The Study of Si Surface Structures and Their Influence Upon
Pb Islands During Growth
Daniel Johnson, REU Program, Physics
W. Yeh, Graduate, Physics
M.C. Tringides, Researcher, Ames National
Laboratory, U.S. DOE
Abstract:
The goal of this study is to determine the ratios of surface type
upon the Si sample. Data is collected from the Si samples using SPALEED. By analyzing the data from dates where combined surfaces are
present it is possible to determine the amount of certain types of
surfaces present on the sample. The ultimate goal is the ability to
control the growth of Pb islands upon the Si crystal. Applications of
this research include the design of advanced microprocessors and
quantum wires.
Introduction:
The goal of the REU project to which I was assigned is
to determine the amounts of certain types of crystalline
surface structures present upon the Si crystal that had
been used in previous experiments involving the formation
of Pb islands at low temperature.
This is accomplished by
analyzing certain aspects of the data collected by Spot
Profile Analysis Low Energy Electron Diffraction (SPALEED).
The data sets are taken from different experimental
runs involving various surfaces and are compared to one
another.
These surfaces include combined types of surface
structures as well as those with only a single surface
structure present.
Background and Experimental Procedure:
Within ultra-high vacuum and at very low temperature,
a Si crystal is placed.
This crystal is heated by use of a
tungsten
filament
high
eliminate
impurities.
to
The
temperatures
Si
is
then
in
order
heated
again
to
at
certain temperatures and for specific durations of time in
order to create particular crystalline surface structures.
Atoms of Pb are then deposited upon the Si crystal by a
process
of
evaporation.
The
amount
of
Pb
deposited
is
known and is measured in terms of monolayers, or layers of
Pb upon the surface measuring one atom in height. These Pb
atoms upon deposition form islands of a particular height.
Electrons at low, known energies are then used to analyze
the surface via their diffraction off the surface.
The diffraction utilized in the laboratory is similar
to that used in optical applications. The differences lie
in
the
execution
of
the
diffraction
and
in
the
use
of
electrons at low energy as opposed to using photons. In
typical optical diffraction the photons are analyzed using
a known diffracting surface that can be altered in order
gather information about the nature of the photons. In the
process utilized by the experimenters here it is the beam
that is known and the surface that is to be investigated.
The energy of the electrons and the areas upon which they
strike the surface can be altered in order to investigate
the nature of the surface. The interference created by the
electron interaction can be analyzed in order to form a
picture of the surface. The data from the SPA-LEED then
undergoes a Fourier transform in order to display visibly
the locations of the highest intensities present and most
prominent features of the surface.
The aspects of the data collected that are of most
interest are the locations and intensities of particular Si
atoms
and
deposited
Pb
islands.
The
types
of
surfaces
present can be determined by the intensities and locations
of certain Si atoms. The surfaces that are known to be
present are the 7 x 7 surface and the sqrt (3) x sqrt (3)
surface. The intensities of specifically investigated atoms
can allow to be known the amounts of each surface type
present in the sample for which data was taken. The data
concerning
the
intensity
of
determine
their
height,
width
the
and
Pb
spots
shape.
is
used
Together,
to
the
scans of the Si and the Pb spots form a picture of what is
happening and what has formed upon the surface of the Si
sample.
Analysis:
The
aspect
analysis
of
the
of
work
the
data
completed
collected
this
is
the
summer.
primary
Data,
taken
earlier in the year from experiments which involved the
combination of the 7 x 7 and sqrt(3) x sqrt(3) surfaces,
contained many un-addressed questions.
This summer’s work
was done to determine the amounts of each type of surface
upon the Si, both before and after the deposition of Pb.
Within
the
data
collected
form
various
experiments,
surface scans certain resolutions, positions, angles, and
energies are found.
Those of most interest are those scans
that are of certain spots upon the surface. Data scans of
matching
energies,
angles,
and
resolutions
are
then
gathered and analyzed.
The analysis of the data involves fitting the data
peaks that result from the Fourier analysis. These peaks
are fit using either a gaussian or a lawrencian 3/2 fitting
model of either a “single scan” or a “double ring” fit.
The
“single
scan”
is
used
primarily
for
simple
and
symmetrical peaks. The “double ring” model is used for more
complicated overlapping peaks that can be found in certain
data
sets.
project
was
All
of
done
the
so
lawrencian 3/2 model.
data
with
a
that
was
“single
analyzed
scan”
fit
for
this
using
a
The fittings allow the areas and widths of the peaks
to
be
accurately
known.
The
areas
under
the
peaks
correspond to the intensities of the peaks and thus to the
amounts of Si or Pb atoms present.
The width of the peaks
speaks of their with upon the Si surface.
By comparing the intensities of specific Si spots with
each other and at various values of Pb coverage the amount
of each surface type can be determined.
common
to
all
scans
reference point.
and
surface
The Si (0,0) spot,
types
is
used
as
a
The Si (-1/7,0) is commonly analyzed in
order to determine the presence of the 7 x 7 surface. The
Si (-1/3, -1/3) as well as the Si (-2/3,-2/3) spot is used
to determine the amount of sqrt (3) x sqrt (3) surface is
present upon the sample.
as
well
plotted
as
the
against
The intensities of these spots,
intensities
each
other
of
the
with
Pb(1,0)
varying
spots,
values
of
are
Pb
coverage and patterns are searched for.
Problems arise in some of the data points. Some of the
peaks
suffer
from
poor
resolution
and/or
poor
symmetry.
These data sets are commonly unable to be fit with the
models available.
Other problems arise due to a lack of
data of certain scans for certain points. This results in
an inability to compare the data points due to the lack of
matching resolutions.
Conclusion:
The research of this summer has yet to yield results.
Further comparisons need to be made between the combined
surface and surface that is composed of a pure sqrt (3) x
sqrt (3) surface.
another
type
Further
of
It is also possible that there may be
surface
investigation
intended.
present
into
up
this
the
sample
possibility
surface.
is
also
Additional data concerning the Si spots is also
to be compared to each other concerning the dates on which
the data was taken.
The goal of the research in general is to be able to
control the Pb islands’ height, width and position upon the
silicon.
It is known through previous experiments that the
surface of the Si upon which the Pb is deposited can change
the
height
of
the
Pb
islands
formed.
By
altering
the
silicon surface upon which the Pb is deposited it is hoped
that the Pb islands can be controlled.
The hope is that the work here can help pave the way
for computer chip architecture in the next fifty years.
If
the
in
Pb
islands
can
be
deposited
evenly
and
placed
specific locations it is believed that they can be use as
quantum
wires,
dramatically
silicon chip computer.
increasing
the
speed
of
the
At this time the data is promising.
With more data
and further analysis the problems of controllable Pb island
formation
are
sure
to
be
solved.
The
use
of
Scanning
Tunneling Microscopy and SPA-LEED are the keys to unlocking
the mysteries of the Si surface and it’s connection to the
formation of the Pb islands.