Effects of doping on the transport properties of CoSb3 have been systematically investigated using Ni, Pd, and Pt as donor impurities. It is shown that the Hall mobility, the Seebeck coefficient, and the electrical conductivity depend strongly not only on the carrier concentration but also on these donor impurities. Our theoretical analysis suggests that the electron effective mass and the conduction band deformation potential are significantly affected by both the doping levels and the donor impurities. These doping effects in CoSb3 can be attributed to (1) the changes in the electronic structure with doping and (2) the specific nature of the conduction band structure, in particular, the nonparabolicity of the band which can be explained in terms of a two-band Kane model. The observed changes in the electronic properties with doping are also consistent with the predictions of a recent band structure calculation of CoSb3. On the other hand, the lattice thermal conductivity decreases markedly with increasing carrier concentration, and is almost independent of the donor impurities. Our analysis based on the Debye model indicates that the coupling of the point-defect (alloy) scattering with the electron-phonon scattering plays an important role in reducing the lattice thermal conductivity in heavily doped n-type CoSb3. The effects of doping on the phonon scattering are also discussed on the basis of a model calculation as a function of the electronic properties and the impurity properties (atomic mass and size). As a result, it is found that the strength of the electron-lattice interaction (the electron-phonon coupling), which is closely related to the effective mass and the deformation potential, is an important factor affecting the scattering of phonons as well as charge carriers in heavily doped n-type CoSb3.

The structural and electronic transport properties of polycrystalline p-type CoSb3 with different grain sizes (about 3 and 3×102 μm) were investigated. The magnetic susceptibility was also measured. Samples were characterized by x-ray diffractometry, electron-probe microanalysis, and optical microscope observation. Samples were found to be stoichiometric and homogeneous. The Hall carrier concentration of the samples is of the order of 1018 cm−3 and weakly dependent on the temperature. The temperature dependence of the Hall mobility suggests that the predominant scattering mechanism drastically changes depending on grain size: for large grain size a combination of the neutral impurity scattering and the acoustic phonon scattering, and for small grain size the ionized impurity scattering. The magnetic susceptibility was found to be essentially diamagnetic independently of grain size, and to vary slightly with temperature. The weak temperature dependence of the susceptibility can be explained by taking into account the three contributions of ion cores, conduction electrons, and trace amounts of magnetic impurities. From the analysis of the susceptibility due to conduction electrons, the band gap energy was determined to be about 70–80 meV, consistent with a recent band structure calculation. Although the effects of nonmagnetic impurity phases segregated (Sb, etc.) on the scattering mechanism are not clear, the grain size is one of the key factors determining the transport properties of polycrystalline CoSb3.

The thin film growth of CoSb, on a semi-insulating GaAs(100) substrate was made by using magnetron If-sputtering, and the electrical and thermoelectric properties of the films were studied with relation to annealing temperature and film thickness. Polycrystalline films with the skutterudite structure were successfully grown on the GaAs(100) substrate. The obtained films were found to be p-type, and their hole mobility, electrical conductivity, and Seebeck coefficient significantly changed depending on the annealing temperature and the thickness. A huge Seebeck coefficient of 600 pV/K, which is about three times as large as the value of ap-type single crystal, was obtained for a thin film annealed at 750 "C with thickness of 71 nm, and the power factor reached the value of 2X 10" W/cmK2. The variation in the thermoelectric properties with the annealing temperature and the thickness can be explained well in terms of a model which takes into account a carrier energy filtering effect by potential barriers at grain boundaries. According to the model, the Seebeck coefficient increases with increasing potential barrier height and also decreases with increasing carrier concentration.

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