Composition dependence of morphology, structure, and thermoelectric properties of FeSi 2 films prepared by sputtering de
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Composition dependence of morphology, structure, and thermoelectric properties of FeSi2 films prepared by sputtering deposition Tatsuo Tsunoda, Masakazu Mukaida, Akio Watanabe, and Yoji Imai Department of Inorganic Materials, National Institute of Materials and Chemical Research, Higashi 1-1, Tsukuba, Ibaraki 305, Japan (Received 31 October 1995; accepted 7 February 1996)
Direct b –FeSi2 film preparation from gaseous phase was examined using a radio-frequency (rf) sputtering deposition apparatus equipped with a composite target of iron and silicon. Films composed of only b –FeSi2 phase were formed at substrate temperatures above 573 K when the chemical composition of the film was very close to stoichiometric FeSi2 . The b –FeSi2 films thus formed showed rather large positive Seebeck coefficient. When the chemical composition of the films were deviated to the Fe-rich side, e –FeSi phase was formed along with b –FeSi2 . On the other hand, a –FeSi2 phase, which is stable above 1210 K in the equilibrium phase diagram, was formed at the substrate temperature as low as 723 K when the chemical composition was deviated to the Si-rich side. The formation of a –FeSi2 phase induced drastic changes in the morphology and thermoelectric properties of the films. The a –FeSi2 phase formed in the films was easily transformed to b –FeSi2 phase by a thermal treatment.
I. INTRODUCTION
b –FeSi2 has been a candidate material for a thermoelectric energy conversion since it has relatively large Seebeck coefficient and low electrical resistivity. It has advantages such as chemical stability against high temperature oxidation and abundance of its raw materials in natural resources. Therefore, various studies on the thermoelectric properties of b –FeSi2 have been carried out.1–3 However, most of these works were related to the FeSi2 doped with Co, Mn, or Al, and the properties of pure b –FeSi2 have not been investigated intensively. Powder metallurgical process and/or smelting methods have been generally used to prepare b –FeSi2 in these studies. In these high-temperature preparation processes, e –FeSi and a –FeSi2 phases were formed at first stage during formation of Fe–Si alloy above 1300 K, and a thermal annealing process for at least 24 h to transform those phases into b –FeSi2 should follow. In the case of sintering process, impurities such as additives and/or oxygen intruded into a sintered body from surface contaminants of powdered raw materials are inevitable. Effects of morphologies on the properties can hardly be investigated. Films prepared by vacuum process have an advantage from a fundamental point of view over the conventional methods above described. That is, the vacuum process provides lower contamination by impurities. Morphologies can be also controlled by changing the supersaturation of reactants. In addition, thin films of b –FeSi2 would have several advantages over the bulk material in the following respects as an energy conversion material: (i) as a thermal controlling dev
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