Phonon and electron transport through interfaces and disordered structures
Author(s)
Song, Qichen
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Advisor
Chen, Gang
Henry, Asegun
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Understanding phonon and electron transport is of great significance for designing efficient solid-state devices such as transistors, laser diodes and thermoelectric energy converters. The structural randomness is inevitable in solid-state devices, and it is often regarded as the undesirable scattering source for phonons and electrons. This thesis studies manipulating phonon and electron flow using structural randomness via mode-resolved Green’s function calculations and pump-probe optical characterizations.
Interface roughness is a common type of randomness in heterostructures, which strongly affects electron and phonon transport across interfaces. We find that atomically rough interfaces can scatter short-wavelength electrons and assist the transmission between mismatched valleys. The contact resistance is reduced by over an order of magnitude. Our study provides new insights on the conventional wisdom to improve the interfacial transport using graded interfaces. We also use the atomistic Green’s function to simulate phonon transport across rough interfaces to show that the basic assumption that phonons lose memories in the often-used diffuse phonon scattering model is questionable.
The coherent backscattering of waves in disordered structures can lead to Anderson localization, where the waves are spatially localized and cannot propagate. Anderson localization has been observed in electronic, photonic and acoustic systems. However, observing its impact on heat conduction is challenging due to the broadband nature and three-dimensional transport of phonons. We use the aperiodicity as a type of randomness to enhance phonon Anderson localization. Our calculation predicts that aperiodic Si/Si0.2Ge0.8 superlattice can induce coherent backscattering for low-frequency phonons and limit the contribution to transport of high-frequency phonons. The interferences among scattered low-frequency phonons lead to a peak in the thermal conductivity versus length curve, a characteristic feature of phonon Anderson localization. Using frequency-domain thermoreflectance, we validate our theoretical predictions and find that the phonon Anderson localization exists up to 200 K. Our findings provide an efficient approach to localize phonons at moderate temperatures using randomness.
Date issued
2022-02Department
Massachusetts Institute of Technology. Department of Mechanical EngineeringPublisher
Massachusetts Institute of Technology