Design and evaluation of n-type Si 1-x Ge x as a thermoelectric-conversion material
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1044-U06-04
Design and evaluation of n-type Si1-xGex as a thermoelectric-conversion material Tomohiro Imai1, Tsutomu Iida2, Yuki Miyata2, Takashi Itoh2, Hiroki Funashima1, Yoshifumi Takanashi2, and Noriaki Hamada1 1 Department of Physics, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba, 2788510, Japan 2 Department of Materials Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba, 278-8510, Japan ABSTRACT Si1-xGex alloys show the high power generating efficiency as a thermoelectricconversion material. We evaluate the thermoelectric power of the n-type SiGe system on the basis of the first principles calculations. The electronic-band-structure calculation is performed using all-electron full-potential linearized augmented-plane-wave (FLAPW) method within the local density approximation (LDA). The Seebeck coefficient is analyzed by the BlochBoltzmann equation. We find that the ordered rhombohedral SiGe has high Seebeck coefficient in comparison with zincblende SiGe. The efficiency of the thermoelectric power in Si1-xGex is gained by the local atomic configuration rather than the Ge concentration. INTRODUCTION Alloys of silicon and germanium (Si1-xGex) are ecologically friendly semiconductors and important materials, not only for microelectronic devices, but also for solid-state power generators such as solar cells and thermoelectric devices. This is mainly due to their chemical stability, mechanical strength at elevated temperatures, and a close match of the n-/p-type alloys which enables better device operation. Since Si1-xGex with x~0.3 to 0.8 can minimize the thermal conductivity due to the random ordering of the constituent atoms in the crystal[1,2], Si1-xGex alloys are more advantageous than simple Si and Ge for the thermoelectric figure of merit. Regarding the valence bands, Si1-xGex has the band edge composed of the almost triply degenerate bands at Γ point. This band edge hardly changes with Ge concentration x ; therefore, the Seebeck coefficient of the p-type Si1-xGex is not intimately related to x . This paper describes the investigation of n-type SiGe thermoelectric power based on the first principles calculations, and we have designed the optimum SiGe as a thermoelectric-conversion material from the viewpoint of Computational Materials Design (CMD). The first-principles calculations were carried out by using All-electron Band-structure CAlculation Package (ABCAP), in which the full-potential linearized augmented-plane-wave (FLAPW) method is employed within the local density approximation (LDA). The thermoelectric power has been evaluated on the basis of the Bloch-Boltzmann theory. BAND-STRUCTURE CALCULATION SCHEME The FLAPW method is employed within the LDA. The analytical form of Vosko, Wilk and Nusair is used for the LDA exchanged-correlation energy[3,4]. The linearization[5] for the APW method is done by the scheme of Takeda and Kübler[6]. The mauffin-tin(MT) radii are 2.0 a.u. for Si and 1.8 a.u. for Ge. The energy cutoff parameters are 6Hr for the wave functi
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