Spectroscopic surface scattering of confined acoustic phonons in silicon nanostructures

The specularity of phonons at rough crystal surfaces is a fundamental aspect of phonon transport in nanostructures. It directly impacts engineering problems such as heat conduction in nanostructures and dissipation in nanomechanical resonators. At room temperature, the available guidance from theory is limited to fully diffuse transport under the condition of small surface roughness. Recent experiments in thermal transport suggest that there may exist large gaps in understanding phonon interactions with rough surfaces, especially when the roughness dimensions are comparable to phonon wavelengths. To consider such refinements, this thesis focuses on spectroscopic measurements of specularity in silicon nanostructures with well-characterized surface morphologies. We employ a femtosecond laser pump-probe setup to excite and detect confined acoustic phonons (∼ 18 - 200 GHz) in freely- suspended silicon membranes and nanowires. Surface scattering dominates intrinsic Akhiezer damping at frequencies > 60 GHz, thereby enabling us to probe phonon-boundary interactions over wavelengths ∼ 42 - 140 nm. To quantitatively understand the dependence of boundary scattering on RMS roughness and correlation length, we obtained detailed statistics of the surfaces using HRTEM and AFM imaging. For silicon membranes, we find that both Ziman and perturbation approach for roughness scattering successfully explain the nearly specular reflection of ∼ 0.1 THz phonons from surface with ∼ 1-nm scale roughness. The measured phonon specularities for silicon nanowires, however, are significantly lower in comparison to membranes for the frequency range ν ∼ 18−100 GHz. The reduction in specularity is caused by additional scattering from multiple surfaces introduced in a nanowire. Using a remarkably simple normalization scheme, we show that the scattering from multiple surfaces can be effectively decoupled. The magnitudes of the (normalized) phonon lifetimes are in good quantitative agreement with the predictions of Ziman approach but does not perfectly explain the frequency dependence. The τ ∼ ν^−1.7 dependence observed in our experiments is suggestive of weak phonon localization which cannot be understood within the existing framework of single scattering (or equivalently first Born approximation). This work helps to advance the fundamental understanding of phonon scattering at the surfaces of nanostructures.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2019

Author dc:creator

• Gelda, Dhruv

Contributors dc:contributor

• Sinha, Sanjiv
• Bahl, Gaurav
• Li, Xiuling
• Matlack, Kathryn

• boundary scattering, acoustic phonons, silicon nanostructures

Statement dc:rights

• Copyright 2018 Dhruv Gelda

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/102912

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/102912