Carrier Transport In Nanostructures: Probing Phonons And Electrons

Abstract

Spectrally resolved phonon transport is important for understanding nanoscale heat flow, which have implications for the realization of efficient thermoelectric and microelectronic cooling devices, for the realization of efficient cryogenic particle detectors, and for the realization of robust implementations of quantum computers. This thesis describes the development and utilization of a microscale phonon spectrometer. Aluminum superconducting tunnel junctions (STJ) are utilized for the emission and detection of non-thermal phonons with frequencies ranging from ~100 to ~870 GHz in silicon nanostructures. The energy resolution of the spectrometer is ~6080 [mu]eV, corresponding to a frequency resolution of ~15-20 GHz, which is about 20 times better than the energy resolution of conventional thermal transport measurements that rely on a Planck distribution of phonons. The spectrometer is utilized to probe surface scattering and phonon backscattering in silicon. To probe surface scattering, silicon nanosheets were fabricated, their surface roughness (~1 nm) was determined using atomic-force microscopy, and their phonon scattering rates were measured. Our results indicate that the well-known Ziman theory, which takes into account the roughness of the surface, underestimates the probability for totally diffusive scattering in nanostructures. To probe phonon backscattering, phonon 'enhancers' (~90 [mu]m deep) were etched around the STJ detectors, and the measured backscattered phonon signal increases with the number of enhancers. Using a geometric analysis of the phonon pathways, we show that the mechanism of the backscattered phonon enhancement is due to confinement of the ballistic phonon. These results have implications for ballistic phonon transport, phonon-mediated detection, and thermal transport studies, and highlight the important effects of phonon scattering from surfaces and interfaces in nanoscale geometrical designs. Finally, a facile room-temperature method, comprising ammonium sulfide treatment and electrophoretic deposition, was developed for assembling colloidal copper sulfide (Cu2-xS) nanoparticles into highly electrically conducting films. Electronic properties of the treated films are characterized with a combination of Hall Effect measurements, field-effect transistor measurements, temperature-dependent conductivity measurements, and capacitance-voltage measurements, revealing their highly-doped p-type semiconducting nature. In addition to being important for solutionprocessed electronics, the periodicity introduced by nanoparticles and their arrays presents a model system for probing phonon transport in complex interfaces.