Abstract
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.