Thermoelectric properties of polycrystalline Si1-xGex grown by die-casting vertical Bridgman growth technique
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Thermoelectric properties of polycrystalline Si1-xGex grown by die-casting vertical Bridgman growth technique Takashi Baba, Tsutomu Iida, Hisashi Hirahara, Takashi Itoh, Masayasu Akasaka, and Yoshifumi Takanashi Department of Materials Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan ABSTRACT The die-casting growth process combined with an advanced version of the Bridgman method was employed for manufacturing the multicrystalline bulk crystal of Si1-xGex. This process provides a form of phase transformation which is completely different from that predicted by the Si-Ge phase diagram. By combining this growth with subsequent heat treatment of the precipitated sample, the variation in the germanium content obtained was within ± 4 % for Si0.65Ge0.35 sample with a carrier concentration in the mid-1018 cm-3. The power factor obtained exhibited a quite flat characteristic over the temperature range of room temperature to 800 K. However, there was a drop in the Seebeck coefficient at about 800 K, which corresponded to a rise in the electrical conductivity. The value of the thermal conductivity was about 0.04 W/cmK at temperatures ranging from 600 to 900 K. The maximum value of the figure of merit obtained for the grown Si0.65Ge0.35 sample was 0.19 at 773 K. 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 in terms of their thermal and electrical characteristics, which enable better device operation with no noticeable variation in efficiency. For thermal-to-electric energy-conversion, Si1-xGex with x~0.2 to 0.8 can minimize the thermal conductivity due to the random ordering of the constitutent atoms in the crystal.[1,2] However, since the Si-Ge system exhibits a complete series of solid solutions with a phase relationship, it is not easy to selectively precipitate crystals possessing a particular composition of silicon or germanium using conventional Bridgman or Czochralski methods.[3-8] As a candidate for the energy conversion material, it is necessary that the materials should possess the advantages of abundance of their constituent elements in the earth’s crust and non-toxicity of their processing by-products. This requirement can be met by a combination of Si1-xGex in a die-casting growth process. To achieve a bulk crystal growth of Si1-xGex, we applied a die-casting growth technique, combined with an advanced version of the Bridgman method. The grown samples were coin-shaped Si1-xGex polycrystalline crystals. By combining this growth with subsequent heat treatment of the precipitated samples, Si1-xGex with x= 0.35 was obtained. Regarding the concentration variation in the grown Si1-xGex , the deviation in the germanium con
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