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LEEM
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One of the special facilities here at the University of Wisconsin
is the Wisconsin Low Energy Electron Microscopy Center, or the
“LEEM” for short. We take advantage of the microscope’s
capability of imaging surfaces at high temperature (greater
than 1000 °C if desired) to study in-situ and in real-time the
deposition of various materials at the atomic level.
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In many ways, this type of microscope is very
similar to its higher-energy, and more common, cousin, the Transmission
Electron Microscope (TEM). Both machines employ an electron
gun and a set of condensor lenses to direct a parallel beam
of electrons onto the desired specimen. After interacting with
the specimen, the electrons pass through an objective lens,
a series of apertures, and finally a set of projector lenses
to magnify the image originally produced by the objective lens.
With lenses arranged in this way and steering coils placed in
appropriate locations along the beam path, both a LEEM and TEM
can perform electron diffraction and bright- and dark-field
imaging. (Click
here to see a schematic of the electron optical paths.) |
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To see some of the other interesting
things we are doing, visit the following:
MRSEC
IRG
#1
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Obviously, differences exist between the
two microscopes. The
important distinction is in the electron energy used for imaging.
A TEM uses electrons with energies on the order of 100,000 eV
or greater to acquire an image, sending electrons through a
specimen to yield information about bulk, or 3-D, properties.
LEEM instead uses electron energies on the order of only a few
eV (typically about 3-10 eV) for imaging, “reflecting”
electrons off the first few atomic layers of the specimen surface,
providing 2-D information. This electron reflection in turn
leads to another major difference: a LEEM has an objective lens
and a section of beam path through which electrons travel in
anti-parallel directions.
To be capable of image magnification, a LEEM must be
able to separate these “incoming” and “outgoing”
beams; this is accomplished by passing the beams through the
same uniform magnetic field which, in the Wisc-LEEM, bends the
beams 120°. In a TEM, though, the imaging electrons simply pass
through a column unidirectionally with no separation required. |
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What makes this microscopy technique so powerful
and exciting is that we can view the deposition of materials
in real-time with a vertical resolution of a single atomic layer
and a lateral resolution of about 150 Å. We can therefore determine
many thermodynamic, kinetic, and morphological characteristics
of growth in-situ at temperature. Previous techniques were limited
to “archeological” studies in which the data had to
be taken after the growth process at much different temperatures. |
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Ongoing studies include the investigation
of growth of GaN surfaces, as well as various silicon and Silicon-on-insulator
surfaces before, during, and after growth of silicon and germanium.
Please visit our other sites to learn about some of the interesting
things we’ve found lately. |
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