Impurity band in SnBi4Se7:: thermoelectric power and electrical resistivity measurements

Thermoelectric power and electrical resistivity measurements on polycrystalline samples of Bi2Se3 and stoichiometric ternary compound in the quasi-binary system SnSe-Bi2Se3 in the temperature range of 90-420 K are presented and explained assuming the existence of an impurity band. The variation of the electron concentration with temperature above 300 K is explained in terms of the thermal activation of a shallow donor, by using a single conduction band model. The density of states effective mass m* = 0.15m(0) of the electrons, the activation energy of the donors, their concentration, and the compensation ratio are estimated. The temperature dependence of the electron mobility in conduction band is analyzed by taking into account the scattering of the charge carriers by acoustic phonon, optical phonon, and polar optical phonon as well as by alloy and ionized impurity modes. On the other hand, by considering the two-band model with electrons in both the conduction and impurity bands, the change in the electrical resistivity with temperature between 420 and 90 K is explained.