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A5: Nanoscale and low dimensional effects
Nanostructuring of thermoelectric (TE) materials has attracted much attention because of TE performance enhancement. Recently, we proposed that Si films including epitaxial Ge nanodots (NDs) were a high performance TE material. Ge NDs as phonon scatterers effectively contribute to reduction of thermal conductivity k. Epitaxial growth of Ge NDs is expected to maintain bulk-like electrical conductivity s. We have reported the fabrication technique of the above structure using ultrathin SiO2 film technique [1] and measured their k [2]. In this study, we investigate the thermal conductivity and electrical properties in Si films including epitaxial Ge NDs.
Clean Si(001) surfaces prepared in an ultrahigh vacuum chamber were oxidized to form ultrathin SiO2 films with one monolayer thickness. Ge was deposited on the ultrathin SiO2 films to form epitaxial Ge NDs with an ultrahigh density (>1012 cm-2) [1]. Si layers were deposited on the Ge NDs, and then oxidized to form ultrathin SiO2 films. These processes were repeated to fabricate Si films including epitaxial Ge ND. Phosphorous doping was carried out by implantation (<1×1015 cm-2). The k was measured by 2w method.
For ~90-nm Si films with 10-nm size Ge NDs, the k value were reduced down to ~1.2 W/mK, which is lower than the reported values in SiGe alloy nanocomposite systems [3]. The reduction was considered to come from the thermal resistance per Si/Ge/Si (SiO2) interface, Rth. The Rth value strongly depended on the Ge NDs size. On the other hand, suppression of s degradation was confirmed. These results demonstrated that Ge NDs prevented mainly phonon propagation, while the influence on electron transport was relatively small.
[1] Y. Nakamura et al., Appl. Phys. Lett., 87, (2005) 133119.
[2] S. Yamasaka et al., ICT2013, Kobe, Japan, July 1, 2013.
[3] G. Joshi et al., Nano Lett. 8, (2008) 4670.