Raman scattering in α-In2Se3 crystals
Introduction
In2Se3 is a semiconductor that exhibits a number of structural modifications. Four phases have been detected: α, β (rhombohedral), γ and δ (the last two are hexagonal) [1]. The α-phase can be either rhombohedral or hexagonal [1]. For the hexagonal type, the space group P63/mmc was assigned [1], [2], [3], [4]. There is no agreement about the space group of the rhombohedral type of the α-In2Se3 phase. According to Popovic [1], [2], the space group of the rhombohedral α-In2Se3 is the centrosymmetric Rm (No. 166), and the structure contains one formula unit in a Bravais cell (ZB = 1). According to Osamura [5], the rhombohedral α-In2Se3 has a noncentrosymmetric structure with the R3m (No. 160) space group, with one formula unit per Bravais cell.
In this study, the Raman spectra of the crystals with a well-defined cleavage plane have been measured with polarized light and analyzed in terms of Raman tensors of two space groups—Rm and R3m.
Section snippets
Crystal symmetry and lattice vibrations
The X-ray powder diffraction measurements showed that the investigated samples belong to the rhombohedral α-phase. However, there are two possible space groups: the centrosymmetric Rm and the R3m without inversion symmetry. We used a correlation method [6] to find Raman (R) and infrared (IR) active phonon modes. For the Rm space group (ZB = 1) we have the following irreducible representations: For R3m space group (ZB = 1), the irreducible representations
Experimental
The crystals were grown by the vertical gradient freezing method. Appropriate amounts of In and Se were carefully mixed and melted for 5 h at a temperature of 1040°C. Then the melt was cooled with a cooling rate of 1°C/h. The crystals cleaved from an ingot had good quality surfaces of approximately 1 cm2 area. The diffraction pattern of a powdered α-In2Se3 crystal is shown in Fig. 1. Rietveld refinement of X-ray diffraction data has been carried out for different models of the hexagonal and
Results and discussion
We present spectra recorded in the k ‖ z configuration in the Fig. 2 (k is a wave vector of the exciting light). The spectra in the k ⊥ z configuration are shown in Fig. 3. General features of the presented spectra bear resemblance to the results of Raman study performed by Kambas et al. [4] for the hexagonal α-In2Se3.
Based on the Raman tensors for both space groups we expect modes of the A1 and E symmetry for (aa) and (bb) configurations, the A1 modes for (zz) configuration, and the E modes
Conclusion
We have successfully assigned features of the Raman spectra of the rhombohedral α-In2Se3 according to the Raman tensors of both the R3m and the Rm group. However, the observed LO-TO splittings indicate a lack of inversion symmetry and R3m as an appropriate space group. This is consistent with the results of the Rietveld refinement of the diffraction data.
Acknowledgements
The support of PBZ KBN 05/T11/98/6A grant is gratefully acknowledged.
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