(Invited) Nanostructure Design for Control of Phonon and Electron Transports
Authors: Nakamura Yoshiaki , Watanabe Kentaro 
Published: February 2020

Thermoelectric materials are required to high electrical conductivity and low thermal conductivity for high performance. The independent control of electrons and phonons is difficult due to their correlation resulting in the difficulty of the enhancement of thermoelectric performance. Recently, nanostructures have drawn much attention in thermoelectric material study. One of the mechanisms of its enhancement is the thermal conductivity reduction due to the phonon scattering at the interfaces introduced by nanostructuring [1]. In some case, however, its interfaces can scatter the carriers leading to the reduction of the carrier conductivity.
In this study, we are proposing the novel nanostructure for independent control of electrons and phonons using nanodots. The nanostructure is ultra-small epitaxial Ge nanodots are embedded in Si layers. It is expected that carrier transports in Si layers with high electrical conductivity and high Seebeck coefficient, while phonons are scattered at the interfaces between nanodots and Si layers [2,3].
Ultra-small Ge nanodots in this nanostructure were grown by our original technique: ultrathin SiO2 film technique. First, we formed the ultrathin SiO2 films with one monolayer thickness by oxidizing clean Si surfaces at~500ºC after introducing the Si substrates into molecular beam epitaxy chamber at the base pressure of ~10-8 Pa. We deposited Ge on the ultrathin SiO2 film to form Ge nanodtos. At the first stage, of Ge deposition the reaction of Ge+SiO2 →GeO↑+SiO↑ occurred to form nanowindows in the ultrathin SiO2 films. Subsequently deposited Ge atoms were trapped at the nanowindows, which worked as nucleation sites. Then, ultras-mall nanodots were formed. The nanodots contacted with Si substrates through nanowindows leading to the epitaxial growth. Si was deposited on the epitaxial Ge nanodots to form Si layers. These formations of the ultrathin SiO2 films, epitaxial Ge nanodots, and epitaxial Si layers were repeated to form the nanostructure that is Si films including Ge nanodots. The thermal conductivity measurements of the nanostructure showed the drastic reduction of thermal conductivity. Hall effect measurements exhibited a similar high electrical conductivity to that of bulk Si. Electrons transports through the interfaces easily by band engineering. On the other hand, ultras-mall nanodots work as phonon scatterers leading to the drastic reduction of thermal conductivity. This phonon scattering is consistent with wave scattering theory. These results demonstrated nanostructure design brought to the independent control of carrier and phonon transport using the Si layers with ultrasmall epitaxial Ge nanodots This work was partially supported by JSPS KAKENHI Grant Number 16H02078 for Scientific Research (A) and 15K13276 for Challenging Exploratory Research. In part, this work was supported by CREST-JST program.
Reference [1] Y. Nakamura, et al., Nano Energy 12, 845 (2015). [2] S. Yamasaka, et al., Sci. Rep. 5, 14490 (2015). [3] S. Yamasaka, et al., Sci. Rep. 6, 22838 (2016).