To investigate the decrease of thermal conductivity (
) of nanoscale Si materials, we conducted the piezoelectric photothermal (PPT) method for the highly periodic Si nanopillar arrays embedded in Si0.7Ge0.3. The PPT is an electrode free method that can measure a heat propagation in the parallel to the nanopillars direction. A distinctive dip was observed in the frequency-dependent PPT signal intensity. By focusing the dip frequency,
was estimated from the comparison with the model analysis based on the one-dimensional multilayer thermal diffusion equation. The estimated
was 0.19 ± 0.07 W m–1 K, in the parallel to the nanopillars direction. Since the considerable decrease of
was confirmed from the non-radiative recombination point of view, we found the present non-destructive PPT method is very useful to estimate
in the nanostructured devices for the thermoelectric application.
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