Abstract
Phonon transport in Si films was controlled using epitaxially-grown ultrasmall Ge nanodots (NDs) with ultrahigh density for the purpose of developing Si-based thermoelectric materials. The Si/Ge ND stacked structures, which were formed by the ultrathin SiO2 film technique, exhibited lower thermal conductivities than those of the conventional nanostructured SiGe bulk alloys, despite the stacked structures having a smaller Ge fraction. This came from the large thermal resistance caused by phonon scattering at the Si/Ge ND interfaces. The phonon scattering can be controlled by the Ge ND structure, which was independent of Si layer structure for carrier transport. These results demonstrate the effectiveness of ultrasmall epitaxial Ge NDs as phonon scattering sources, opening up a route for the realisation of Si-based thermoelectric materials.
Highlights
In order to independently control the phonon and carrier transports, we propose an improved nanoarchitecture where ultrasmall epitaxial nanodots (NDs) are introduced as phonon scattering sources in epitaxial Si layers related to carrier transport
Si layers/Ge NDs stacked structures were fabricated by the aforementioned process, during which epitaxial growth was confirmed by reflection high energy electron diffraction (RHEED) observations
The enlarged image of the region near the SiO2 film in Fig. 2b reveals that hemispherical Ge NDs with a dark contrast existed in the Si layer, and the NDs were on the ultrathin SiO2 films
Summary
In order to independently control the phonon and carrier transports, we propose an improved nanoarchitecture (see Fig. 1a) where ultrasmall epitaxial nanodots (NDs) are introduced as phonon scattering sources in epitaxial Si layers related to carrier transport In this structure, the size and shape of the NDs can be tuned for phonon scattering and the Si layers can be designed to have high carrier mobilities for carrier transport, independently. Ge is one of the promising candidates as a ND material because of its compatibility with conventional Si process technologies and recently we developed the formation technique of the new nanoarchitecture using the ultrathin SiO2 film technique[21] In this nanoarchitecture, ultrasmall Ge NDs were epitaxially grown with identical crystal orientations, which were completely different from the conventional nanostructured SiGe bulk alloys made by sintering.
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