Optical absorption and Raman scattering in doped crystals TlGaSe$_2$ and TlInS$_2$

Background and Objectives. The TlGaSe$_2$ and TlInS$_2$ monocrystals are the $А ^{3}$ $В^{3}$ $С^{6}_2$ type ternary semiconductor compounds. Studies of TlGaSe$_2$ and TlInS$_2$ materials are carried out quite intensively due to the uniqueness of their optical and electrophysical properties and significant prospects for their practical use. At the same time, information about the effect of specific doping impurities on the properties of such materials is insufficient and often contradictory. Thus, the purpose of this paper was to establish the influence of doping Ag, Al, B, Er, Fe, Nd, Tb impurities in concentrations from 0.1 to 3% on the optical absorption and Raman scattering of light in the TlGaSe$_2$ and TlInS$_2$ crystals. Materials and Methods: Crystals of semiconductor ferroelectrics TlGaSe$_2$ and TlInS$_2$, doped with Ag, Al, B, Er, Fe, Nd, Tb at concentrations from 0.1 to 3%, were studied to establish the features of the impurities doping influence on the optical absorption and Raman scattering in the tested crystals. The absorption measurements were carried out by measuring the light transmission and reflection spectra and next calculating the band gap by the Tauc method. Raman spectra were measured with the confocal spectrometer. Solid-state lasers operating at wavelengths of 473 and 532 nm were used as an excitation source. The size of the light spot on the sample was 1 $\mu$m. The backscattered light was detected by a deeply cooled silicon CCD array with a resolution of at least 1 cm$^{-1}$. Results: The measurements of transmittance and reflection spectra of the TlGaSe$_2$ and TlInS$_2$ samples and the calculation of the band gap Eg have shown that when the crystals were the doped with the impurities indicated quantities, the values of Eg, known from the literature data for undoped materials, were retained. These results are confirmed by the Raman scattering data in the temperature range 21.5-300 K.