Urbach Tail and Optical Absorption in Layered Semiconductor TlGaSe_2(1-x)S_2x Single Crystals

TlGaSe_{2(1 - x)}S_2x 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 TlGaSe_{2(1 - x)}S_2x (x = 0, 0.2, 0.4, 0.6, 0.8, 1) samples were calculated as a function of temperature. There is an abrupt change in the energy spectrum of TlGaSe_{2(1 - x)}S_2x in the temperature ranges 90–100, 100, 100–120, 160–180, 220–240, and 240–250 K. 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 TlGaSe_{2(1 - x)}S_2x samples have been investigated. The steepness parameters and Urbach energies for TlGaSe_{2(1 - x)}S_2x samples increased with increasing sample temperature in the range 10–320 K. We have concluded that the compositions x are determined without using the other techniques during crystal growth considering band gaps energies.