Transport phenomena in mixed layered tetradymite-like compounds in the GeTe-Bi2Te3 system

The electrical transport properties of the GeBi2Te4-, Ge3Bi2Te6-, and GeBi4 Te7-layered compounds were characterized as a function of temperature and f or samples with various stoichiometric deviations. The Hall coefficient and electrical resistivity were measured from 77 to 800 K, and the Seebeck coe fficient was determined in the 90-450 K temperature range, The onset of int rinsic conductivity behavior was observed in these ternary compounds at fai rly low temperatures. On the basis of the variation of the electrical condu ctivity 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 compou nds, 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 scatt ering by acoustic phonons increases with temperature. The analysis of exper imental data for the GeBi4T7 compound in the 150-200 K temperature range de monstrated the need for an additional carrier scattering mechanism, probabl y connected to the order-disorder phenomena in the cation layers. An anomal ous temperature dependence of the Hall coefficient was discovered for the " stoichiometric" GeBi2Te4 compound. It is explained by mixed carrier conduct ion effects due to fluctuations of the composition and the existence of p- and n-type conductivity domains in the GeBi2Te4 crystal. (C) 1999 Academic Press.