Urbach tail and optical characterization of gadolinium-doped TlGaSe₂single crystals

Abstract

TlGaSe2 and TlGaSe2:Gd single crystals were grown by the Stockbarger method. There are no cracks or voids on the surface of ingots. The samples were cleaved along the cleavage planes (001). The freshly cleaved crystals had mirror-like surfaces even before using mechanical treatment. The phonon energies in TlGaSe2 and TlGaSe2:Gd have been calculated as (35.7 ± 4) and (38.3 ± 4) meV at 200 K, respectively. At 10 K, the direct band gaps of TlGaSe2 and TlGaSe2:Gd are 2.293 and 2.297 eV and the indirect bandgaps are 2.196 and 2.106 eV, respectively. There is an abrupt change in the energy spectrum of TlGaSe2 and TlGaSe2:Gd in the temperature ranges 105–115 and 210–215 K, respectively. The values obtained from the energy peak change may be related phase transition temperatures. The defect levels were found to be at 2.162, 2.143, 2.136 and 2.108 eV for TlGaSe2:Gd at 10, 100, 200 and 300 K, respectively. The defect levels, which exist in TlGaSe2:Gd, but not in pure TlGaSe2 can be observed by adding Gd in the temperature range of 10–320 K. The steepness parameters and Urbach energies for TlGaSe2 and TlGaSe2:Gd samples increased with increasing sample temperature in the range of 10–320 K. The other important results obtained are: (i) the defect levels and defect absorption peaks due to Gd which have been observed (figure 9(b)). (ii) Gd and Gd related defect levels which exist in Gd-doped TlGaSe2 and can be observed by adding Gd in the range of 10–320 K. Adding of the rare earth elements to the TlGaSe2 causes the new appearance and changing of the absorption peak intensity (figure 9(b)).

Study of optical absorption in TlGaSe2:Zn2+ single crystals

“…These results agree well with the data reported in Ref. [1]. The parameter dE g /dT depends on the expansion of crystal lattice and on the electron-phonon interactions [38].…”

“…Moreover, abnormal behaviour of the quantitative parameters of this phenomenological model has indicated to a presence of two additional phase transitions in TlGaSe 2 that occur at 246 and 101 K, besides of the transitions earlier established at 120 and 107 K [1,14,15]. Apart from the direct and indirect transmission bands and the temperature points of phase transitions, the previous investigations on the subject have also been concerned with the phonon energy, the temperature coefficient and the so-called 'steepness' parameter of the Urbach energy, as well as the Debye parameter of A III B III C 2 VI single crystals [4,[16][17][18][19].…”

“…The coexistence of these transitions in the A III B III C 2 VI crystals has earlier been reported in Refs. [1,16,17,36,37]. We have obtained the Table 2 for the temperatures 100, 150, 200, 250 and 300 K. It is known that the doping leads to decrease in both the direct and indirect bandgaps, when compared to the pure TlGaSe 2 crystals [16].…”

“…Physical properties of layered crystalline materials are extensively studied owing to their promising practical applications [1,2]. These crystals consist of layers with strong (covalent) bonds among the atoms within the layers, whereas much weaker bonds among the layers are mainly of a van der Waals type, which leads to expressed anisotropy of different properties of the crystals and easy formation of structural defects [3].…”

Novel Quaternary TlGaSn2Se6 Single Crystal as Promising Material for Laser Operated Infrared Nonlinear Optical Modulators

“…The complex shape of the absorption edge is clearly seen (Figure 7b). A significant difference in the spectral distribution of the absorption coefficient for TlGaSe2 [52] and TlGaSn2Se6 crystals is observed at energies below the beginning of inter-band transitions, in the spectral range of about ~790 nm (1.57 eV). However, due to a great number of intrinsic defects with energy levels below the conduction band gap, a clear separation of the direct and indirect transitions is not possible.…”