The Shubnikov–de Haas effect and thermoelectric properties of Tl-doped Sb$_2$Te$_3$ and Bi$_2$Se$_3$

The influence of doping with Tl on the Shubnikov–de Haas effect at $T$ = 4.2 K in magnetic fields up to 38 T in $p$-Sb$_{2-x}$Tl$_x$Te$_3$ ($x$ = 0, 0.005, 0.015, and 0.05) and $n$-Bi$_{2-x}$Tl$_x$Se$_3$ ($x$ = 0, 0.01, 0.02, 0.04, and 0.06) single crystals is investigated. Extreme cross-sections of the Fermi surface in both materials decrease upon doping with Tl: the hole concentration decreases in Sb$_{2-x}$Tl$_x$Te$_3$ due to the donor effect of Tl and the electron concentration in $n$-Bi$_{2-x}$Tl$_x$Se$_3$ decreases due to the acceptor effect of Tl. The temperature dependences of the Seebeck coefficient, electrical conductivity, thermal conductivity, and dimensionless thermoelectric figure of merit in a temperature range of 77–300 K are measured. The thermal conductivity and electrical conductivity decrease upon doping with Tl both in $p$-Sb$_{2-x}$Tl$_x$Te$_3$ and in $n$-Bi$_{2-x}$Tl$_x$Se$_3$. The Seebeck coefficient increases in all compositions upon an increase in doping over the entire measured temperature range. The thermoelectric figure of merit increases upon doping with Tl.