Thermoelectrics

The central challenge for silicon-based thermoelectrics is that silicon has an exceptionally high thermal conductivity, which must be reduced to raise the thermoelectric figure of merit ZT without degrading electrical conductivity. We explored several top-down nanofabrication strategies to achieve this: arrays of holes in silicon membranes can reduce thermal conductivity several times through a combination of phonon boundary scattering and coherent phonon dispersion engineering. Surface nanopillars fabricated using damageless neutral-beam etching produce similar reductions, and measurements across the 4–300 K range allowed us to separate the coherent (wave-interference) and incoherent (scattering) contributions of the pillars to thermal conductivity. Silicon fishbone nanowires — nanowires with periodic lateral wings — were also shown to reduce thermal conductivity effectively via local resonances.

A simpler and more scalable approach was demonstrated by depositing an ultrathin amorphous aluminum layer on a silicon membrane surface. This atomic-scale coating strongly suppresses phonon transport through surface scattering while barely affecting electrical conductivity, yielding a significant improvement in ZT. Surface nanocones produced by a large-area maskless process were shown to reduce thermal conductivity of silicon membranes by over 40%, offering another low-cost route to thermoelectric improvement. Together, these results establish a versatile toolkit of thermal conductivity engineering strategies for silicon-based devices.

Building on these findings, we fabricated planar thermoelectric generators that exploit nanostructured silicon membranes to convert temperature gradients into electrical power, as shown in the figure above. The project also extended to silicon-germanium alloys: a planar-type SiGe thermoelectric generator with a double cavity structure was designed and fabricated, achieving improved performance through alloy scattering and geometric optimization. The project continues in Nomura lab.