Research Article  Open Access
Resonance Raman Scattering in TlGaSe_{2} Crystals
Abstract
The resonance Raman scattering for geometries and at temperature 10 K and infrared reflection spectra in and polarizations at 300 K were investigated. The number of () and () symmetry vibrational modes observed experimentally and calculated theoretically agree better in this case than when TlGa_{2}Se_{4} crystals belong to symmetry group. The emission of resonance Raman scattering and excitonic levels luminescence spectra overlap. The lines in resonance Raman spectra were identified as a combination of optical phonons in Brillouin zone center.
1. Introduction
TlGaSe_{2} crystals are triple thallium chalcogenides with a layered structure [1, 2]. One of these crystals features is the strong anisotropy of physical characteristics due to the specificity of the crystals lattice [1–3]. Optical spectra in the absorption edge region [4–11] and resonance Raman scattering for different geometries and temperatures (77–400 K) [12] were investigated in TlGaSe_{2} crystals. Reflection spectra for the 50–4000 cm^{−1} region were studied and polar vibrational modes LO and TO and their parameters were determined. Such crystals had an effect of switching of currentvoltage and acoustooptic characteristics [13–15]. There are a lot of materials dedicated to the investigations of these materials (see [4–16] and the references therein). But resonance Raman scattering in TlGaSe_{2} crystals has not been investigated.
2. Experimental Methods
Raman scattering spectra of TlGaSe_{2} crystals were measured on double highaperture spectrometers DFS32 with linear dispersion of 5 Å/mm and relative aperture of 1 : 5 and resonance Raman scattering spectra on spectrometer SDL1 with dispersion of 7 Å/mm and relative aperture of 1 : 2. The photomultiplier working in the photon counting regime was used as a detector. Resonance Raman spectra had an accuracy of ±0.5 meV. Reflection spectra in and polarizations in the range 50–400 cm^{−1} were measured on a vacuum spectrometer KSDI82 using an acoustooptical receiver with an accuracy of ±1 cm^{−1}. Cleft crystals of TlGaSe_{2} with different thicknesses mounted on a cold finger of a closedcircuit helium cryostat LTS22 C 330 optical cryogenic system were used in the measurements. The Raman scattering was excited by 6328 Å line of a HeNe laser. The resonance Raman scattering was excited by lines 4579 Å and 5145 Å of an Ar^{+} laser.
3. Experimental Results and Discussions
According to the crystallographic data, the TlGaSe_{2} structure is described by the space group (). The unit cell contains 8 formula units of TlGaSe_{2}. The main motive of the structure is formed by tetrahedral polyhedrons of Ga_{4}Se_{10}, consisting of 4 tetrahedrons of GaSe_{4}. These tetrahedrons have common atoms of selenium on the tops of the octahedron [1–3]. These tetrahedral polyhedrons have common vertices of 4 selenium atoms and take up layered positions perpendicular to the axis. The layers are rotated to each other at 90°. The edges of polyhedrons lie in the plain and are situated along the diagonal of the base square. Thus, the TlGaSe_{2} compound has a monoclinic pseudotetragonal structure with the following parameters: Å, Å, and β = 100.0°. The distances between TlSe, SeSe, and TlTl are equal to 3.45 Å, 3.92 Å, and 3.42 Å, respectively. This distance corresponds to the sum of ionic radiuses Tl^{1}Se (3.38 Å) [1–3].
The next vibrational modes should be observed in Brillouin zone center of the abovementioned crystals:The phonons of and symmetry should be observed in Raman spectra and and in IR reflection spectra in and polarizations, respectively. The scattering tensors are next:
Figure 1 shows the Raman scattering of TlGaSe_{2} crystals measured at a temperature of 10 K and in and geometries. The structure of vibrational modes depends on polarization. It was reported in [12] that 8 and 6 modes of symmetry were recognized at temperatures of 77 K and 300 K, respectively. One can see from the analysis of the abovementioned spectra that, even at 10 K, the number of experimentally observed modes is smaller than the number of modes theoretically predicted by grouptheoretic calculations. 14 modes of symmetry and 10 modes of symmetry were observed. Hence, the amount of the observed vibrational modes as in IR reflection spectra as in Raman spectra is lower than expected according to the theory.
The most intensive modes in reflection spectra for both polarizations are highfrequency modes (see Figure 2). Thus, in reflection spectra of TlGaSe_{2} crystals, 23 and 22 modes are expected in the region of singlephonons vibrational modes, but only 5 modes and 8 modes have been observed in and polarizations, respectively (Figure 2).
(a)
(b)
Compounds TlGaS_{2} and TlGaSe_{2} belong to thallium based crystals. This group of crystals (TlMX_{2}, where M = Ga, In and X = S, Se, Te) has a family likeness of optical spectra and energy band structures. The analogs are observed in all wellstudied compounds (Si, Ge, and ). The results of band structure calculations for TlMX_{2} crystals have a common character and reflect only its main features. This leads to proximity of the abovementioned compounds lattices. Crystal structures of TlGaS_{2} and TlGaSe_{2} compounds are different only in the replacement of S atoms by Se atoms in the crystal lattice. The structure of the layer TlGaS_{2} in [12] is symmetrized by an insignificant shift of atoms inside the layer to achieve a tetragonal structure with space group. The hypothetic structure with space group with unit cell comprising two layers was achieved by the authors of [12], introducing the interlayer inversion operation with preserving the elements of layer symmetry. Similarly, we analyze and investigate TlGaSe_{2} crystals. Using the same assumption, one can obtain a better agreement in the number of theoretically predicted and experimentally measured vibration modes in the case of TlGaSe_{2} crystals. Based on the analysis of polarization dependences of Raman and IR reflection spectra, the TlGaS_{2} and TlGaSe_{2} crystals can be attributed to symmetry group or .
The emission lines (1–20) of resonance Raman scattering in the region of excitonic resonances at excitation of 514.5 nm laser line of TlGaSe_{2} crystals at a temperature of 10 K and geometry were observed (see Figure 3 and Table 1). These lines (1–20) skirt the broad emission lines at 2.17–2.19 eV, 2.30 eV, and 2.39 eV in resonance Raman scattering spectra of TlGaSe_{2} crystals. These broad lines are caused by emission of ground states , , and excitons.

(a)
(b)
Figure 3 shows resonance Raman scattering spectra in TlGaSe_{2} crystals measured at a temperature of 10 K in geometry and excited by 496.5 nm Ar^{+} laser line. The narrow lines (1–17) that skirted the line at 2.4 eV were observed in these spectra. The observed lines of resonance Raman scattering and possible combination of phonons responsible for these emission lines are presented in Tables 1 and 2. At high frequencies, these data do not include all possible combinations of phonons responsible for lines of resonance Raman scattering.

4. Conclusions
The resonance Raman scattering in and geometries excited by HeNe laser was investigated at a temperature of 10 K. The energies of phonons with and symmetries were determined. It was shown that the amount of modes in Raman scattering and IR reflection spectra measured at 10 K is half the expected according to group theory calculations. The experimental and theoretical results coincide if the crystal is described by symmetry group . The superposition of excitonic luminescence with resonance Raman scattering emission was observed. The lines of resonance Raman emission were identified and attributed to optical phonons in Brillouin zone center.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
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