Optical characterization of thermal transport in multifunctional materials

Understanding thermal transport properties of materials is essential for both device applications and materials physics. Thermal conductivity and interface thermal conductance are fundamental materials properties and important engineering parameters for small-scale device applications. In addition, the effect of isotopic mass disorder on the vibrational properties of ultrahigh thermal conductivity material becomes important with significantly reduced anharmonicity and the improvements in the quality of crystal growth. In this thesis, I use ultrafast pump-probe laser based optical characterization technique to experimentally investigate anisotropic thermal transport properties of layered InSe that is recently prepared in two-dimensional (2D) structures and have distinct in-plane anisotropies. InSe is a promising candidate for next-generation electronics and optoelectronics, owing to its high electron mobility and direct optical bandgap in the few-layer limit. The knowledge of thermal transport properties is needed for engineering heat dissipation in devices. I report the room-temperature thermal conductivity of exfoliated crystals of InSe along the through-plane and in-plane directions using conventional and beam offset time domain thermoreflectance (TDTR), respectively. InSe crystals with varying thicknesses were prepared by mechanical exfoliation onto Si(100) wafers followed by immediate encapsulation with a 3-nm-thick AlOx passivation layer to prevent ambient degradation prior to coating with metal films for TDTR measurements. The measured thermal conductivity in the in-plane direction, Λin ≈ 8.5 ± 2 W/m-K, is an order of magnitude higher than that in the through-plane direction, Λthrough ≈ 0.76 ± 0.15 W/m-K, which implies a high thermal anisotropy ≈ 11 ± 3. This relatively high anisotropy and low thermal conductivity compared to other layered semiconductors imply that InSe will require unique thermal management considerations when implemented in electronic, optoelectronic, and thermoelectric applications. The second topic I investigate is the effect of isotopic disorder on the vibrational properties of cubic boron arsenide (c-BAs) at room temperature using Raman spectroscopy. Boron arsenide is at the forefront of research on ultrahigh thermal conductivity materials. I report a Raman scattering study of isotopically tailored cubic boron arsenide single crystals for 11 isotopic compositions spanning the range from nearly pure c-10BAs to nearly pure c-11BAs. The results provide insights on the effects of strong mass disorder on optical phonons and the appearance of two-mode behavior in the Raman spectra of mixed crystals, not seen before in isotopically disordered materials. Strong isotope disorder also relaxes the one-phonon Raman selection rules, resulting in disorder activated Raman scattering by acoustic and optical phonons.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Materials Science & Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Rai, Akash

Contributors dc:contributor

• Cahill, David G.

• Materials science

Statement dc:rights

• Copyright 2021 Akash Rai

Language dc:language

en, eng

Handle dc:identifier

http://hdl.handle.net/2142/113839

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F113839