- Figures 3.1,
2,
3,
4,
5, and
6: The
**LEED**I(V) spectra of Si{111}√3x√3-30-B. - Figure 3.7: The H
_{3}-, T_{4}-, and B_{5}-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
H
_{3}-, T_{4}-, and B_{5}-model structures of the Si{111}√3x√3-30-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}√3x√3-30-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}√3x√3-30-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}√3x√3-30-X surfaces as a function of the adsorbate's covalent radius.

## Chapter 3
## B/Si{111}
## Abstract
The past several years have seen many works on the
Si{111}√3x√3-30-B system [40-65].
This is the last
of the simple Si-group-III systems to be studied, probably
because of its difficulty of preparation. Boron is a common
dopant in p-type Si wafers, so Si{111}√3x√3-30-B
promises to be a system of technological significance.
Therefore, a dynamical ## Introduction
The formation of a Si{111}√3x√3-30-B surface has only been realized in
the past few years. This was followed by a rapid
search for its structure. Initially, models
were proposed which involve 1/3 ml of boron in one
of four different sites: T
Korobstov
Thibaudau
Bedrossian
Total energy calculations (
Finally, an analysis
using surface x-ray diffraction ( ## 3.2 Experiment## 3.2.1 Si{111}√3x√3-30-B
Highly-boron-doped Si{111} samples,
nominally 10x20x0.5 mm,
p-type, 0.0016-0.0025 Ohm cm,
and cleaved from a larger wafer were
used for the experiment. Before being mounted on the
goniometer, the samples were HF etched.
The samples were 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 disk was then mounted on the
goniometer which allows heating to
1400 C (
Cleanliness and boron segregation were monitored
with
The above
The Si{111}√3x√3-30-B surface was prepared many times by anneal
of the sample to temperatures between
1200 and 1300 C. Lower anneal
temperatures of 1000 C only produced 7x7 patterns.
Intermediate temperatures produced mixtures of both
7x7 and rt3 surfaces.
When the rt3 surface
was prepared, the
## 3.2.2 Si/Si{111}√3x√3-30-B
The formation of an `all-Si rt3'surface was also
attempted. Bedrossian
Attempts were also made
using solid-phase epitaxy at lower temperatures.
Progressive anneal (500, 550, 600 C etc....)
of amorphous Si on a Si{111}√3x√3-30-B surface
produced 1x1, 7x7, and rt3 patterns.
These patterns often contained high background and
diffuse beams. The spectra of these rt3 patterns were
also equal to those previously collected from the B ## 3.3 LEED Intensity Analysis
Dynamical
The structural parameters for the relaxed
B
The results of the three
studies agree well within the quoted
error margins. The differences may be due to the
inability of the
The main features of the structure involve the
substitution of a boron atom for a Si atom in the second
layer. The replaced Si atom then resides above
the boron atom in the T ## 3.4 Keating Energy Analysis
The stability of the B
The vector i and j. R_{i} is the tetrahedral covalent
radius of atom i.
The scaling constants alpha and beta are
0.210 eV/Ang.^{4} and 0.0183 eV/Ang.^{4} [22], respectively.
Thus, the Keating energy is an elastic energy sensitive
to interatomic bond compression, expansion, and bending.
The length term increases when an interatomic bond
is larger or smaller than the sum of the radii.
The angle term increases when the angle between bonds
differs from 109.5 degrees.
The scaling constant alpha is larger than beta. Hence,
bonds may more easily be bent than relaxed.
The starting place for the Keating analysis is an
unrelaxed Si{111}1x1 surface, see Figure 3.7.
The bulk terminated surface contains only atoms
which are tetrahedrally coordinated with ideal
both lengths of 2.35 Ang. The Keating energy for this
surface is zero. Electronically, this is still a highly
energetic surface due to the presence of the
dangling bonds.
Saturation of these bonds by an adsorbate on the
T
We have modeled the H
The H
The B
The Keating results agree well with the structures determined
by
A Keating analysis for the Ge{111}√3x√3-30-X surfaces also reveals
the same trends as for the Si{111}√3x√3-30-X surfaces.
First, the H ## 3.5 Discussion## 3.5.1 Si{111}√3x√3-30-B
Additional surface science
techniques have been applied by other
researchers toward determining the structure of the
Si{111}√3x√3-30-B surface.
The dynamical
A photoemission and inverse photoemission
study [43] resolved the
predominant states in the valence band, which were identified
as being an unoccupied state from an empty Si dangling-bond
and an occupied state from Si-Si back bonding.
These results are consistent with the boron-B
The boron B
The B
In conclusion, the group-III and -IV metals
form 1/3 ml rt3 structures on the Si{111} surface.
The small boron atom forms the B ## 3.5.2 Si{111}√3x√3-30-B
The attempts to form epitaxial overlayers were
motivated by a desire:
a) to form an `all-Si rt3' phase and
determine its structure, and b) to study the epitaxy itself.
In regard to the `all-Si rt3',
the report of such a structure
by Bedrossian
In regard to Si epitaxy, initially deposited Si goes down
amorphously and does not remove the Si rt3-B
structure. This agrees with
other studies [41,50]
which indicate that
the Si adatoms are removed by the deposited atoms,
but the sub-surface boron
is unaffected. These facts further support the
B
Our results with Si
solid-phase epitaxy are, however, contradictory with
The solid-phase-epitaxy results are also different from those of chemical-vapor deposition [52,54], at lower reaction temperatures (500 C) can form epitaxial overlayers without segregating boron from the interface. This process allows for the preparation of boron-delta-doped layers deep within Si{111} crystals. On Si/SiP001}2x1-B, chemical-vapor deposition does not produce boron-delta-doped layers, but molecular-beam epitaxy does produce boron-delta-doped layers.
In conclusion, boron can be placed in the surface of Si{111}
by segregation from the bulk or by
decomposition of boron compounds.
In both cases, the Si{111} surface-structure changes
from 7x7 to √3x√3-30, with boron atoms located in
the B |

- Figures 3. 1,
2,
3,
4,
5, and
6: The
**LEED**I(V) spectra of Si{111}√3x√3-30-B. - Figure 3.7: The H
_{3}-, T_{4}-, and B_{5}-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
H
_{3}-, T_{4}-, and B_{5}-model structures of the Si{111}√3x√3-30-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}√3x√3-30-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}√3x√3-30-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}√3x√3-30-X surfaces as a function of the adsorbate's covalent radius.

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