Evaluation for Optical and Thermal Properties of Si-nanopillars(NPs)/SiGe Composite Film Intentionally Varied NPs Spacing

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  • ナノピラー（NP）間隔を変化させたSi-NP/SiGe複合膜の熱および光学的特性評価

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Thermoelectric (TE) devices have been attracted attention because of their importance in converting waste heat into electrical energy. Si-based materials are expected to use as low-cost TE devices. By introducing nanostructure into the TE materials, both low thermal (κ) and high electrical conductivities are required. We have succeeded to fabricate a highly periodic Si-nanopillars (NPs) structure embedded in Si0.7Ge0.3. Bio-template and Neutral-Beam Etching techniques enable us to fabricate the NPs of 10 nm diameter. In this study, we have prepared two samples with different NP spacing of 40 and 60 nm. Our preliminary 3 measurements showed that κ of 40 and 60 nm spacing samples were 2.6 and 2.8 W/mK, respectively. Then, these were 35% lower than that of SiGe bulk material. However, this method requires an electrode as thin as the composite film. This is very difficult to prepare and we should use an electrode of 10 m width. Therefore, another evaluation technique is necessary to estimate κ more accurately. Piezoelectric photothermal (PPT) measurements were carried out to study the effect of NPs spacing on the optical and thermal properties from the non-radiative recombination point of view. The observed PPT signal of the 40 nm spacing sample was 10 times smaller than that of the 60 nm spacing sample. Since the PPT signal depends on κ as well as optical absorption coefficient ( ), we carried out a theoretical calculation based on a one-dimensional heat propagation equation to estimate κ and . However, the calculated results gave us unexpectedly lower κ and higher. A detailed 3-dimensional heat propagation model including carrier diffusion of photo excited carriers should be considered.

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