Abstract

The atomic structure of several Si{111}√3x√3-30-metal metal surfaces has been investigated using dynamical low-energy electron diffraction.   A LEED intensity analysis of the Si{111}√3x√3-30-B surface has determined that boron atoms replace second-layer Si atoms and a Si adatom is located above each boron atom, the B₅ site.   The ideal coverage of this structure is 1/3 of a monolayer of boron atoms and the surface is relaxed.   The Si{111}√3x√3-30-Mg surface could not be formed; the Si{111}(2/3)√3x(2/3)√3-30-Mg surface could, however, be created and is most likely that of a reacted silicide.   The formation of a Si{111}3x1-Mg surface is reported with LEED I(V) spectra essentially identical to those of Si{111}3x1-Li, -Na, and -Ag.   Therefore, these metal atoms induce the formation of the Si{111}3x1 surface structure and are not ordered in the unit cell.   The surface structure of the Si{111}√3x√3-30-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 Å away from the center of symmetry.   Si{111}√3x√3-30-Ce and 2x1-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}√3x√3-30-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}√3x√3-30-B

3.2.2 Si/Si{111}√3x√3-30-B

3.3 LEED Intensity Analysis

3.4 Keating Energy Analysis

3.5 Discussion

3.5.1 Si{111}√3x√3-30-B

3.5.2 Si/Si{111}√3x√3-30-B

4 Mg/Si{111}

4.1 Introduction

4.2 Experiment

4.3 Discussion

4.3.1 Si{111}(2/3)√3x(2/3)√3-30-Mg

4.3.2 Si{111}3x1-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}2x1-Ce

Si{111}√3x√3-30-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 B₅-model surface of Si{111}√3x√3-B, as determined by LEED, SXD, TEC, and Keating analyses.

3.2: The bond lengths and the Keating energies for the relaxed B₅-model surface of Si{111}√3x√3-B, as determined by LEED, SXD, TEC, and Keating analyses.

3.3: Three reliability factors (½R_P, r_ZJ, and R_VHT) for the optimized B₅-model Si{111}√3x√3-B structure.

3.4: The structural parameters for the relaxed H₃-, T₄-, and B₅-model surfaces of Si{11}√3x√3-Al and -B, as determined by a Keating analysis.

3.5: The bond lengths and the Keating energies for the H₃-, T₄-, and B₅-model relaxed surfaces of Si{11}√3x√3-30-Al and -B, as determined by a Keating analysis.

3.6: The structural parameters for the relaxed T₄-model surfaces of Si{111}√3x√3-30-X, as determined by a Keating analysis.

3.7: The structural parameters and Keating energies for the relaxed T₄-model surfaces of Si{111}√3x√3-30-X, as determined by a Keating analysis.

3.8: The structural parameters for the H₃-, T₄-, and B₅-model surfaces of Ge{11}√3x√3-30-Al and -B, as determined by a Keating analysis.

3.9: The bond lengths and the Keating energies for the relaxed H₃-, T₄-, and B₅-model relaxed surfaces of Ge{11}√3x√3-30-Al and -B, as determined by a Keating analysis.

3.10: The structural parameters for the relaxed T₄-model surfaces of Ge{111}√3x√3-30-X, as determined by a Keating analysis.

3.11: The bond lengths and the Keating energies for the relaxed T₄-model surfaces of Ge{111}√3x√3-30-X, as determined by a Keating analysis.

5.1: The structural parameters for Si{111}√3x√3-30-Au, as determined by a Keating analysis, using several of the models described in the text.

5.2: The structural parameters for Si{111}√3x√3-30-Au, as determined by a Keating analysis, using several of the models described in the text.

5.3: The structural parameters for Si{111}√3x√3-30-Au, as determined by a Keating analysis, using several of the models described in the text.

5.4: Three reliability factors (½R_P, r_ZJ, and R_VHT) for the optimized MTL-H₃-T₁-model Si{111}√3x√3-30-Au structure.

C.1: The model parameters for the Si{111}√3x√3-30-Sb surface, as determined by a Keating analysis.

C.2: The model parameters for the Si{111}√3x√3-30-Bi surface, as determined by a Keating analysis.

C.3: The model parameters for the Ge{111}√3x√3-30-Sb surface, as determined by a Keating analysis.

C.4: The model parameters for the Ge{111}√3x√3-30-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}1x1

Figure 1.4: The Periodic Table of Elements

Figure 1.5: The T₁ model for Si{111}√3x√3-30-X

Figure 1.6: The H₃ model for Si{111}√3x√3-30-X

Figure 1.7: The T₄ model for Si{111}√3x√3-30-X

Figure 1.8: The B₅ model for Si{111}√3x√3-30-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}√3x√3-30-X.

Figure 2.4: A LEED pattern of Si{111}7x7, 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}√3x√3-30-B.

Figure 3.7: The H₃-, T₄-, and B₅-model structures of the Si{111}√3x√3-30-X surfaces in the unrelaxed (starting) state.

Figure 3.8: The minimum Keating energies for the H₃-, T₄, and B₅-model structures of the Si{111}√3x√3-30-X surfaces as a function of the adsorbate's covalent radius.

Figure 3.9: The H₃, T₄-, and B₅-model structures of the Si{111}√3x√3-30-Al surface in the relaxed state, as determined by a Keating analysis.

Figure 3.10: The H₃, T₄-, and B₅-model structures of the Si{111}√3x√3-30-B surface in the relaxed state, as determined by a Keating analysis.

Figure 3.11: The minimum Keating energies for the H₃, T₄-, and B₅-model structures of the Ge{111}√3x√3-30-X surfaces as a function of the adsorbate's covalent radius.

Figure 4.1: The T₄² model for Si{111}√3x√3-30-Mg. The Mg atoms form a close-packed layer centered on T₄ sites.

Figure 4.2: The H₃² model for Ge{111}√3x√3-30-Pb.

Figure 4.3: The T₁² model for Si{111}√3x√3-30-Pb.

Figure 4.4: The H₃-T₄ model for Si{111}√3x√3-30-Au.

Figure 4.5: The H₃-T₁ model for Si{111}√3x√3-30-Bi, -Sb, and -Au.

Figure 4.6: The T₄-T₁ model for Si{111}√3x√3-30-Bi, -Sb, and -Au.

Figure 4.7: Top: The superposition of schematic 1x1 (closed circles) and Si{111}(2/3)√3x(2/3)√3-30-Mg (open circles) LEED patterns.

Figure 4.8: Schematic LEED patterns for Si{111}3x3-Mg and 3x1-Mg.


Figure 4.9 and 4.10: The I(V) spectra of Si{111}3x1-Ag, -Li, -Na, and -Mg.

Figure 5.1: The MCT-T₁-T₁-T₄ model for Si{111}√3x√3-30-Au.

Figure 5.2: The MCT-T₁-T₁-H₃ model for Si{111}√3x√3-30-Au.

Figure 5.3: The MCT-T^V₁-T₁-T₄ model for Si{111}√3x√3-30-Au.

Figure 5.4: The MCT-T^V₁-T₁-H₃ model for Si{111}√3x√3-30-Au.

Figure 5.5: The honeycomb (HC) model for Si{111}√3x√3-30-Au.

Figure 5.6: Experimental AES data for Au/Si{111}.

Figure 5.7: The MTL-T₄-H₃ model for Si{111}√3x√3-30-Au.

Figure 5.8: The MTL-T₄-T₁ model for Si{111}√3x√3-30-Au.

Figure 5.9: The relaxed MTL-T₁-H₃ model for Si{111}√3x√3-30-Au, also denoted conjugate honeycomb chained-trimer, as determined by a Keating energy analysis.

Figure 5.10: The relaxed MTL-H₃-T₁ model for Si{111}√3x√3-30-Au, also denoted conjugate honeycomb chained-trimer, as determined by a Keating energy analysis.

Figure 5.11: The relaxed MTL-T₁-H₃ model for Si{111}√3x√3-30-Ag, also denoted honeycomb chained-trimer, as determined by a Keating energy analysis.

Figure 5.12: The relaxed MTL-H₃-T₁ model for Si{111}√3x√3-30-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}√3x√3-30-Au.

Figure 5.19: The experimental LEED spectra of Si{111}√3x√3-30-Au and Si{111}6x6-Au.

Figure 5.20,5.21: The experimental LEED I(V) spectra for the Si-√3-Au and -Ag surfaces, and the calculated spectra for the Si-√3-Ag HCT-model.

Figure 6.1: Schematic LEED patterns for Si{111}2x2-X and 2x1-X.

Figure 6.2: Schematic unit-cells (a,c) and LEED patterns (b,d) for the ThSi₂ (a,b) and AlB₂ (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:

B₅ : Figure 1.8 : The five-fold coordinated adsorption-site created by the reversal of the adsorbate and second-layer Si atom in the T₄-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 H₃ 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-T₁-H₃ and MTL-H₃-T₁ 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 H₃ sites.

HCT : Figures 5.11 and 5.12 : A honeycomb chained-trimer model is an alternate description of both the MTL-T₁-H₃ and MTL-H₃-T₁ 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.

H₃ : 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.

H₃² : Figure 4.2 : The close-packed layer with 1/3 ml of adsorbate atoms centered on H₃ sites and 1 ml of adsorbate atoms between T₄ and T₁ sites.

H₃-T₁ : Figure 4.5 : A trimer model wherein the trimer is centered on a H₃ site and the individual adsorbate atoms are on off-T₁ sites (radially displaced toward the trimer center).

H₃-T₄ : Figure 4.4 : A trimer model wherein the trimer is centered on a H₃ site and the individual adsorbate atoms are on off-T₄ 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-T₁-T₁-T₄ model has a Si honeycomb-layer registered on T₁ sites and a Au trimer centered on a T₁-site composed of Au atoms on off-T₄ 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-T₄-H₃ model has no first-layer Si atoms and has a Au trimer centered on a T₄-site composed of Au atoms on off-H₃ sites.

T₁ : Figure 1.5 : The one-fold coordinated adsorption-site on top of a first-layer Si atom.

T₁² : Figure 4.3 : The close-packed layer with 1/3 ml of adsorbate atoms centered on T₁ sites and 1 ml of adsorbate atoms between T₄ and H₃ sites.

T₁-H₃ : A trimer model wherein the trimer is centered on a T₁ site and the individual adsorbate atoms are on off-H₃ sites (radially displaced toward the trimer center).

T₁-T₄ : A trimer model wherein the trimer is centered on a T₁ site and the individual adsorbate atoms are on off-T₄ sites (radially displaced toward the trimer center).

T₄ : Figure 1.7 : The four-fold coordinated adsorption-site on top of a second-layer Si atom.

T₄² : Figure 4.1 : The close-packed layer with 1/3 ml of adsorbate atoms centered on T₄ sites and 1-ml of adsorbate atoms between T₁ and H₃ sites.

T₄-T₁ : Figure 4.6 : A trimer model wherein the trimer is centered on a T₄ site and the individual adsorbate atoms are on off-T₁ sites (radially displaced toward the trimer center).

T₄-H₃ : A trimer model wherein the trimer is centered on a T₄ site and the individual adsorbate atoms are on off-H₃ sites (radially displaced toward the trimer center).

T^V₁ : The vacancy site created by the removal of a first-layer Si atom.   For example, the MCT-T^V₁-T₁-T₄ 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 √3-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 √3-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.