Electrical properties of bismuth telluride

Abstract

Measurements have been made of the electrical conductivity, thermoelectric power and quasi-adiabatic Hall and Nernst coefficients over the temperature range 100°K to 600°K on n- and p-type specimens of Bi2Te3 with a wide variation of impurity content. Zone-refined material and single crystals were used with the current flow parallel and magnetic field either perpendicular or parallel to the cleavage planes. For one n- and one p-type specimen the isothermal coefficients were measured from 100°K to 450°K. An original method has been developed for measuring the isothermal Galvano magnetic effects accurately which is simpler than A.C. methods. Values of the isothermal Nernst coefficient were used to calculate the Ettingshausen coefficient. The Righi-Leduc effect, however, does not appear to obey simple semiconductor theory. The measured coefficients exhibit the anisotropy expected from the crystal structure and their temperature variation the general behaviour expected for a semiconductor, although the Hall coefficient is anomalous in the extrinsic region. Possible scattering mechanisms have been examined, taking into account the partial degeneracy of the specimens. Over the temperature range studied acoustical mode lattice scattering is the predominant mechanism, while at lower temperatures ionized impurity scattering becomes important. Conductivity measurements yield the temperature variations of mobility, T -1.63 for electrons and T -1.94 for holes, and an energy gap of (0.21 -+ 0.02) ev at 0°K. Optical and thermal values for the gap differ and possible reasons are discussed. The Nernst coefficient agrees qualitatively with theory. The experimental results are in general agreement with previously published data on Bi2Te3. In many cases the interpretation differs from that in previous work and the differences are discussed. It is thought that the activation energy for impurities is zero, and that even in the extrinsic range a two (or more) band model must be used. Lack of knowledge about the basic parameters in Bi2Te3 makes the interpretation of the results, even in the extrinsic region, uncertain.