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A5: Nanoscale and low dimensional effects

  Stacking structures of epitaxial Si nanodots and their thermal conductivity

Yoshiaki Nakamura1, 2, *, Masayuki Isogawa, Tomohiro Ueda1, Jun Kikkawa1, and Akira Sakai1
1 Osaka University, 2PRESTO-JST, *Corresponding author: Y. Nakamura

 Recently, nanostructures have been intensively studied for interest in enhancement of thermoelectric performance. We have been focusing on stacking structures of Si nanodots (NDs) with identical crystal orientation for thermoelectrical materials using our original ultrathin SiO2 film technique. In this paper, we develop the formation technique of the stacked Si ND structures using ultrathin SiO2 film technique [1-2] and investigate the ND interface effect on thermoelectric properties.

 The ultrathin SiO2 film technique is written everywhere as follows [1]. Clean Si surfaces were oxidized at 500°C for 10 min at the O2 pressure of 2×10-4 Pa to form ultrathin (~0.3 nm) SiO2 films. Si was then deposited to form ultrahigh density (>1012 cm-2) Si NDs on Si substrate. Si NDs were epitaxially grown on Si substrates through nanowindows in the ultrathin SiO2 films which were formed at the first stage of Si deposition. The above Si ND formation process and oxidization process were repeated to form the stacking structure of Si NDs.

 High resolution transmission electron microscope observation showed that the Si NDs were stacked. We confirmed that NDs were epitaxially grown on Si substrates. In the case of small Si NDs, thermal conductivity was drastically reduced down to about 1 W/mK. The thermal conductivity of Si can be controlled by tuning ND size. The detailed analysis indicated that the ND interface played an important role of thermal resistance increase. This demonstrates that our proposed nanostructure can be a good thermoelectric material composed only of ubiquitous elements. We will talk about the thermal conductivity related to ND structure.

 This work was supported by PRESTO-JST program. It was also supported by a Grant-in-Aid for Scientific Research B (25286026) and a Grant-in-Aid for Exploratory Research (25600016), and by the Tepco Memorial Foundation Research Grant (Basic Research).

[1] Y. Nakamura, Phys. Rev. B 72, 075404 (2005). [2]Y. Nakamura, et al., Appl. Phys. Lett. 87, 133119 (2005).