The Hall and Seebeck Effects in Nonstoichiometric Bismuth Telluride

Detailed measurements of electrical conductivity (σ), Hall coefficient (R H ), and Seebeck coefficient (α) were made on seven bismuth telluride samples of composition Bi 2 Te 3+x , with x ranging from −0.06 to +0.48. Above room temperature the slope of R H T 3/2 gave an apparent energy gap about 50% higher than that from the σ slope, and the Hall mobility decreased steeply with a T −4 dependence. These effects could be explained by an electron-to-hole mobility ratio (b) near unity, but the magnitude of α at higher temperatures appeared to require a b value of about 2. At the highest temperature the Hall mobility was found to increase with increasing tellurium content. The electron effective mass (m n ) was found to be less than that of the holes (m p ), at least below room temperature. Both m n and m p increased with extrinsic carrier concentration, and to some extent with temperature. These facts may be due to a stronger warping of the valence band at higher energies than the conduction band. The Hall mobility below room temperature could be explained by acoustic lattice scattering and degeneracy. At the Seebeck coefficient maxima, the value of αT was found to be roughly constant. The temperature slopes of α below the maxima were generally greater than expected classically. Anomalies were found in R H , R H σ, and α above room temperature in a near intrinsic n-type sample.