Composition dependence of morphology, structure, and thermoelectric properties of FeSi2 films prepared by sputtering deposition

Direct beta-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 beta-FeSi2 phase were formed at substrate temperatures above 573 K when the chemical composition of the film was very close to stoichiometric FeSi2. The beta-FeSi2 films thus formed showed rather large positive Seebeck coefficient. When the chemical composition of the films were deviated to the Fe-rich side, epsilon-FeSi phase was formed along with beta-FeSi2. On the other hand, alpha-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 alpha-FeSi2 phase induced drastic changes in the morphology and thermoelectric properties of the films. The alpha-FeSi2 phase formed in the films was easily transformed to beta-FeSi2 phase by a thermal treatment.