Fabrication and Characterization of Heterogeneous Nanowires

Nanoscience and nanotechnology research has provided us with new paradigms of technologies to improve human life, but still there is plenty of room to expand its frontiers. In order to do so, we need to pursue the development and study of novel nanostructures with the main goal of understanding the physical properties and finding potential applications. Understanding the physics of low-dimensional systems is the first step to fostering the corresponding technological applications. Considering this premise, the goal of this dissertation is to study two distinct classes of heterogeneous nanowires (NWs): phosphorous-doped Si NWs with an axial doping gradient and metal NWs grown on DNA templates. The Si NWs grown by vapor-liquid-solid chemical vapor deposition were utilized to fabricate Schottky barrier-limited field-effect transistors (FETs), which have shown significant promise in the areas of electronics and sensing because of their unique characteristics. The idea of utilizing the modulation of the nano Schottky junction at a metal-semiconductor interface promises higher performance for chemical and biomolecular sensor applications when compared to conventional FETs with Ohmic contacts (exponential versus linear responses). However, the fabrication of such asymmetric FETs presents challenges such as reproducibility through complications in the fabrication processes. We have been able to circumvent the fabrication difficulties and reproducibility problems by utilizing our Si nanowires synthesized by a chemical vapor deposition process which yields a pronounced doping gradient along the length of the NWs. This inhomogenous doping in NWs has typically been seen as a detrimental characteristic; however, we have taken advantage of this doping profile as the basis of our approach. The graded doping profile facilitates definition of a series of metal contacts on a single NW that systematically evolve from Ohmic to Schottky with increasing effective barrier height along the axial direction. The study of this systematic variation is presented in this dissertation as a proposal to obtain devices for sensing and electronic applications. The main results of our research is recently published. The fabrication and study of metal NWs is the second effort discussed in this dissertation. The main motivation is to address the fundamental question of whether a true superconducting state could exist in one dimension. The answer to this question lies in the nature of superconducting fluctuations of the order parameter that describe the coherent behavior of the Cooper pairs. In a superconducting system, the order parameter has a well-defined amplitude and phase. The superconducting fluctuations occur in the form of phase slips which can be either thermally activated or quantum mechanical. Although much experimental and theoretical work has been done on the topic, an unambiguous resolution of this issue remains elusive mainly due to the challenge of producing NWs having the dimensions of the cross-section of the NW smaller than the superconducting coherence length or the size of the Cooper pairs. Our approach to overcome the fabrication difficulties to reach the true 1D limit is a unique combination of DNA templates and low temperature quench-condensation for in situ fabrication and measurement of superconducting NWs with a width of just a few nanometers. In this dissertation, details on the fabrication and our initial results demonstrating the capability of our DNA molecular templates to reach small cross-section metal NWs are presented; also, we present systematic characterizations of the electrical properties of metal nanowires with respect to in situ variation of the geometry of the nanowire. This effort has laid a full foundation for a comprehensive examination of superconductivity in 1D reaching unprecedentedly small cross-sections. A manuscript summarizing these results is in preparation.