The effect of surface roughness on thermal and acoustic phonon transport in nanowires and nanomembranes
Phonons are the primary carriers of heat in dielectric solids. In the limit of long wavelength, phonons are essentially sound waves in the solid. The interaction of phonons with crystal surfaces is of fundamental interest in thermal transport as well as in acoustics. This thesis focuses on understanding how surface roughness influences phonon transport through thermal and acoustic transport measurements in nanostructures with well-characterized surface morphologies. Results from the experiments are interpreted through wave scattering theory. This work investigates Si nanowires and nanomembranes with characteristic sizes smaller than 200 nm. Using a MEMS-based measurement platform, we observe the low-temperature dependence of thermal conductivity in silicon nanowires fabricated using a two-step, metal-assisted chemical etch. Using experimentally characterized surface roughness, we show that a multiple scattering theory is able to explain the observed low-temperature trends in thermal conductivity without employing any fitting parameters. A surface finish with a 5-10 nm roughness correlation length is typical in metal-assisted chemical etching. Such surfaces resonantly scatter dominant phonons in silicon, leading to the observed ~T^{1.6 - 2.4} behavior and drive the thermal conductivity below the Casimir limit. The second part of this thesis measures the dissipation lifetimes of acoustic waves in suspended Si membranes and nanowires. We employ a femtosecond laser pump-probe setup to excite and measure the lifetime of sub-THz longitudinal acoustic modes in suspended Si membranes with thickness down to 36 nm (~118 GHz). We show that the lifetime of acoustic phonons is Akhieser-limited for thick membranes while surface scattering is the dominant scattering mechanism for thin membranes. Perturbation-based spectral scattering theory does not seem to reproduce the observed trend in phonon lifetime even in the limit of small correlation lengths. A surface specularity parameter based on Kirchhoff's approximation correctly predicts the observed frequency trend but underestimates the lifetimes by a constant multiplicative factor. This work paves the way toward designing higher quality thin films acoustic resonators which are indispensable to the telecommunication and cell phone industries.
Phonons, Surface Scattering, Surface Specularity, Acoustics, Thermal Transoport
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign