Thermoelectric properties of beta-FeSi2 single crystals and polycrystalline beta-FeSi2+x thin films

The resistivity and thermoelectric power of beta -FeSi2 single crystals and polycrystalline beta -FeSi2+x thin films doped with 3d transition metals, have been investigated for dependence on the deviation from strict stoichio metry and on the purity of the source material. It has been found that in s ingle crystals, the thermoelectric power is characterized by a large phonon drag effect which is more pronounced in samples grown with the source mate rial of higher purity, and by large absolute values of > 500 muV/K in a bro ad temperature range. The values of the resistivity prepared at the Si-rich and Fe-rich phase boundaries do not show any correlation with the deviatio n from strict stoichiometry, but its temperature dependence rho (T) is cont rolled by native defects. Polycrystalline beta -FeSi2+x thin films doped wi th Co and Cr, and prepared by electron beam evaporation and magnetron sputt ering, were found to have maximum power factors outside the homogeneity reg ion at approximately x = 0.15 +/- 0.05. Their thermoelectric power remains below that of single crystals. (C) 2001 Elsevier Science B.V. All rights re served.