Characterisation of Bi2Se3 crystals highly doped with Pb

Pb-doped Bi2Se3 crystals were prepared from starting elements Bi, Se and Pb of 5N purity in the concentration interval c(Pb) = 0 - 4 x 10(25) Pb atoms m(-3) by a modified Bridgman method. The measured values of the transmittance and reflectance were used to determine the dependence of the absorption coefficient K on the photon energy for crystals with various values of c(Pb) and to prove the shift of the short-wavelength absorption edge with c(Pb). On the basis of the assumption of the validity of the ''single valley'' model, which can describe the lowest conductivity band of Bi2Se3, and using the values of the free-carrier effective mass in the directions perpendicular and parallel to the trigonal axis c we determined the value of the reduced Fermi energy eta as 300 K for crystals with various values of c(Pb). Using the value of eta, we calculated the dependence of the Seebeck coefficient on c(Pb) and compared it with the experimentally determined values. The comparison has shown that the increasing content of Pb atoms in the Bi2Se3 lattice leads to a suppression of the role of the mechanism of scattering by ionised impurities; at higher concentrations of Pb in the crystal the mechanism of scattering of free carriers by acoustic phonons becomes dominant. Further, the ideas on the nature of the point defects in the Bi2Se3(Pb) crystals are presented and the "anomalous" dependence of the free-electron concentration on c(Pb) is qualitatively accounted for.