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 PbTe_1-xS_xaloys (0.04≤x≤0.12), and its effect on the thermoelectric figure of merit ZT. The x=0.08 sample has ZT∼1.55 at 773 K. The magnetic field dependence of the high-temperature Hall resistivity of heavily p-type (>1019 cm^-3) Na-doped PbTe_1-xS_x 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 Σ-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 S2σ of p-type PbTe are optimal, e.g., 700-800 K.