Optical absorption edge and Urbach tails for Tl_0.999GaPr_0.001Se₂, <span style="font-size:14.0pt;line-height:115%;font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";mso-ansi-language:EN-IN;mso-fareast-language: EN-IN;mso-bidi-language:HI">Tl_0.995GaPr_0.005Se₂ and TlGaSe₂</span>

Abstract

TlGaSe₂, Tl_0.999GaPr_0.001Se₂, and Tl_0.995GaPr_0.005Se₂, single crystals were grown by the Stockbarger method. The absorption measurements were carried out in these samples in temperature range 10-320 K with a step of 10K. The phonon energies calculated in TlGaSe₂, Tl_0.999GaPr_0.001Se₂and Tl_0.995GaPr_0.005Se₂are 60.0±5, 55.0±5 and 130.0±5 meV respectively. The first defect levels (n=1) have been found as 2.259, 2.235, 2.200 and 2.149 eV for Tl_0.999GaPr_0.001Se₂and 2.254, 2.225, 2.189 and 2.149 eV for Tl_0.995GaPr_0.005Se₂at 10,100, 200 and 300 K. At 300 K direct band gap of TIGaSe₂ is 2.156 eV, and indirect band gap is 2.075 eV. There are abrupt changes in the Urbach energy peaks for Tl_0.999GaPr_0.001Se₂at 100 and 200 K, and Tl_0.995GaPr_0.005Se₂at 200 and 260 K. There is an abrupt change in the σ₀ values for Tl_0.999GaPr_0.001Se₂and Tl_0.995GaPr_0.005Se₂in the temperature range 140-180 K and 220-260 K. These temperatures obtained from the changing of Urbach energy and σ₀ values may be phase transition temperatures.