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 × 10²⁵ 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 cpb and to prove the shift of the short-wavelength absorption edge with cpb. 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 freecarrier effective mass in the directions perpendicular and parallel to the trigonal axis c we determined the value of the reduced Fermi energy η 300 K for crystals with various values of cpb. Using the value of η, we calculated the dependence of the Seebeck coefficient on cpb 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 cpb is qualitatively accounted for.