Valence-band structure of highly efficient p-type thermoelectric PbTe-PbS alloys

Experimental evidence is given relevant to the temperature dependence of the valence band structure of PbTe and PbTe1-xSx alloys ( 0.04 <= x <= 0.12), and its effect on the thermoelectric figure of merit ZT. The x = 0.08 sample has ZT similar to 1.55 at 773 K. The magnetic field dependence of the high-temperature Hall resistivity of heavily p-type (>10(19) cm(-3)) Na-doped PbTe1-xSx reveals the presence of high-mobility electrons. This casts doubts on prior analyses of the Hall coefficient suggesting that temperature induces a rapid rise in energy of the "heavy" hole relative to the "light" hole bands. The electron-like behavior is likely induced by the topology of the Fermi surface when the L- and Sigma-bands merge. Negative values for the low-temperature thermopower are also observed. The data show that PbTe continues to be a direct-gap semiconductor at temperatures where the ZT and S-2 sigma of p-type PbTe are optimal, e.g., 700-800 K. DOI: 10.1103/PhysRevB.87.045203