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-xSₓ alloys (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-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 {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.