Composition Dependence of Crystal Structures and Electrical Properties of Ca-Mg-Si Films Prepared by Sputtering
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https://doi.org/10.1007/s11664-020-08510-2 Ó 2020 The Minerals, Metals & Materials Society
Composition Dependence of Crystal Structures and Electrical Properties of Ca-Mg-Si Films Prepared by Sputtering ATSUO KATAGIRI,1 MUTSUO UEHARA,2 MAO KUROKAWA,2 KENSUKE AKIYAMA,1,3 TAKAO SHIMIZU,1 MASAAKI MATSUSHIMA,2 HIROSHI UCHIDA,4 YOSHISATO KIMURA,1 and HIROSHI FUNAKUBO1,5 1.—Department of Materials Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midoriku, Yokohama 226-8502, Japan. 2.—Department of Innovative and Engineered Materials, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midoriku, Yokohama 226-8502, Japan. 3.—Kanagawa Institute of Industrial Science and Technology, 705-1 Shimoizumi, Ebina, Kanagawa 243-0435, Japan. 4.—Department of Materials and Life Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 1028554, Japan. 5.—e-mail: [email protected]
Ca-Mg-Si ternary films were deposited at 300–335°C on (001)Al2O3 substrates by a radio-frequency magnetron sputtering. Amorphous films were obtained for a wide range of compositions but not for single-phase CaMgSi. For all compositions, the electrical conductivity of the as-deposited films increased with the increase in temperature up to 400°C. The conduction type was controlled mainly by changing the Si/(Ca + Mg + Si) ratios of the films, and films with p- and n-type conductions were observed respectively with Si/(Ca + Mg + Si) ratios below 0.6 and above 0.7 along a fixed Ca/(Ca + Mg) ratio of about 0.50. A high thermoelectric power factor above 140 lW/(m K2) with ptype conduction was obtained at 400°C for an amorphous-phase film. Key words: Thermoelectricity, silicide, thin film, sputtering method, environmentally friendly material
INTRODUCTION Thermoelectric power generation is an attractive way to reuse waste heat through the Seebeck effect.1–3 For practical applications, nontoxic and low-environmental-impact materials are essential. To satisfy these requirements, Mg2Si is a promising candidate because of its reported good figure of merit and its constituents of nontoxic Mg and Si with large Clark numbers.4–6 Hence, the improvement of the thermoelectricity of Mg2Si has been widely investigated.7–9 In terms of demand for thermoelectric devices, both p-type and n-type conductive Mg2Si materials are required. Non-doped Mg2Si is widely known to show n-type conduction as a bulk form,10,11 and ZT of n-type Mg2Si is reported to show above unity.6 ZT is a dimensionless figure of
(Received March 12, 2020; accepted September 22, 2020)
merit of thermoelectric materials12,13 calculated from the Seebeck coefficient, electrical conductivity, thermal conductivity, and measurement temperature. While p-type conduction of Mg2Si by doping was obtained in several studies, the ZT value of Mg2Si is still lower than that of the n-type one.9,14,15 To increase the thermoelectric performance of both conduction types, we tried to expand the materials system to a ternary system by introducing a third element into the Mg-Si system.16 We sel
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