Transport Phenomena in Mixed Layered Tetradymite-like Compounds in the GeTe-Bi2Te3 System

The electrical transport properties of the GeBi2Te4-, Ge3Bi2Te6-, and GeBi4Te7-layered compounds were characterized as a function of temperature and for samples with various stoichiometric deviations. The Hall coefficient and electrical resistivity were measured from 77 to 800 K, and the Seebeck coefficient was determined in the 90-450 K temperature range. The onset of intrinsic conductivity behavior was observed in these ternary compounds at fairly low temperatures. On the basis of the variation of the electrical conductivity at elevated temperatures, energy gap values of 0.24, 0.20, and 0.22 eV were calculated at 0 K for the Ge3Bi2Te6, GeBi2Te4, and GeBi4Te7 compounds, respectively. To explain the carrier mobility temperature dependence, a mixed carrier scattering mechanism on acoustic phonons and point defects is proposed at low temperatures. It is found that the contribution of scattering by acoustic phonons increases with temperature. The analysis of experimental data for the GeBi4Te7 compound in the 150-200 K temperature range demonstrated the need for an additional carrier scattering mechanism, probably connected to the order-disorder phenomena in the cation layers. An anomalous temperature dependence of the Hall coefficient was discovered for the "stoichiometric" GeBi2Te4 compound. It is explained by mixed carrier conduction effects due to fluctuations of the composition and the existence of p- and n-type conductivity domains in the GeBi2Te4 crystal.