Ultrafast Terahertz Response of Epitaxial and Chemical-Vapor-Deposited Graphene and of Germanium and Germanium/Silicon Core/Shell Nanowires.

[摘要] Nanotechnology has recently enjoyed a tremendous interest, and many emerging nanoscale and low-dimensional materials and devices are under active investigation. This dissertation presents research work on the physics of the ultrafast terahertz (THz) response and the hot-carrier dynamics in two sample classes of state-of-the-art materials, namely graphene and semiconductor nanowires, by using ultrafast time-resolved THz spectroscopy. Thorough understanding of this physics is indispensable for the implementation of novel high-speed nanoelectronic and nanophotonic devices based on these materials. The dynamic THz response of a wide variety of high-quality graphene, including epitaxial and chemical-vapor-deposited, is systematically studied. In pristine epitaxial and chemical-vapor-deposited graphene, the dynamics depend critically on the doping density with relaxation times ranging from a few picoseconds in highly doped to hundreds of picoseconds in quasi-neutral layers; a microscopic density-matrix theory of carrier-carrier and carrier-phonon interactions accounts quantitatively for the experimental results. In multilayer epitaxial graphene with a few highly doped and many lightly doped layers, low-energy relaxation times of hundreds of picoseconds are measured in the lightly doped layers consistent with a first-principles theory of interlayer energy transfer via screened Coulomb interactions. In semiconducting epitaxial graphene, the relaxation of the dynamics is up to two orders of magnitude faster relative to metallic epitaxial graphene attributed to large enhancement of the phase-space available for carrier scattering due to an energy gap. In epitaxial graphene nanoribbons, a highly anisotropic response arises from two-dimensional quantum confinement with carrier lifetimes of tens of picoseconds in the high-energy subbands. The dynamic THz response of quasi-one-dimensional high-quality single-crystal germanium and germanium/silicon core/shell nanowires is also extensively investigated. The dynamics are characterized by fast few-picosecond intraband carrier relaxation followed by slower sub-nanosecond interband carrier recombination processes. Clear evidence for ballistic carrier transport on tens of nanometers is observed, and surface defect states are identified as the dominant carrier recombination and scattering mechanism limiting carrier lifetime and transport. Three possible physical origins giving rise to the observed highly anisotropic response are considered including free-carrier response, exciton response and quantum confinement effects.

[发布机构] University of Michigan

[效力级别] terahertz spectroscopy

[关键词] ultrafast optics;terahertz spectroscopy;terahertz response;carrier dynamics;graphene;semiconductor nanowires;Electrical Engineering;Physics;Engineering;Science;Electrical Engineering