1. Introduction

Cu-based quaternary chalcogenides are a family of compounds with composition, Cu₂-II-IV-VI₄ (II = Zn, Cd; IV = Si, Ge, Sn; VI = Te, Se, S) that are receiving an increasing interest because of their good properties as absorbers for the development of advanced solar cells. From these semiconductors, the telluride compounds have been much less studied [1–3] than the corresponding selenides and sulfides. The forbidden gap (E_g) for tellurides is expected to be smaller than for the selenides and sulfides, and for Cu₂ZnSiTe₄ a value of E_g = 1.47 eV [1] has been reported, which matches well the optimum value for solar energy converters. The orthorhombic Si containing quaternary compounds were recently studied and structural [4,5], optical [5–8], electrical [9], luminescent [10] and Raman scattering [10–12] data for Cu₂ZnSi(S,Se)₄ are already available in the literature. However, Cu₂ZnSiTe₄ (CZSiTe) belongs to the tetragonal space group and thus may possess physical properties similar to those from other Cu-based quaternary chalcogenides with tetragonal lattice.

Herein we report, for the first time to our knowledge, the growth of bulk crystal of CZSiTe by Bridgman method, and its detailed structural study and Raman scattering analysis. The investigation of the crystals by Raman scattering has allowed observation of nine modes characteristic of this compound, that are analyzed in terms of the derived irreducible representation for the zone center phonons. Comparison with the experimental data reported for compounds with very similar crystalline structure, as the stannite Cu₂FeSnS₄ as well as with the theoretical calculations reported for the stannite structure of the close related Cu₂ZnSnSe₄ compound has allowed proposing a first identification of the symmetry of the experimental peaks.

2. Experimental data

The compound CZSiTe was synthesized by direct reaction of elements (all 99.999 purity) in evacuated and welded quartz glass tubes. Vacuum inside tubes was 0.0133 Pa. Synthesis was carried out in a vertical furnace with gradient 5 °C/cm along furnace axes (higher temperatures in upper part of furnace).The maximum temperature of synthesis was 950 °C.

After 24 h keeping at this temperature a slow (10 K/h) cooling until 500 °C was applied. At the end of the process a gray ingot with metallic luster was obtained. The content of elements in the sample from the lower part of the ingot as measured by Energy Dispersive X-Ray Analysis (EDAX) was Cu:Zn:Si:Te = 28.1:10.6:12.1:49.2. The upper part of the ingot was inhomogeneous. The samples for X-ray and Raman scattering analyses were selected from the homogeneous lower part of the ingot.

To clarify the type of the melting and melting temperature of Cu₂ZnSiTe₄ a differential thermal analysis (DTA) was performed. The DTA curve exhibits only one peak at 725 °C during heating till 900 °C. This peak has first order phase transition features (differences in cooling and heating processes) and is also observed in [1] as a melting point. However in our case some pieces of material were not completely melted despite of 900 °C upper heating temperature.

Single crystal X-ray data for CZSiTe were obtained at 293 K using an Xcalibur E diffractometer equipped with an EOS CCD space detector and a monochromatic source of MoKα radiation (graphite monochromator). The data were collected and processed using the program CrysAlisPro (Oxford Diffraction Ltd., version 1.171.33.66) and were corrected for the Lorentz and polarization effects and absorption [13]. The structure was refined by the full matrix least squares method on F² with anisotropic displacement parameters using the program SHELXL [14].

Raman spectra were measured using a LabRam HR800-UV Horiba Jobin Yvon spectrometer with a solid state laser (line 785 nm) system as laser excitation source. The spectrometer was calibrated using a reference single crystal Si sample (Raman peak at 520.7 cm⁻¹). The spectra were measured in backscattering configuration in the spectral range 50 – 600 cm⁻¹; excitation and light collection were made through an Olympus metallographic microscope, with a laser spot on the sample of about ~1 μm and a laser power ~1 mW. These conditions allow avoiding the presence of thermal effects in the Raman spectra. Non-polarized spectra were measured in several points of the sample and no changes in Raman spectra were observed, corroborating the high uniformity of the sample.

3. Results and discussion

3.1 Structural analysis

The results of the structure refinement and experimental details of X-ray measurements for CZSiTe single crystal, atomic coordinates, and equivalent isotropic displacement parameters are presented in Tables 1 and 2. The values of lattice parameters are in good agreement with previously measurements reported in [1] and [2]. Refinements of the data converged rapidly with the atomic position of the closely related compound Cu₂ZnSnSe₄ (CZTSe) [15] as starting parameters.

Table 1. Results of Refinement and Experimental Details of X-ray Measurements.

View Table | View all tables in this article

Table 2. Atomic Coordinates^a and Equivalent Isotropic Displacement Parameters^b.

View Table | View all tables in this article

The crystal structure of CZSiTe is shown in Fig. 1. The Cu(1) and Zn(1) atoms occupy the same 4d Wyckoff positions of the unit cell statistically with equal probability, while Cu(2) atoms are situated in 2b position. A similar structure with randomly distributed Cu and Zn in 4d positions has also been reported for CZTSe and Cu₂ZnSnS₄ compounds [15, 16], and is designed as the “disordered kesterite” structure [16].

 

Fig. 1 The crystal structure of CZSiTe.

Download Full Size | PDF

3.2. Raman scattering investigation

Based on the results of group theoretical analysis performed in [17] for the zone center phonons of each of 32 space groups, the irreducible representation for tetragonal CZSiTe, with respect to Wyckoff position of all atoms (Table 2), was derived and is given in Table 3. From all 24 calculated modes: 20 are Raman active${\text{(2A}}_{\text{1}}\oplus {\text{2B}}_{\text{1}}\oplus {\text{4B}}_{\text{2}}\oplus \text{6E)}$, 16 from them are also IR active${\text{(4B}}_{\text{2}}\oplus \text{6E)}$, three are acoustic ${\text{(B}}_{\text{2}}\oplus \text{E)}$ and one ${\text{(A}}_{\text{2}}\text{)}$is silent. Note that, E modes are double degenerate and in addition ${\text{B}}_{\text{2}}$ and $\text{E}$ modes exhibit TO\LO splitting.

Table 3. Irreducible Representations for Atoms of Tetragonal CZSiTe.

View Table | View all tables in this article

It is interesting to remark that the irreducible representation obtained for the CZSiTe zone center phonons is similar to that of stannite type Cu-based quaternary chalcogenides [18,19]. This can be explained by group theory, where the result depends only on the site symmetry of each atom and on the number of atoms in the unit cell. On the other hand, the Wyckoff positions of the cations are occupied by different atoms in CZSiTe and in the unit cell of Cu-based stannite quaternary chalcogenides, so that cation like modes are different in nature. For example, the B₁, B₂ and 2E modes originate from Cu(1), and Zn(1) atoms at 4d position for CZSiTe (Table 3), while only motion of Cu atoms at 4d position are responsible for these modes in stannite type Cu-based quaternary chalcogenides [20,21].

Raman spectra of CZSiTe crystal exhibit two dominant peaks at 134 cm⁻¹ and 151 cm⁻¹ (see Fig. 2), which can be considered as a fingerprint of the investigated material. This also correlates with results published in [11], where two dominant fingerprint peaks were observed in quaternary compounds with tetragonal lattice in comparison of three dominant fingerprint peaks in quaternary compounds with orthorhombic lattice. Some additional peaks with weaker intensity were also detected (see Fig. 2 and Table 4). The small value of full width at half maximum (FWHM) for most of the Raman active peaks shown in Table 4 gives a clear evidence on the high crystalline quality of the grown sample.

 

Fig. 2 Fitting (red line) of CZSiTe Raman spectra (black line) with Lorentzian curves (green line). The breaks in the intensity scale were done to show better the low intensity peaks. The untreated Raman spectrum of CZSiTe crystal is shown in the inset.

Download Full Size | PDF

Table 4. Proposed Symmetry Assignment of Raman Active Modes in CZSiTe Compound.

View Table | View all tables in this article

Taking into account the similar space group and irreducible representation of the zone center phonons from CZSiTe and the stannite type Cu-based quaternary chalcogenides, from the comparison of the Raman spectra from both kinds of compounds relevant information can be obtained for the symmetry assignment of the observed Raman peaks. As reported in [19], the experimental Raman spectra from stannite Cu₂FeSnS₄ (CFTS) show two dominant peaks located at 285 cm⁻¹ and 318 cm⁻¹. From Raman scattering measurements performed under different polarization configurations, these peaks were identified with the A₁ symmetry modes from the stannite structure, which originate from pure anion vibrations, i.e. pure S vibrations in 8i-position (Table 3). According to the similarities in the Raman spectra from both compounds, the dominant peaks at 134 cm⁻¹ and 151 cm⁻¹ are also assigned to the A₁ symmetry modes related to pure Te vibrations in CZSiTe. This assignment is supported by the fact that the ratio of the frequency of these peaks in relation to that of the A₁ modes from the stannite CZFS compounds is of the order of 0.47, correlates well with the value of square root ratio of molar masses of Te and S, ${\left(M_S/M_{Te}\right)}^{1/2}$, which is equal to 0.50. This can be explained taking into account that for the close related I-III-VI₂ chalcopyrite’s the pure anion (VI-atoms) vibration mode frequency (ν_VI) can be estimated by the following expression:

Additional support to the identification of the dominant CZSiTe Raman peaks with the A₁ symmetry modes can be derived from the comparison with the theoretical values calculated for the zone center phonons of the Cu₂ZnSnSe₄ compound assuming a stannite crystalline structure [20,21]. In this case, the frequency ratio between the experimental values measured in this work for CZSiTe and those theoretically calculated for the stannite crystalline structure of CZTSe ranges from 0.74 to 0.78. This is again very close to the value of square root ratio of molar masses of Te and Se, ${\left(M_{Se}/M_{Te}\right)}^{1/2}$, which is equal to 0.79, in agreement with the behavior predicted by Eq. (1).

On the other hand, neglecting the effect of the force constants, the B₁ mode originating from Cu and Zn cation motions in CZSiTe (Table 3) is expected to be similar to B₁ mode related only to Cu motions in the stannite CZTSe crystal as the molar masses of Cu and Zn atoms are very close. In contrast, under the same assumptions on similarity of force constants, the anion-like B₁ mode in CZSiTe (Table 3) is expected to be strongly shifted in relation to the anion-like B₁ mode in the stannite CZTSe crystal due to the anion mass effect. Theoretical calculations yielded position of B₁ symmetry modes of the stannite CZTSe at 69.2 cm⁻¹ and 220.2 cm⁻¹ [21] which can be well correlated with Raman peaks observed in CZSiTe at 69 cm⁻¹ and 167 cm⁻¹ under above made assumptions and might be assigned to the cation-like B₁ mode and to the anion-like B₁ mode, respectively.

Based on a conclusion from IR measurements on the Cu-based quaternary chalcogenides with space group $I\overline{4}2m$ that IR-active modes with the highest wavelength number are due to four-valent cation motions we may assume that the highest vibration modes at 331 cm⁻¹ and 351 cm⁻¹ in CZSiTe originate from the motions of SiTe₄ units and might be assigned to B₂/E modes [18]. The remaining peaks at 81 cm⁻¹, 84 cm⁻¹, and 182 cm⁻¹ of CZSiTe have also been ascribed to B₂/E symmetry modes (Table 4). These peaks are more likely related to modes originating from vibration of different kinds of atoms, rather than to the symmetry modes involving only one sort of atoms (Cu, Zn, Si or Te) [18,23].

Here it is worth mentioning that proposed assignment of CZSiTe Raman active mode should be considered as tentative and polarization dependent Raman measurements are required in order to determine the symmetry of the observed peaks.

4. Conclusions

The bulk crystals of CZSiTe were grown by modified Bridgman method. From the homogenous part of the ingot a single crystal was selected and full X-ray single crystal structural analysis has been carried out. In tetragonal space group $I\overline{4}2m$ Cu(1) and Zn(1) atoms randomly distribute at 4d positions with equal probability. Based on the refined crystal structure, the irreducible representation for the zone center phonons of CZSiTe compound was derived. From the comparison of CZSiTe Raman spectra to the experimental and theoretical data from close related stannite type Cu-base quaternary chalcogenides a first symmetry assignment of the Raman active modes is proposed.