Optical properties of (Bi_1−xSb_x)₂Se₃ single crystals

(Bi_1−xSb_x)₂Se₃ (x = 0 to 0.2) single crystals prepared from elements of 5N purity by means of a modified Bridgman method were characterized by measurements of infrared reflectance and transmittance. Values of the plasma resonance frequency ϵ_p, optical relaxation time τ, and high-frequency permittivity ϵ_∞ were determined by fitting the reflectance spectra. It was found that the substitution of Sb atoms for Bi atoms in the Bi₂Se₃ crystal lattice leads to an increase in the ω_p values, whereas in the range of higher Sb content the ω_p values decrease. This effect is accounted for by a model of point defects in the crystal lattice of (Bi_1−xSb_x)₂Se₃. The dependence of the absorption coefficient K on the energy of incident photons was determined from the transmittance spectra. The position of the short-wavelength absorption edge is discussed. With regard to the longwavelength absorption edge it was found that the values of α in the relation K ∼ λ^α lie in the interval 2.2 to 2.5, hence the dominant scattering mechanism of the free charge carriers in (Bi_1−xSb_x)₂Se₃ crystals at room temperature is the scattering by acoustic phonons.