- Figure 5.1: The
*MCT*-T_{1}-T_{1}-T_{4}model for Si{111}√3x√3-30-Au. - Figure 5.2: The
*MCT*-T_{1}-T_{1}-H_{3}model for Si{111}√3x√3-30-Au. - Figure 5.3: The
*MCT*-T^{V}_{1}-T_{1}-T_{4}model for Si{111}√3x√3-30-Au. - Figure 5.4: The
*MCT*-T^{V}_{1}-T_{1}-H_{3}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*-T_{4}-H_{3}model for Si{111}√3x√3-30-Au. - Figure 5.8: The
*MTL*-T_{4}-T_{1}model for Si{111}√3x√3-30-Au. - Figure 5.9: The relaxed
*MTL*-T_{1}-H_{3}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*-H_{3}-T_{1}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*-T_{1}-H_{3}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*-H_{3}-T_{1}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.

## Chapter 5
## Au/Si{111}
## AbstractThe structure of the Si{111}√3x√3-30-Au surface has received much attention owing to the use of gold in the semiconductor industry and to the presumed relationship between the Si-√3-Au and the Si-√3-Ag surfaces. Plausible models suggested for these structures can be divided into two groups: honeycomb models and trimer models. We demonstrate conclusively that the Si-√3-Au surface contains 1-ml of Au atoms which form trimers [82]. The trimers have a 2.8 Ang. edge length, are located above fourth-layer Si atom sites, and consist of Au atoms registered on off-first-layer Si atom sites. The first layer of Si atoms is missing. The Au trimer layer is 0.56 Ang. above the second-layer Si atoms. The second-layer Si atoms are radially displaced 0.48 Ang. away from the three-fold axis. The remaining Si atoms are presumed to be in bulk sites. This structure is closely related to the structure of the Si{111}6x6-Au surface. Presented below is a description of the background information, the experimental details, and the structural analysis. ## 5.1 IntroductionA complex morphology is developed when thick Au films, of the order of 10 ml, are deposited on a Si{111} surface. For growth on a room-temperature substrate the interface between the bulk Si substrate and the Au film is not sharp, Si atoms migrate through the Au, and a Au/Si layer is formed on top of the Au films. For growth on a substrate above 400 C the initial interface is a 6x6 structure, islands of Au grow above this interface structure, and the tops of the islands are a reacted Au/Si surface [1]. Little is known about the structure of the interface, the bulk, and the surface of such thick Au films. Even less is known about the surface structures of near- and sub-monolayer growth of Au on Si{111}. These structures, in particular a Si{111}√3x√3-30-Au structure, are the subject of this chapter. A historical overview is presented next. For the aid of the reader, Glossary B contains definitions of the structure acronyms.
An initial
Subsequent studies
[84] - [88] led to the
reporting of three
structures when Au was evaporated onto the
Si{111}7x7 surface and then heated to 700 C
for several minutes. These structures were:
a 5x1 phase at coverages reaching 2/5 ml;
a mixed 5x1 and √3 surface at coverages
reaching 4/5 ml;
a √3 surface at coverages reaching 1-ml;
and a 6x6 surface above 1-ml.
The 5x1 pattern also contained weak streaks
between and parallel
to the rows of the fractional-order beams,
which suggest [84] that
the 5x1 structure was actually a disordered 5x2 phase.
The √3 surface was reported [89] in varying degrees of
order,
Further structural studies by
Oura
Huang
A recent
Total-energy calculations, by Ding Finally, a study of the lower-coverage 5x1 surface [105] has suggested a model involving trimers of Au atoms. Although the 5x1 and √3 surfaces are clearly different, it is quite possible that the building blocks of the surface unit-meshes are the same.
At this stage, some general remarks should be made about
the applicability of the various surface techniques,
remarks which are particularly important for the problem
discussed here.
First, the mere observation of
a
The purpose of the present study was to determine
the √3 structure or at least to rule out many
of the proposed models for a range of structures.
These models, in summary, are the
Au honeycomb and
centered-honeycomb models registered
on H ## 5.2 Experiment
A Si{111} sample cleaved from a larger
wafer (phosphor, n-type, 8-12 Ohm cm) was
used for the experiment. Prior to mounting on a
goniometer, the sample was HF etched.
The sample was then loosely mounted on a 0.025-mm thick
Ta disk, 25 mm in diameter, with care given so that
no other materials were forward of the sample surface.
This sample disk was mounted on a
goniometer which allows heating to
1400 C (
For deposition, Au foils were mounted on
two separate W-coils,
approximately 10 cm from the sample face.
The sources were resistively heated to
approximately 1000 C, yielding deposition rates
on the order of 1/2 ml per minute, as determined
by
After the initial baking of the vacuum system and outgassing
of all filaments, a pressure below
2•10
After an initial bombardment
(2 uA, 375 eV, 5•10
The deposition of Au on the room-temperature Si substrate
gradually obscured the Si{111]7x7
From a set of sixty-nine ## 5.3 Keating Energy Analysis
A Keating energy analysis for
several of the Si-√3-Au model structures was
undertaken,
The models considered may be subdivided into five groups.
The first group contains the
2/3-ml Au honeycomb and the
1-ml Au-centered honeycomb
models.
The other four groups contain
Au trimers with various registries to the Si surface
and the addition (or depletion) of Si atoms in the surface,
- The
*MCT*-T_{1}-T_{1}-T_{4}and*MCT*-T_{1}-T_{1}-H_{3}models, see Figures 5.1 and 5.2, are plausible, but unlikely. Each Si adatom satisfies a T_{1}dangling-bond and bonds to three Au atoms. There exists, however, an additional T_{1}dangling-bond below each trimer. - The
*MCT*-T_{4}-T_{4}-T_{1},*MCT*-T_{4}-T_{4}-H_{3},*MCT*-H_{3}-H_{3}-T_{1}, and*MCT*-H_{3}-H_{3}-T_{4}models are implausible. Each Si adatom satisfies three T_{1}dangling-bonds. There exists, however, only one adatom bond per three Au atoms. - The H
_{3}-T_{1}and T_{4}-T_{1}models, see Figures 4.5 and 4.6, are plausible. Each T_{1}dangling-bond is satisfied by one Au atom. - The T
_{1}-H_{3}, T_{1}-T_{4}, H_{3}-T_{4}, and T_{4}-H_{3}models, see Figure 4.4 as an example, are implausible. The first two require one T_{1}dangling-bond per three Au atoms and two unsatisfied T_{1}dangling-bonds. The second two require six bonds per Si-T_{1}atom. - The
*MCT*-T^{V}_{1}-T_{1}-T_{4}model, see Figure 5.3, is plausible, but unlikely. There are two unsatisfied T_{1}dangling-bonds, but the T_{4}dangling-bonds are each satisfied by a Au trimer-atom. - The
*MCT*-T^{V}_{1}-T_{1}-H_{3}model, see Figure 5.4, is implausible. There are two unsatisfied T_{1}dangling-bonds, and the T_{4}dangling-bonds must each be split for two Au trimer-atoms. - The
*MTL*-T_{4}-H_{3}and*MTL*-T_{4}-T_{1}models, see Figures 5.7 and 5.8, are plausible, but unlikely. The nine T_{4}dangling-bonds are satisfied, but substantial reconstruction can be expected. - The
*MTL*-T_{1}-T_{4}and*MTL*-H_{3}-T_{4}models are implausible. Each T_{4}atom can only bond once to a Au atom. Hence, there are six unsatisfied T_{4}dangling-bonds. - The
*MTL*-T_{1}-H_{3}and*MTL*-H_{3}-T_{1}models, see Figures 5.9 and 5.10, are plausible. Each T_{4}atom can satisfy all of its dangling-bonds.
The honeycomb (
The
As an extension to the Keating energy analysis, we
considered the adjustment of the Au-trimer's size
for the two plausible ## 5.4 LEED Intensity Analysis
The
The following models were tested:
T
Each of the above models failed to yield calculated I(V)
spectra which agreed with the experimental spectra, but
the
R-factor analysis confirmed our visual
assessment, that the
In further calculations,
the size of the Au trimer and the second
interlayer-spacing for several of the above models
was also varied, but
there was still no agreement.
Calculations which twisted the
The I(V) spectra for the
A final set of - The Au trimer has an edge size of 2.8 Ang..
We initially chose the value specified by the
x-ray analysis, since
**LEED**is generally insensitive to small in-plane displacements. We later varied this variable, but found no significant reduction in the R-factor values. - The first Si-layer is missing.
- The Au trimer is centered above the fourth-layer Si atoms.
- The Au atoms are on off-first-layer Si atom sites.
- The Au-trimer plane is 0.56 Ang. above of the plane
of second Si-layer.
This is the value specified by Ding
*et. al.*'s total-energy calculation analysis. Variations of up to +-0.2 Ang. did not produce any significant improvement in our R-factor analysis. - The second Si-layer atoms are radially displaced away from the trimer center by 0.48 Ang. Variations, again, did not produce detectable improvements.
- The interlayer spacing between the second-layer and the third-layer Si atoms is unchanged with respect to the bulk value.
- The real part of the inner potential is -6 eV.
- We have not determined if there is a symmetric out-of-plane motion of the fourth-layer Si atoms.
- We have not determined if there is a radial motion of the third-layer Si atoms.
- The Au-Si distances are 2.405 Ang. and 2.416 Ang. Without the radial displacement of the second-layer Si atoms the Au-Si bonds would be of unequal length, namely, two would be highly compressed and one would be highly expanded.
The visual agreement between the experimental and theoretical (for the optimized structure) spectra can be examined in Figures 5.13, 14, 15, 16, 17, and 18. Corresponding R-factors are listed in Table 5.4. Generally, the agreement is not ``outstanding'', but it is significantly better than for any other attempted structure. We expect that the agreement may improve if calculations using twelve atoms in the surface layer could be done, because this would allow for the radial motion of third-layer Si atoms, the buckling of the fourth-layer, etc.... to be explored. Additionally, we have yet to explore the twisting of the second-layer Si atoms. ## 5.5 Discussion
The 1/3 1/3, 10, and 01 beams
for the 6x6 and √3 surfaces have remarkably
similar
In regard to the √3 surface,
there exist in the literature, see Appendix D,
several model structures that have been applied to both
the Si{111}√3x√3-30-Au and -Ag surfaces. This linkage is most likely
due to the isoelectronic properties of Au and Ag.
However, the growth of thin films of Au and Ag on Si is
very different. For instance, Ag islands grow on the
partially exposed and sharp Si-√3-Ag interface, whereas
Au islands grow on a diffuse Si6x6-Au interface and the
island tops contain Si atoms [1].
Additionally, the fractional-beam
The problem is, however, that there is a consensus
in the literature that the Si-√3-Ag surface
has the
Additional complications arise when discussing
Au or Ag on Ge{111} surfaces. There are no
reports of structure studies for Ge-√3-Au,
but Dornisch
In conclusion, the Si{111}√3x√3-30-Au surface is
a trimer-based structure. The surface
is missing the first layer of Si atoms.
The Au atoms are registered on first-layer Si atom sites,
form trimers centered on fourth-layer Si atoms sites,
and are 0.56 Ang. above the second-layer Si atoms.
These second-layer Si atoms relax radially
away from the Au trimer by 0.48 Ang.
We denote this structure |

- Figure 5.1: The
*MCT*-T_{1}-T_{1}-T_{4}model for Si{111}√3x√3-30-Au. - Figure 5.2: The
*MCT*-T_{1}-T_{1}-H_{3}model for Si{111}√3x√3-30-Au. - Figure 5.3: The
*MCT*-T^{V}_{1}-T_{1}-T_{4}model for Si{111}√3x√3-30-Au. - Figure 5.4: The
*MCT*-T^{V}_{1}-T_{1}-H_{3}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*-T_{4}-H_{3}model for Si{111}√3x√3-30-Au. - Figure 5.8: The
*MTL*-T_{4}-T_{1}model for Si{111}√3x√3-30-Au. - Figure 5.9: The relaxed
*MTL*-T_{1}-H_{3}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*-H_{3}-T_{1}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*-T_{1}-H_{3}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*-H_{3}-T_{1}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.

*The "content" has not been modified, but the
HTML code was last modified on 07/02/96, by
me.
*

*
Please notice the copyright and
disclaimer.
*