Optical absorption edge and Urbach tails for Tl0.999GaPr0.001Se2, Tl0.995GaPr0.005Se and TlGaSe2

TlGaSe2, Tl0.999GaPr0.001Se2 and Tl0.995GaPr0.005Se2 single crystals were g rown by the Stockbarger method. The absorption measurements were carried ou t in these samples in temperature range 10-320 K with a step of 10 K. The p honon energies calculated in TlGaSe2, Tl0.000GaPr0.001Se2 and Tl0.995GaPr0. 005Se2 are 60.0 +/-5, 55.0 +/- 5 and 130.0 +/- 5 meV respectively. The firs t defect levels (n=1) have been found as 2.259, 2.235, 2.200 and 2.149 eV f or Tl0.999GaPr0.001Se2 and 2.254, 2.225, 2.189 and 2.149 eV for Tl0.995GaPr 0.005Se2 at 10, 100, 200 and 300 K. At 300 K direct band gap of TlGaSe2 is 2.156 eV, and indirect band gap is 2.075 eV. There are abrupt changes in th e Urbach energy peaks for Tl0.999GaPr0.001Se2 at 100 and 200 K, and Tl0.995 GaPr0.005Se2 at 200 and 260 K. There is an abrupt change in the sigma (0) v alues for Tl0.999GaPr0.001Se2 and Tl0.995GaPr0.005Se2 in the temperature ra nge 140-180 K and 220-260 K. These temperatures obtained from the changing of Urbach energy and sigma (0) values may be phase. transition temperatures .