Charge carrier transport in semiconducting carbon nanotubes

Carbon nanotubes (CNTs) are cylindrical structures of hexagonally bonded carbon atoms with diameters of a few nanometers. CNTs are generated with different structures characterized by their diameter and chirality (twist). Two types of semiconducting CNTs are zigzag (chirality theta = 0°) and chiral (0° < theta < 30°). CNTs have demonstrated potential to be the key building blocks in the next generation of nano-scale electronic and opto-electronic devices. However, the CNT fabrication process still suffers from lack of controllability of diameter and chirality. Hence, transport experiments also suffer from lack of reproducibility and a systematic theoretical study became essential.; This thesis presents a comprehensive study of the charge carrier transport in semiconducting CNTs. The low- and high-field transport characteristics are explored using Rode's method and Ensemble Monte Carlo simulation technique, respectively. The basis for the transport calculations is provided by electronic structure calculations within the framework of a simple tight binding model. The principal scattering mechanisms considered are due to the electron-phonon interactions involving longitudinal acoustic, longitudinal optical, and radial breathing mode phonons. The effects of structural parameters, i.e. diameter, chirality, and group on electronic structure, phonon dispersion, and transport properties are explored. Low-field mobility is found to depend strongly on diameter compared to either chirality or group. For high fields, both transient and steady-state phenomena are explored for various temperatures. Due to the smaller Brillouin zone, Umklapp scattering processes occur with much greater frequency in chiral carbon nanotubes than in zigzag nanotubes. The results show interesting transient phenomena that are caused by the limited phase space of these dynamically one-dimensional structures. The steady-state velocity saturates due to optical phonon scattering, and negative differential mobility is obtained at large electric fields. The effects of chirality and group are stronger for small diameter tubes and become negligible for large diameters. The two zigzag CNTs (groups +1 and -1) set upper and lower bounds for important transport parameters. The overall effect of diameter on transport properties is stronger than those of chirality and group. The work thus establishes a clear correlation between the structural and the transport characteristics of semiconducting CNTs.