Urbach Tail and Optical Investigations of Gd Doped and Undoped InSe Single Crystals

Undoped InSe and Gd doped InSe (InSe:Gd) single crystals were grown by using the stockbarger method. Ingots had no cracks and voids on the surface. There was no process to polish and clean treatments at cleavage faces of these samples because of the natural mirror-like cleavage faces. Absorption measurements were carried out in InSe and InSe:Gd samples in the temperature range 10–320 K with a step of 10 K. The first exciton energies for n = 1 were calculated as 1.323, 1.239 eV in InSe and were 1.317, 1.235 eV in InSe:Gd at 10 K and 300 K, and the second exciton energies for n = 2 in InSe were calculated as 1.338, 1.328 eV and in InSe:Gd were 1.331, 1.321 eV at 10 K and 80 K, respectively. Binding energies of InSe and InSe:Gd were calculated as 20.0 and 18.0 meV, respectively. The direct band gaps are estimated as 1.343, 1.259 eV in InSe and 1.335, 1.253 eV in InSe:Gd at 10 and 300 K, respectively. The steepness parameters and Urbach energy for InSe and InSe:Gd samples increased with increasing sample temperature in the range 10–320 K.