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Coexistence of Indirect and Direct Optical Transitions, Refractive Indices, and Oscillator Parameters in TlGaS2, TlGaSe2, and TlInS2 Layered Single Crystals

The optical properties of TlGaS2; TlGaSe2, and TlInS2 crystals have been investigated through transmission and reflection measurements in the wavelength range of 400 - 1100 nm. Optical indirect and direct band gap energies of 2.45 and 2.63 eV (TlGaS2), 1.97 and 2.26 eV (TlGaSe2), 2.27 and 2.47 eV (TlInS2) were found by analyzing the absorption data at room temperature. The transmission measurements carried out in the temperature range of 10 - 300 K revealed that the rates of change of the indirect band gaps with temperature were gamma = -5.3 x 10(-4), -4.2 x 10(-4), and -9.2 x 10(-4) eV/K for the TlGaS2, TlGaSe2, and TlInS2 crystals, respectively. The dispersion of the refractive index is discussed in terms of the single oscillator model. The refractive indices, the oscillator and dispersion energies, and the zero-frequency dielectric constants were determined.