The optical investigation of TlGa 0.999Pr 0.001Se 2 and TlGaSe 2 single crystals

Abstract

TlGaSe 2 and TlGa 0.999Pr 0.001Se 2 single crystals were grown by a method which is similar to the direct freezing method. They did not have cracks and voids on their surface. The freshly cleaved crystals had a mirror-like surface and there was no need for mechanical treatment. The absorption measurements were carried out on TlGaSe 2 and TlGa 0.999Pr 0.001Se 2 samples in the temperature range 10–320 K with a step of 10 K. The phonon energies calculated for TlGaSe 2 and TlGa 0.999Pr 0.001Se 2 are 34 and 26 meV, respectively. At 320 K, the direct band gaps of TlGaSe 2 and TlGa 0.999Pr 0.001Se 2 are 2.173 and 2.155 eV and the indirect band gaps are 2.054 and 2.044 eV, respectively. There are abrupt changes in the Urbach energy for TlGaSe 2 at 180 and 210 K, and for TlGa 0.999Pr 0.001Se 2 at 140 K. These temperatures obtained from the changing of Urbach energy may be phase transition temperatures.