Sintered degenerate n-type PbTe samples with small grain sizes ranging from 0.7 to 4 μm were prepared and the effects of grain size on their thermoelectric properties were then investigated. The Seebeck coefficient of the sintered samples increased almost double when the grain size decreased from 4 to 0.7 μm. On the other hand, their electrical and thermal conductivity decreased with decreasing grain size. Accordingly, decreasing their grain size increased their thermoelectric figure-of-merit. A maximum value of the figure-of-merit of the obtained small grain-size samples was significantly higher than that of large grain-size samples with the same carrier concentration reported. This favorable result was caused mainly by the increase in the Seebeck coefficient. The influences of grain boundaries on the increase in the Seebeck coefficient were discussed. It is concluded that the Seebeck coefficient was increased by potential barrier scattering, which occurred at the grain boundaries in the sintered samples.

IV-VI diluted magnetic semiconductor Ge0.92Mn0.08Te epilayers are grown on BaF2 substrates by molecular beam epitaxy. The ferromagnetic behaviors, such as the spontaneous magnetization, the coercive field, and the Curie temperature TC, are altered by the hole concentration p. In the Ge0.92Mn0.08Te layer with high p, strong magnetic anisotropy and the temperature dependence of the magnetization expected for homogeneous ferromagnets are observed, implying that long-range ordering is induced by the holes. The maximum TC reaches 190 K for 1.57×1021 cm−3.

Thermoelectric properties of polycrystalline degenerate n-type PbTe films have been investigated in order to understand potential barrier scattering. The Seebeck coefficients of the PbTe films obtained in this study were larger than those of bulk samples having the same carrier concentrations in the temperature range from 100 to 450 K. Some of their power factors were larger than those of bulk samples having the same carrier concentrations in the temperature range from 200 to 450 K, while their electrical conductivities were smaller than those of bulk samples. From a comparison of these results with those previously reported, we concluded that potential barrier scattering occurred at grain boundaries in our films, resulting in the above favorable changes in thermoelectric properties. By analyzing their properties using the energy filtering model, we estimated the height of grain-boundary potential barriers, which probably influenced the increases in the Seebeck coefficient. We also examined the origin of the potential barriers accordingly. Consequently, we consider that the origin of the potential barriers was mainly due to point defects, probably Te vacancies, and that the barrier height may be controlled, for example, by the preparation conditions.

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We reported the thermoelectric properties of the sintered type-II clathrate K8Ba16Ga40Sn96 in a previous paper [S. Koda et al., J. Appl. Phys. 116, 023710 (2014)]. The clathrate had a high dimensionless figure of merit ZT, namely, 0.93. In this study, we optimized the carrier concentration n by modifying the chemical compositions of (K, Ba)24(Ga, Sn)136 samples, and heat treated the sintered samples. The carrier mobilities μ were improved because of the reduction in potential barrier scattering at grain boundaries. The room-temperature (RT) n values varied from 7.7 × 1017 to 3.7 × 1019 cm−3; the maximum RT μ value was 170 cm2V−1s−1. Consequently, we obtained a high ZT value of 1.19 at 630 K for n = 2.5 × 1019 cm−3. This material therefore has good thermoelectric properties.