi surface, as determined by a Keating analysis.

List of Figures

• Figure 1.1: Schematic of several surface unit-meshes
• Figure 1.2: Schematic of relaxed surface plane
• Figure 1.3: Model of the ideal surface for Si{111}1*1
• Figure 1.4: The Periodic Table of Elements
• Figure 1.5: The T_1 model for Si{111}rt3*rt3-30-X<
• Figure 1.6: The H_3 model for Si{111}rt3*rt3-30-X
• Figure 1.7: The T_4 model for Si{111}rt3*rt3-30-X
• Figure 1.8: The B_5 model for Si{111}rt3*rt3-30-X
• Figure 2.1: Schematic LEED-AES retarding-field analyzer (RFA).
• Figure 2.2: Schematic LEED pattern for Si{111} 1x1.
• Figure 2.3: Schematic LEED pattern for Si{111}rt3*rt3-30-X.
• Figure 2.4: A LEED pattern of Si{111}7*7, at 100 eV.
• Figure 2.5: Schematic LEED Data Acquisition System.
• Figure 2.6: Reliability-factor (R_VHT, r_ZJ, and R_P) contour plots for the surface of TB{11-20}.
• Figure 2.7: Schematic AES Data Acquisition System.
• Figure 2.8: Typical Auger electron spectrum for Au deposited upon a Si{111} surface.
• Figure 2.9: Schematic LEED-AES Surface Analysis UHV System.
• Figures 3.1, 3.2, 3.3, 3.4, 3.5, and 3.6: The LEED I(V) spectra of Si{111}rt3*rt3-30-B.
• Figure 3.7: The H_3-, T_4-, and B_5-model structures of the Si{111}rt3*rt3-30-X surfaces in the unrelaxed (starting) state.
• Figure 3.8: The minimum Keating energies for the H_3-, T_4-, and B_5-model structures of the Si{111}rt3*rt3-30-X surfaces as a function of the adsorbate's covalent radius.
• Figure 3.9: The H_3-, T_4-, and B_5-model structures of the Si{111}rt3*rt3-30-Al surface in the relaxed state, as determined by a Keating analysis.
• Figure 3.10: The H_3-, T_4-, and B_5-model structures of the Si{111}rt3*rt3-30-B surface in the relaxed state, as determined by a Keating analysis.
• Figure 3.11: The minimum Keating energies for the H_3-, T_4-, and B_5-model structures of the Ge{111}rt3*rt3-30-X surfaces as a function of the adsorbate's covalent radius.
• Figure 4.1: The T_4^2 model for Si{111}rt3*rt3-30-Mg. The Mg atoms form a close-packed layer centered on T_4 sites.
• Figure 4.2: The H_3^2 model for Ge{111}rt3*rt3-30-Pb.
• Figure 4.3: The T_1^2 model for Si{111}rt3*rt3-30-Pb.
• Figure 4.4: The H_3-T_4 model for Si{111}rt3*rt3-30-Au.
• Figure 4.5: The H_3-T_1 model for Si{111}rt3*rt3-30-Bi, -Sb, and -Au.
• Figure 4.6: The T_4-T_1 model for Si{111}rt3*rt3-30-Bi, -Sb, and -Au.
• Figure 4.7: Top: The superposition of schematic 1*1 (closed circles) and Si{111}(2/3)rt3*(2/3)rt3-30-Mg (open circles) LEED patterns. Bottom: Schematic top-view of the Si{111} surface. The large and small circles are fist and second layer atoms. The reciprocal and real unit-meshes are indicated.
• Figure 4.8: Schematic LEED patterns for Si{111}3*3-Mg and 3*1-Mg.
  
• Figure 4.9 and 4.10: The I(V) spectra of Si{111}3*1-Ag, -Li, -Na, and -Mg.
• Figure 5.1: The MCT-T_1-T_1-T_4 model for Si{111}rt3*rt3-30-Au.
• Figure 5.2: The MCT-T_1-T_1-H_3 model for Si{111}rt3*rt3-30-Au.
• Figure 5.3: The MCT-VT-T_1-T_4 model for Si{111}rt3*rt3-30-Au.
• Figure 5.4: The MCT-VT-T_1-H_3 model for Si{111}rt3*rt3-30-Au.
• Figure 5.5: The honeycomb (HC) model for Si{111}rt3*rt3-30-Au.
• Figure 5.6: Experimental AES data for Au/Si{111}.
• Figure 5.7: The MTL-T_4-H_3 model for Si{111}rt3*rt3-30-Au.
• Figure 5.8: The MTL-T_4-T_1 model for Si{111}rt3*rt3-30-Au.
• Figure 5.9: The relaxed MTL-T_1-H_3 model for Si{111}rt3*rt3-30-Au, also denoted conjugate honeycomb chained-trimer, as determined by a Keating energy analysis.
• Figure 5.10: The relaxed MTL-H_3-T_1 model for Si{111}rt3*rt3-30-Au, also denoted conjugate honeycomb chained-trimer, as determined by a Keating energy analysis.
• Figure 5.11: The relaxed MTL-T_1-H_3 model for Si{111}rt3*rt3-30-Ag, also denoted honeycomb chained-trimer, as determined by a Keating energy analysis.
• Figure 5.12: The relaxed MTL-H_3-T_1 model for Si{111}rt3*rt3-30-Ag, also denoted honeycomb chained-trimer, as determined by a Keating energy analysis.
• Figure 5.13, 5.14, 5.15, 5.16, 5.17, and 5.18: The LEED I(V) spectra of Si{111}rt3*rt3-30-Au.
• Figure 5.19: The experimental LEED spectra of Si{111}rt3*rt3-30-Au and Si{111}6*6-Au.
• Figure 5.20,5.21: The experimental LEED I(V) spectra for the Si-rt3-Au and -Ag surfaces, and the calculated spectra for the Si-rt3-Ag HCT-model.
• Figure 6.1: Schematic LEED patterns for Si{111}2*2-X and 2*1-X.
• Figure 6.2: Schematic unit-cells (a,c) and LEED patterns (b,d) for the ThSiR_2 (a,b) and AlB_2 (c,d) polymorphs.

Glossary A. Technique Acronyms

In alphabetical order, the surface technique acronyms are:

• AES : Auger electron spectroscopy

• ARPES : angle-resolved photoemission electron spectroscopy

• kRIPES : k-resolved inverse photoemission spectroscopy

• LEED : low-energy electron diffraction

• M/LEIS : medium- and low-energy ion scattering

• RHEED : reflection high-energy electron diffraction

• SEXAFS : surface extended x-ray absorption fine structure

• STM : scanning tunneling microscopy

• SXD : surface x-ray diffraction

• TEC : total energy calculations

• TED : transmission electron diffraction

• UPS : ultraviolet photoemission spectroscopy

• XPS : x-ray photoemission spectroscopy

• XPD : x-ray photoelectron diffraction

• XSWM : x-ray standing wave methods

Glossary B. Structure Acronyms

In alphabetical order, the structure acronyms are:

• B_5 : Figure 1.8 : The five-fold coordinated adsorption-site created by the reversal of the adsorbate and second-layer Si atom in the T_4-model, i.e., the adsorbate is below the surface.

• CHC : The centered-honeycomb model is the HC model with an additional 1/3 ml of adsorbate atoms located on H_3 sites, at a different elevation.

• CHCT : Figures 5.9 and 5.10 : A conjugate honeycomb chained-trimer model is an alternate description of both the MTL-T_1-H_3 and MTL-H_3-T_1 models. The second-layer Si atoms relax away from the center symmetry, which causes the appearance of Si trimers that are chained together. Additionally, the adsorbate atoms relax toward the center of symmetry, thus forming adsorbate trimers.

• HC : Figure 5.5 : The honeycomb model contains 2/3 ml of adsorbate atoms located on H_3 sites.

• HCT : Figures 5.11 and 5.12 : A honeycomb chained-trimer model is an alternate description of both the MTL-T_1-H_3 and MTL-H_3-T_1 models. The adsorbate atoms relax away from the center symmetry, which causes the appearance of adsorbate trimers that are chained together. Additionally, the second-layer Si atoms relax toward the center of symmetry, thus forming Si trimers.

• H_3 : Figure 1.6 : The three-fold coordinated adsorption site-above of a fourth-layer Si atom, in the hollow created by first- and second-layer Si atoms.

• H_3^2 : Figure 4.2 : The close-packed layer with 1/3 ml of adsorbate atoms centered on H_3 sites and 1 ml of adsorbate atoms between T_4 and T_1 sites.

• H_3-T_1 : Figure 4.5 : A trimer model wherein the trimer is centered on a H_3 site and the individual adsorbate atoms are on off-T_1 sites (radially displaced toward the trimer center).

• H_3-T_4 : Figure 4.4 : A trimer model wherein the trimer is centered on a H_3 site and the individual adsorbate atoms are on off-T_4 sites (radially displaced toward the trimer center).

• MCT-X-Y-Z : as an example see Figure 5.1 : The modified coplanar-trimer models are composed of a Si honeycomb layer on X sites, an adsorbate trimer centered on a Y site, and the individual adsorbate atoms on off-Z sites (radially displaced toward the trimer center), i.e., the MCT-T_1-T_1-T_4 model has a Si honeycomb-layer registered on T_1 sites and a Au trimer centered on a T_1-site composed of Au atoms on off-T_4 sites.

• MTL-Y-Z : as an example see Figure 5.7 : The missing top-layer models contain a trimer centered on a Y site and the individual adsorbate atoms are on off-Z sites (radially displaced toward the trimer center), i.e., the MTL-T_4-H_3 model has no first-layer Si atoms and has a Au trimer centered on a T_4-site composed of Au atoms on off-H_3 sites.

• T_1 : Figure 1.5 : The one-fold coordinated adsorption-site on top of a first-layer Si atom.

• T_1^2 : Figure 4.3 : The close-packed layer with 1/3 ml of adsorbate atoms centered on T_1 sites and 1 ml of adsorbate atoms between T_4 and H_3 sites.

• T_1-H_3 : A trimer model wherein the trimer is centered on a T_1 site and the individual adsorbate atoms are on off-H_3 sites (radially displaced toward the trimer center).

• T_1-T_4 : A trimer model wherein the trimer is centered on a T_1 site and the individual adsorbate atoms are on off-T_4 sites (radially displaced toward the trimer center).

• T_4 : Figure 1.7 : The four-fold coordinated adsorption-site on top of a second-layer Si atom.

• T_4^2 : Figure 4.1 : The close-packed layer with 1/3 ml of adsorbate atoms centered on T_4 sites and 1-ml of adsorbate atoms between T_1 and H_3 sites.

• T_4-T_1 : Figure 4.6 : A trimer model wherein the trimer is centered on a T_4 site and the individual adsorbate atoms are on off-T_1 sites (radially displaced toward the trimer center).

• T_4-H_3 : A trimer model wherein the trimer is centered on a T_4 site and the individual adsorbate atoms are on off-H_3 sites (radially displaced toward the trimer center).

• VT : The vacancy site created by the removal of a first-layer Si atom. For example, the MCT-VT-T_1-T_4 model has a honeycomb of first-layer Si atoms; the honeycomb itself is centered on the missing first-layer Si atoms.

Acknowledgements

In the book The Agony and the Ecstasy by Irving Stone, Pope Julius repeatedly asks Michelangelo, `When will you make an end of it?' Michelangelo repeatedly responds, `When I am finished.' My family, friends, and co-workers have often asked a similar question. Well, I am finished and `thank you' for all the support and assistance you have provided.

I gratefully acknowledge the support and assistance given to me by the students, faculty, administrative staff, and technical support staff of the Departments of Materials Science, Physics, Chemistry, Earth & Space Sciences, Computing Services, and Biology, both past and present. In particular, Yuesheng Li, Marty Helfand, Gary Halada, and Joan Pidot have been very helpful. `Thank you' to the `boys in the basement' who made the holes, found the bolts, and screwed them in. A special `thanks' to Fran, Bronwen, and Lenny, who always went the extra yards, even though it was not in their job description. A special debt of appreciation is owed to members of my defense committee; it is very kind of them to grill, probe, direct, and abet me. Dr. S.Y. Tong and H. Huang, from the University of Wisconsin at Milwaukee, performed the LEED intensity calculations of Si rt3-B; for this, I am eternally grateful to these gentlemen. IBM provided the CPU time and Don Jepsen provided the program for the solution of the Si rt3-Au structure; thank you `Big-Blue.' Funding for this research has been provided by the Department of Energy and the National Science Foundation; I would like to thank both organizations for financial assistance. On the other side of the coin, I would like to express my disgust and outrage to the Stony Brook bureaucrats who know nothing about research, take the credit, and squash the efforts.

Finally, I would like to thank my Advisor, Franco Jona, for which words would not suffice to describe my gratitude. I remain his student, colleague, and friend.

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