Urbach tail and optical absorption in layered semiconductor TlGaSe2(1-x)S2x single crystals

TIGaSe2(1-x)S2x single crystals were grown by the modified Bridgman Stockbarger method. None of the grown crystals had cracks and voids on the surface. The freshly cleaved crystals had mirror-like surfaces and there was no need for mechanical or chemical polishing treatments. The measurements were performed in steps of 10 K if changes were small, and with steps of 3 and 5 K if changes were large in the direct and indirect band gaps energies. The direct and indirect band gaps for TIGaSe2(1-x)S2x (x = 0, 0.2, 0.4, 0.6. 0.8, 1) samples were calculated as a function of temperature. There is an abrupt change if) the energy spectrum of TIGaSe2(1-x)S2x in the temperature ranges 90-100, 100, 100-120, 160-180, 220-240, and 240-250K. The values obtained from the energy peak change may be phase transition temperatures. It is the first time that Urbach's rule and steepness parameters of TIGaSe2(1-x)S2x samples have been investigated. The steepness parameters and Urbach energies for TIGaSe2(1-x)S2x samples increased with increasing sample temperature in the range 10-320K. We have concluded that the compositions x are determined without using (he other techniques during crystal growth considering band gaps energies.