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Dr. Alison A. Baski
Silicon Surfaces
One of my primary areas of interest, silicon, is a material that has spawned
a multibillion-dollar industry. Properties of silicon which have led to its
dominance in the electronics industry include: a nearly perfect SiO2 passivating
interface, large wafer size, robustness, and low cost. Although silicon is the
material of choice for VLSI technology, it does lack a few highly desirable
properties, such as laser action and high mobility. To overcome this limitation,
there has been a strong effort to integrate materials having such properties
with standard Si integrated circuit technology, i.e. to grow heterostructures on
silicon. Recent examples include GaAs grown on Si for optoelectronic and high
speed applications, HgCdTe grown on Si for far-infrared detection, and Si/Ge
heterostructures for heterojunction bipolar transistors. The challenge of thee
systems lies in understanding and controlling the interface between the
various materials, where lattice mismatch and different thermal expansion
coefficients can lead to misfit dislocations which impair device performance. A
number of approaches have been explored to minimize the effect of such
dislocations, ranging from manipulation of the growth conditions to the choice
of substrate orientation, but much remains to be done.
Dr. John A. Carlisle
Synchrotron-radiation-based materials characterization
After completing my doctoral work in October 1993, I was a postdoctoral research
associate in the Chemistry and Materials Science Department at Lawrence
Livermore National Laboratory (LLNL). Working under Dr. Louis J. Terminello, and
in collaboration with a variety of researchers at LLNL, as well as other
national labs, industry, and academia, I have conducted cutting-edge research at
the newly commissioned Advanced Light Source (ALS) at Lawrence Berkely National
Laboratory. Utilizing the brightest tunable source of soft X-rays available, I
have used photoemission, photoabsorption, and soft-x-ray flourescence to probe
the geometric and electronic structure of many novel material systems and
surfaces. These systems include diamond and diamond-like thin-films, buried
monolayers, magnetic multilayers, dopants, polymers, aerogels, and other
thin-film systems. In particular, I have been involved in ground-breaking work
probing inelastic x-ray scattering processes in several of the above systems,
using a technique called resonant soft-x-ray fluorescence
It is this emerging technique of resonant soft-x-ray fluorescence that I wish
to develop into a powerful characterization tool of novel surface and material
systems. Soft x-ray-fluorescence (SXF) spectroscopy, using synchrotron radiation
as a tunable excitation source, offers several advantages for probing the
electronic structure of complex, multi-elemental materials [D. L. Ederer et
al., Synchrotron Radiat. News 7, 29 (1994)]. As a photon-in,
photon-out spectroscopy, SXF is intrinsically bulk-sensitive. This is due to the
long mean-free path of photons in solids (~1000A). Also, since core levels are
involved in both the photon absorption and emission processes, SXF is both an
element- and angular-momentum-selective probe of the occupied electronic
structure. Consequently, SXF measures the local particle density of states
projected onto each constituent element of the material. Note that this allows
one to probe the electronic structure of elements which are buried deeply in a
sample, such as dopants, buried thin films, and multilayers. The chief limitatin
of SXF has been the low fluorescence yield for photon emission, particularly for
light elements. However, third-generation light sources such as the ALS now
offer the high brightness that have made high-resolution SXF experiments
practical. In particular, this high brightness has enabled detailed studies of
the excitation energy dependence of emission spectra when the excitation energy
is tuned through a core level binding energy. In these robust resonant SXF
experiments, the presence or lack of long-range order, the degree of
localization, the energy-separation between the major symmetry points in the
band structure, and the crystal momenum-resolved electronic structure, may be
determined.
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