Optical parameters of anisotropic chain-structured Tl2InGaTe4 crystals by spectroscopic ellipsometry

Spectroscopic ellipsometry measurements were performed on Tl2InGaTe4 single crystals in the 1.2-6.2 eV range for orientations of electric field, parallel (E//c) and perpendicular (E⊥c) to optic axis c. Spectral dependence of optical parameters; real and imaginary components of the dielectric function, refractive index and extinction coefficient were obtained from the analyses of experimental data using an ambient-substrate optical model. The analysis of the absorption data calculated using the extinction coefficient showed the existence of indirect transitions in the crystal with an energy band gap of ∼0.72 eV for both orientations. Interband transition (critical point) energies were revealed using second-energy derivative spectra of the dielectric function. The results showed the presence of five each interband transition structures for E//c and E⊥c orientations.