Dr Felix Hofmann, Department of Engineering Science, Oxford University

Coherent phonon lifetime measurements in GaAs-AlAs superlattices and Si membranes
When Jan 21, 2013
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865 283302
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Phonons play a central role in determining the thermal conductivity of nano-structured semiconductor materials. Their lifetime is of particular interest for thermoelectric devices where thermal conductivity must be minimised for best performance. Characterising the interactions of phonons with material defects and interfaces is a substantial challenge. Indeed it is not possible to extract details of these interactions through thermal transport measurements that yield information integrated over the entire phonon spectrum. A way forward is provided by coherent phonon measurements that allow one to examine behaviour at specific frequencies.
In this seminar I will present results from recent sub-THz phonon lifetime measurements in GaAs/AlAs superlattices. Here, superlattices are used as a nano-structure material model system. Using measurements at different temperatures the effects of both intrinsic (i.e. lattice anharmonicity) and extrinsic (i.e. defect) scattering mechanisms on phonon lifetime can be separated. A transition from intrinsically dominated to extrinsically dominated behaviour occurs at sub-THz frequencies. This agrees with predictions based on lattice dynamics calculations. Extrinsic scattering appears to depend strongly on defect correlation lengths in the sample. This strong dependence of phonon lifetime on interface quality is further illustrated by lifetime measurements of higher order thickness resonances in Si membrane samples.