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

TlGaSe2 and TlGa0.999Pr0.001Se2 single crystals were grown by a method whic h is similar to the direct freezing method. They did not have cracks and vo ids on their surface. The freshly cleaved crystals had a mirror-like surfac e and there was no need for mechanical treatment. The absorption measuremen ts were carried out on TlGaSe2 and TlGa0.999Pr0.001Se2 samples in the tempe rature range 10-320 K with a step of 10 K. The phonon energies calculated f or 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. The re are abrupt changes in the Urbach energy for TlGaSe2 at 180 and 210 K, an d for TlGa0.999Pr0.001Se2 at 140 K. These temperatures obtained from the ch anging of Urbach energy may be phase transition temperatures. (C) 2001 Else vier Science B.V. All rights reserved.