INFLUENCE OF TEMPERATURE AND PHASE-TRANSITIONS ON THE URBACHS TAILS OF ABSORPTION-SPECTRA FOR TLINS2 SINGLE-CRYSTALS

Optical absorption spectra of TlInS2 single crystals were investigated in the temperature range of 10-340 K. The exponentially increasing ab sorption tails were explained as Urbach tails. TlInS2 single crystals exhibited five different convergent groups of lines ln alpha(E,T) havi ng different convergence points of E-0 and alpha(0) between 120 and 34 0 K. Characteristic Urbach's parameters such as steepness parameter [s igma(T)], Urbach's energy [E-u(T)], and effective phonon energy (h nu( p)) were obtained as a function of temperature. sigma(T) and E-u(T) ex hibited a number of jump-like trends at the identified temperature reg ions. Jump-like changes of sigma(T) and E-u(T) were explained to be du e to the phase transitions (PTs) and contributions of electron-(excito n)-phonon interaction caused by dissociation. The temperature dependen ce of the band gap (BG) exhibited six clearly distinct regions and dif fers from the temperature dependent BG behavior of known semiconductor s. Absorption edge and band gap anomalies in TlInS2 samples at PT were interpreted using the theory proposed by Zametin [Phys. Status Solidi B 124, 625 (1984)]. Measured BG jumping values, Delta E-g(alpha), are greater than those calculated from this theory in the 340-200 and 175 -120 K regions, while one of them is smaller than that of the calculat ed in the 198-180 K region. These differences were explained as due to PTs and simultaneous influence of phonon coupling with electrons (exc itons) induced by exciton dissociation, and the existence of the chaot ic states caused by the coexistence of different phases which have opp osite sign contributions below the ferroelectric phase transition, res pectively. (C) 1998 American Institute of Physics. [S0021-8979(98)0461 9-2].