ii
Abstract

One-dimensional (1D) nano-structures comprising of a wide spectrum of
materials ranging from Carbon nanotubes to molecular and lithographically patterned
wires are potential candidates for the next generation electronic devices. In such
structures, confinement of electron motion results in very fascinating transport
phenomena which cannot be explained by the classical conduction theory. The
purpose of this thesis is to present the interesting findings in the quantum transport
phenomena in silicon and Indium nitride based single electron transistors.

The first introductory chapter gives few rudimentary ideas in general quantum
transport theories. The experimental methods for fabricating quantum transport
devices are then discussed. The key transport phenomena manifesting as
experimentally measured Magneto Resistance Fluctuations, Electron localization,
ballistic conductance and strong (coulomb) localized electron systems with tunnel
barriers as Single Electron Transistors are briefly described.

In the second chapter, the basic experiments on the measurements of
Magnetoresistance fluctuations in a weak disorder indium nitride nanowire are
presented. These fluctuations are reproducible, aperiodic and symmetric in magnetic
field reversal but are asymmetric upon reversal of bias direction of the current flow.
The fluctuations are analyzed for both perpendicular and parallel external magnetic
field configurations in the light of tunnel Magnetoresistance at low field and impurity
scattering at higher field. The asymmetry in bias reversal has been attributed to the
breakdown of time reversal symmetry.

The third chapter deals with fabrication and tunneling transport characteristics
of Silicon based single electron transistor with lateral succession of a big island and
small quantum dots. The big island gives rise to a small period Coulomb oscillation
riding on the large irregular oscillation arising from the small quantum dots. The
peaks of the latter shift in the presence of magnetic field which is analyzed in the
context of field-induced Landau level shift with a soft-wall confinement potential.
Furthermore, the current peak was suppressed for fields beyond a threshold value. An
explanation based on cyclotron localization at non-interacting Landau levels is
iii
presented and consistently described with numerical estimates.

In the fourth chapter, as new aspect of electron transport phenomena in a single
electron transistor based on an individual indium nitride nanowire is presented.
Meticulous Coulomb oscillations are observed at low temperatures. While the device
shows single period Coulomb oscillation at high temperatures or at high bias voltages,
additional satellite peaks along with the main Coulomb peak appear at low
temperatures and low bias voltages. The quasi-periodic structure is attributed to the
mixing of dissimilar Coulomb oscillations arising from two serially coupled islands
embedded inadvertently in the surface metallic states of the nanowire. The proposed
model is numerically simulated with good agreement with the experimental data.

In the fifth chapter, the physics of single electron transistor fabricated in Double
Quantum dot geometry are presented and discussed in detail. At around 2K, these
devices showed clear Coulomb blockade structures. An external perpendicular
magnetic field was found to enhance the resonant tunneling peak and was used to
predict the presence of two laterally coupled quantum dots in the narrow constriction
between the source-drain electrodes. The proposed model and measured experimental
data were consistently explained using numerical simulations.

Chapter 6 presents ongoing work on InN nanobelt device showing signatures of
superconductivity in tunnel junction geometry. It was found that superconducting
transition takes place at temperature of 1.2K and the critical magnetic field is
measured to be about 5500Gs. The energy gap extrapolated to absolute temperature is
about 110μeV. The measured temperature and magnetic field dependences of the
superconducting gap agree well with the reported dependences for conventional
metallic superconductors. As the magnetic field is decreased to cross the critical
magnetic field, the device shows a huge zero-bias magnetoresistance ratio of about
400%. This is attributed to the suppression of subgap tunneling in the presence of
superconductivity.

The overall summary and conclusions are presented in the last chapter 7.

63
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