The Effect of Ultrasmall Grain Sizes on the Thermal Conductivity of
Nanocrystalline Silicon Thin Films
Naval Research Laboratory
Wed, February 27, 2019 - 4:00 PM
Karl Herzfeld Auditorium of Hannan Hall - Rm 108
Nanocrystallization has been an effective approach to reduce thermal conductivity in thermoelectric materials. In general, the amount of reduction is highly dependent on the grain sizes as boundary scattering dominates the phonon mean-free path. In this work, we report on the thermal conductivity of nanocrystalline silicon thin films with the average grain sizes varying from 3 nm to 10 nm, prepared by plasma-enhanced chemical-vapor deposition (PECVD). The crystallinity and grain sizes are controlled by the amount of hydrogen dilution in silane gas during growth process.
Structural analysis shows that the as-grown nanocrystalline silicon is approximately 80% crystalline, including nanograins and grain boundaries. The nanograins are embedded in an amorphous matrix. Thermal conductivity was measured by the 3ω technique from 80 K to room temperature. The thermal conductivity of the film with 10 nm grain size roughly follows the minimum thermal conductivity predicted for amorphous silicon. As the grain size decreases to 3 nm, its thermal conductivity is reduced to one third of the minimum thermal conductivity of silicon. We extend the model of grain boundary scattering of phonons with non-Debye dispersion relations to explain our result of nanocrystalline silicon, strong grain size dependence of heat transport for nanocrystalline silicon. However, the similarity in thermal conductivity between amorphous and nanocrystalline silicon suggests the heat transport mechanism in both structures may not be as dissimilar as we currently understand.
Refreshments served at 3:45 PM
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