| Summary: | In this thesis we have used a combination of theory and experiment to explore the
electronic structure of two nanoscale semiconductor systems, namely porous silicon and the
metal-semiconductor tunnel junction in a scanning tunneling microscope (STM). The
experimental techniques used to study these systems are complementary in the sense that STM
provides atomic scale information on surface structure while X-ray absorption and fluorescence
provide abundant information on electronic structure. STM provides a unique way to locally
probe the electronic structure of a surface with atomic resolution.
We have used a linear combination of atomic orbitals model to explain the features seen
in X-ray absorption and fluorescence spectra of bulk silicon and porous silicon. We can
explain why the band edges shift in porous Si relative to bulk Si using a quantum confinement
model. These band shifts can also be modeled by effective mass theory. The linear
combination of atomic orbitals model has also shown that X-ray fluorescence spectra are
sensitive to the structure of porous Si on a nanometer length scale. We show that the observed
spectra are in good agreement with the nanostructures being very thin <100> oriented Si
sheets.
In order to perform cross-sectional STM measurements on semiconductor
heterostructures an ultra-high vacuum STM with a precise lateral sample motion capability was
constructed as part of the thesis. This STM also has variable sample temperature capability
from room temperature down to 120 K. As well as imaging, with atomic resolution, a
GaAs/AlGaAs interface in cross-section we have imaged the following various heterostructures
in cross-section for the first time; a CoSi₂layer buried in Si, an InGaAs quantum well and a
ZnSe layer on GaAs. Spatially resolved STM spectroscopy of the CoSi₂layer distinguished
between the metallic nature of the CoSi₂and the semiconductor nature of Si. From STM on
Si(l 10) we have proposed a "sextet" model for the atomic structure of the clean reconstructed
Si(110) surface. A GaAs/ZnSe interface was also imaged for the first time with atomic resolution. The conduction band offset of this interface was obtained by measuring the width
of the depleted region on the ZnSe side of the interface.
In order to gain information on the electronic structure of semiconductor surfaces we
performed current-voltage measurements with our STM. We show that STM current-voltage
measurements on clean (110) GaAs can give information about the presence of material on the
end of the STM tip. The turn-on voltage of the current at positive tip-sample bias can be
explained by assuming there is an insulating layer on the end of the STM tip. This
contamination which has the effect of increasing the tunnel-junction resistance at negative tipsample
bias but not at positive bias may be GaAs which is deposited on the tip from the sample
during STM operation. A thin layer of GaAs is sufficient to fit the data. STM current-voltage
measurements performed on clean n-type (110) GaAs also showed structure at both forward
and negative bias which arises from tip-induced band-bending as well as the very low density
of states at the bottom of the conduction band of GaAs and the transition to much higher
density of states at the L6-point located 0.25 eV above the conduction band minimum.
Variable temperature current-voltage characteristics show an exponential dependence from
room temperature down to 125 K that cannot be explained by thermionic emission. The
measured temperature dependence can be explained by assuming the affinity of the (110) GaAs
surface is =0.2 eV smaller at 125 K than at room temperature. === Science, Faculty of === Physics and Astronomy, Department of === Graduate
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