H. Anno

Effects of doping on the transport properties of CoSb3

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 increasi...

Structural and electronic transport properties of polycrystalline p-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 ...

Electronic transport properties of the Skutterudite CoSb 3 and mixed alloys

We present some results on physical and electrical properties of the Skutterudites CoSb/sub 3/ and mixed alloys. Most of samples measured were polycrystalline, which have been prepared by hot-pressing of powders under 10/sup 7/ Pa at /spl sim/650/spl deg/C and /spl sim/750/spl deg/C, and a few measurements on CoSb/sub 3/ single crystals were performed. We report some results of the Hall mobility, Seebeck coefficient and electrical conductivity measurements on CoSb/sub 3/ and mixed alloys. Based on the experimental results, we make clear important factors which have influence on the electronic transport properties, in particular, on the Hall mobility of the materials.

Electron tunneling experiments on skutterudite Co1−xFexSb3 semiconductors

Electron tunneling experiments were performed on p-Co1−xFexSb3–Al-oxide–Al junctions for x=0 and x=0.1 at 4.2 K. A U-shaped tunneling conductance curve obtained for polycrystalline p-CoSb3 clearly shows an energy-band gap of ∼50 meV. For p-Co0.9Fe0.1Sb3 skutterudites, a strong zero-bias conductance anomaly is observed. This anomaly may be due to a structural disorder arising from defects such as vacancies and interstitial Fe atoms.

Phonon behaviors and electronic structures of the filled skutterudite YbyCo4Sb12 compounds: An electron tunneling study

Electron tunneling experiments were performed on YbyCo4Sb12–Al oxide–Al junctions for y=0–0.25 at 4.2 K. In the second derivative tunneling spectrum of CoSb3 compound (y=0), three peaks were observed at around 5, 20, and 33 mV, which are closely related to an optical phonon mode with a rigid rectangle, Sb–Sb bond bending and bond stretching, and a large Co atomic motion, respectively. Appearance of the strong peak at 7 mV observed in Yb-filled samples corresponds to a rattled phonon mode of Yb ions. The peak energy due to the Sb–Sb bonds is unchanged, whereas the one due to Co motions shifts to lower with increasing Yb concentration. This fact indicates that the filled Yb ions strongly interact to the host framework Co atoms, which were clearly observed in the change of tunneling conductance.

Preparation and thermoelectric properties of CoSb/sub 3/ thin films on GaAs(100) substrate

The thin film growth of CoSb/sub 3/ on a semi-insulating GaAs(100) substrate was made by using magnetron rf-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 /spl mu/V/K, which is about three times as large as the value of a p-type single crystal, was obtained for a thin film annealed at 750/spl deg/C with thickness of 71 nm, and the power factor reached the value of 2/spl times/10/sup -4/ W/cm K/sup 2/. 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.

Effects of doping Pd/Pt in CoSb/sub 3/ crystals on electrical and thermoelectric properties

Double doping of palladium and platinum is Skutterudite CoSb/sub 3/ has been studied to improve thermoelectric properties. It was found that the solubility limit of Pd and Pt in CoSb/sub 3/ crystal amounted to about 10 at% and substitution of Pd and Pt for Co occurred. The carrier concentration increased to about 5/spl times/10/sup 20/ cm/sup -3/ due to a higher degree of solutioning of Pd and Pt by double doping than that of Pd or Pt by single doping. Electrical conductivity of /spl sigma/=1950 S/cm at room temperature could be obtained for the double doping of 5 at% Pd and 5 at% Pt. Heavy doping by Pd and Pi was found to significantly reduce the lattice thermal conductivity, leading to the decrease in thermal conductivity from 0.1 (non-doped crystal) to 0.05 W/cm K. As a result, the figure of merit value Z was about 1.2/spl times/10/sup -3//K at temperatures between 500 and 800 K, and the maximum ZT=0.9 was obtained at 750 K.

Thermoelectric properties of Co(M x Sb/sub 1-x/) 3 (M=Ge, Sn, Pb) compounds

The thermoelectric properties of CoSb/sub 3/, which was doped with IV elements of Sn, Ge and Pb, were investigated in order to improve the p-type thermoelectric performance of non-doped CoSb/sub 3/. Co(M/sub x/Sb/sub 1-x/)/sub 3/ (M=Ge, Sn, Pb) compounds were prepared by hot-pressing. The solubility limit of these dopants was determined from measurements of the X-ray photoelectron spectroscopy (XPS). The solubility limit of Ge and Sn are estimated to be about 1.4 and 2.4%, respectively, while Pb hardly substitutes for Sb. The Hall measurements reveal that the maximum carrier concentrations at room temperature were 1.5/spl times/10/sup 19/ cm/sup -3/ for Ge and 2.9/spl times/10/sup 19/ cm/sup -3/ for Sn. The change in the mobility with x can be explained qualitatively by taking account of conduction of both holes and electrons in doped CoSb/sub 3/. The electrical conductivity /spl sigma/ and Seebeck coefficient S of these samples were measured in the temperature range from 300 to 800 K. The p-type conduction is achieved in the whole temperature range by introducing Ge and Sn in CoSb/sub 3/. The values of the power factor S/sup 2//spl sigma/ for Co(Sn/sub x/Sb/sub 1-x/)/sub 3/ with x=1% are of the order of 10/sup -5/W/cmK/sup 2/ and 2/spl sim/10 times as high as those of the non-doped CoSb/sub 3/ in the wide temperature range.

Thermoelectric properties of rf-sputtered CoSb 3 films

Film growth and electronic transport properties of CoSb/sub 3/ with the cubic skutterudite structure were studied. The skutterudite CoSb/sub 3/ films were successfully grown on Si(100) and quartz substrates by using a magnetron rf-sputtering. A preferential orientation growth along [310] plane occurs when the pressure of sputtering gas decreases. Polycrystalline films with the skutterudite structure are also obtained by annealing sputtered films at low temperature. Electrical conductivity of the films changes with the film growth conditions. Electronic structure of the films was investigated by X-ray photoelectron spectroscopy. Observed valence band structure agrees well with a recent band calculation. It is revealed that the valence band structure of CoSb, consists of the hybridized bands between Co d and Sb p states and a peak structure derived from localized Co d state.

The Effect of Magnetic Atom Substitution on the Tunneling Conductance in Skutterudite CoSb3 Semiconductors

Electron tunneling experiments were performed on n-Co 0.9 Ni 0.1 Sb 3 -Al oxide-Al junctions measured at 4.2 K. A V-shape tunneling conductance curve with small zero bias offset is observed, which can be associated with disorder states. Comparing with the results on p-Co 0.9 Fe 0.1 Sb 3 samples, it has been clarified that the Fe substitution leads to a stronger three-dimensional disorder state as compared to the case of Ni substitution. Magnetic susceptibility measurements suggest that the observed three-dimensional disorder state is mainly magnetic in nature.