Nanowire Thermoelectrics

Phonon transport is expected to be greatly impeded in thin (i.e., d<Λ, where d is the diameter and Λ is the phonon mean free path) 1D nanostructures as a result of increased boundary scattering and reduced phonon group velocities stemming from phonon confinement. Detailed models of phonon heat conduction in cylindrical semiconducting nanowires that consider modified dispersion relations and all important scattering processes predict a large decrease in the lattice thermal conductivity of wires tens of nanometers in diameter. Size-dependent thermal conductivity in nanostructures presents a major hurdle in the drive toward miniaturization in the semiconductor industry. Yet poor heat transport is advantageous for thermoelectric materials, which are characterized by a figure of merit (ZT = α2T/[ρ(κp + κe)], with α, T, ρ, κp and κe the Seebeck coefficient, absolute temperature, electronic resistivity, lattice thermal conductivity and electronic thermal conductivity, respectively) that improves as phonon transport worsens. A decade ago, the Dresselhaus group predicted that ZT can be increased above bulk values in thin nanowires by carefully tailoring their diameters, compositions and carrier concentrations.  Recent work in our laboratories has focused on understanding the thermal transport properties of pure silicon and Si/SiGe superlattice nanowires as the first step in the experimental verification of enhanced ZT values in these complex nanostructures.


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