Optical investigation of TlGa0.999Pr0.001Se2 and TlGaSe2 single crystals

Abstract

TlGaSe2 and TlGa0.999Pr0.001Se2 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 TlGaSe2 and TlGa0.999Pr0.001Se2 samples in the temperature range 10-320 K with a step of 10 K. The phonon energies calculated for TlGaSe2 and TlGa0.999Pr0.001Se2 are 34 and 26 meV, respectively. At 320 K, the direct band gaps of TlGaSe2 and TlGa0.999Pr0.001Se2 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 TlGaSe2 at 180 and 210 K, and for TlGa0.999Pr0.001Se2 at 140 K. These temperatures obtained from the changing of Urbach energy may be phase transition temperatures.