Theoretical Study of Carbon-Based Materials and Their Applications in Nanoelectronics

Abstract

Continual scaling down of silicon device, which is the main driving force in device performance enhancement, is not sustainable as we approach the physical limits of silicon and it is foreseen that new materials and novel device structures will be required for future device improvements. In this regards, research in carbon electronics has been intensified since 2004 due to the physical realization of thermodynamically stable planar graphene. Two-dimensional monolayer graphene sheets have unique electrical and physical properties which can be exploited in new device structures. However, due to its semi-metallic nature, much focus has been given to converting graphene based materials into semi-conducting material, such as applying a perpendicular electric field to a bilayer graphene and impurity adsorption on the graphene surface. A more commonly studied method involves cutting two-dimensional graphene sheets into one-dimensional narrow ribbons, i.e. graphene nanoribbons (GNRs), where the quantum confinement introduced by the physical edges generate an energy bandgap that is closely related to the width and edge configurations of the ribbon. Such semi-conducting GNRs can be relatively easy to integrate into existing device structures and the unique electronic properties can be used in new device applications. Both experimental and theoretical studies have been carried out extensively on integrating GNRs into existing device technologies such as metal-oxide-semiconductor field-effect transistors. In addition, bilayer GNRs, which combine the unique electrical properties of GNRs and bilayer graphene, show great potential as versatile materials which can enable new device designs that take advantage of tuneable energy bandgap such as nanoelectromechanical devices. Recent development in obtaining GNRs by unzipping carbon nanotubes has made the prospect of fabricating GNR-based electronic devices in large quantities more promising and hence, detailed understanding of the device physics of GNR-based devices are much needed. This thesis, therefore, summarizes the investigation of the electronic structures of GNRs, both monolayer and bilayer, and materials with graphene-like atomic structure such as boron-nitride-carbon (B-N-C) compound. In addition, potential devices that can be implemented with these materials are also studied in details. Using various methods for the calculation of the electronic structure of the material, such as density functional theory, p-orbital tight-binding model and the Dirac equation model and utilizing the general non-equilibrium Green?s function approach to simulate the electron transport for device evaluations, with the inclusion of acoustic and optical phonon scattering, the performance of various devices such as Schottky Barrier field-effect transistors (FET), nanoelectromechanical switches, resonant tunnelling dioides and the effects of heterojunction, fringing field, and phonon scattering on tunneling FET based on GNRs are evaluated. This exploration on the device physics and performance of carbon electronics serves to enhance the knowledge for post-silicon device investigations.