| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Nanomaterials | Tuesday Sessions |
| Session NM-TuM |
| Session: | Nanofabrication and Nanodevices I |
| Presenter: | Yoshiaki Nakamura, Osaka University, Japan |
| Correspondent: | Click to Email |
Thermoelectric (TE) conversion has been expected to be one of the ideal energy sources, where waste heat is converted to be usable electrical energy. Therefore, TE performance enhancement had drawn much attention. TE performance is related to dimensionless figure of merit, ZT=S2σT/κ, where S is Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, and T is absolute temperature. This means that the materials with high S, σ, and low κ are required. However, high σ and low κ are difficult to achieve simultaneously because they are correlated. Therefore, the identical control of σ and κ has been a vital goal in thermoelectric study for long.
We proposed the nanoarchitecture where phonon was scattered effectively at the nanostructure interfaces, but carriers could go through them using ultrasmall epitaxial nanodots (ND) with well-controlled interfaces. Herein, carriers feel that nanomaterials are single crystal because crystals of NDs are oriented. On the other hand, the interfaces of ultrasmall NDs scatter phonon. In this study, we use our original technique for forming epitaxial ND structures. We achieve the identical control of phonon and carrier transports using the above nanoarchitecture design.
Clean Si surfaces were oxidized at 500°C at the O2 pressure of 2×10-4 Pa to form ultrathin (~0.3 nm) SiO2 films. Si or Ge was then deposited to form epitaxial Si or Ge NDs grown on Si substrates with ultrahigh density of ~1012 cm-2 as the following. At first stage of Si of Ge deposition, nanowindows were created in the ultrathin SiO2 films through the reaction of Si+SiO2→2SiO↑, or Ge+ SiO2→GeO↑+SiO↑. By subsequent deposition, Si or Ge NDs formed on the ultrahigh density nanowindows. In the case of Ge NDs, Si layers were formed on Ge NDs at 400°C. The above formation of Si NDs or Si layer/Ge NDs and the oxidization process were repeated to fabricate the ND stacked structures.
The formation of the epitaxial nanoarchitectures including NDs was succeeded. The κ values were measured by 2ω method. The smallest κ value of stacked Si NDs with 3 nm diameter was close to the amorphous value. In the case of Ge NDs, the κ values were also drastically reduced compared with bulk Si. The σ of Ge ND structures was enhanced to be a similar value to the bulk Si cases by the interface designing, indicating the independent control of κ and σ. This demonstrates the possibility of our nanostructure as a Si-based thermoelectric material with high ZT.
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.