Bi2Te_xSe_y series studied by resistivity and thermopower

We study the detailed temperature and composition dependence of the resistivity, $\rho (T)$, and thermopower, $S(T)$, for a series of layered bismuth chalcogenides Bi${}_2$Te${}_{3-x}$Se${}_x$, and report the stoichiometry dependence of the optical band gap. In the resistivity of the most compensated member, Bi${}_2$Te${}_{2.1}$Se${}_{0.9}$, we find a low-temperature plateau whose onset temperature correlates with the high-temperature activation energy. For the whole series $S(T)$ can be described by a simple model for an extrinsic semiconductor. By substituting Se for Te, the Fermi level is tuned from the valence band into the conduction band. The maximum values of $S(T)$, bulk band gap as well the activation energy in the resistivity are found for $x \approx 0.9$.