Features of reflection spectra of single crystals of Bi2Te3-Sb2Te3 solid solutions in the region of plasma effects

Reflectance spectra of single crystals of Bi2Te3-Sb2Te3 solid solutions containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, and 100 mol % of Sb2Te3 have been studied in the range of 400–4000 cm−1 at the temperature T = 291 K and with orientation of the vector of the electric-field strength E perpendicular to the trigonal axis of the crystal C 3 (E ⊥ C 3). The shape of the spectra is characteristic of plasma reflection; the spectra include the features in the range 1250–3000 cm−1 corresponding to the optical band gap E g opt. The features become more pronounced as the content of Bi2Te3 is increased to 80 mol % in the composition of the Bi2Te3-Sb2Te3 solid solution. A further increase in the content of Sb2Te3 is accompanied by discontinuities in the functional dependences of the parameters characterizing the plasma oscillations of free charge carriers on the solid-solution composition and also by a sharp increase in E g opt.