Specific features of the IR reflectance and Raman spectra of Sb$_{2}$Te$_{3-x}$Se$_{x}$ crystals

A comprehensive study (X-ray phase analysis, Raman spectroscopy, and IR reflectance measurements) of $p$-Sb$_{2}$Te$_{3-x}$Se$_{x}$ (0 $\le x\le$ 0.10) crystals synthesized by the Czochralski method is carried out. The X-ray phase data and Raman spectra show that all of the alloy samples under study are rhombohedral crystals in structure, with unit-cell parameters close to those of Sb$_{2}$Te$_{3}$ crystals. At the same time, the formation of regions with different degrees of the replacement of Te atoms with Se atoms is observed. The reflectance spectra of the crystals, R ($\nu$), have a characteristic minimum at about 1000 cm$^{-1}$. The minimum is attributed to hole plasma vibrations. The plasma minimum systematically shifts to higher frequencies, as the Se content is increased. Calculation of the reflectance spectra in the context of the Drude–Lorentz model satisfactorily describes the experimental data; in this case, it is necessary to take into account also the contribution of optical crystal-lattice vibrations to spectra in the low-frequency region. The calculation makes it possible to determine the optical parameters of the crystals and to estimate the hole effective mass and the optical conductivity.