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Polarized Raman spectra of ordered-vacancy Ga 2 Se 3 were calculated using the sp 3 s * tightbinding and vibration models. We found a large peak around 150 cm −1 which originates from the breathing-like movement of Se atoms around the vacancy. Reflecting the localized nature of vibration and the dangling-bond direction of three-coordinated Se atoms, this peak shows large intensity and anisotropy; the intensity is large for the light polarization perpendicular to the vacancy array, in contrast to the anisotropy observed in the photo-absorption and luminescence. These results well explain the experiments.Due to the misvalency between III and VI atoms, III 2 IV 3 compounds show a variety of crystal polymorphism depending on the growth condition. 1-3) When epitaxialized, for example, Ga 2 Se 3 has an orthorhombic defect zinc-blende structure, 4-6) where every atom is located at the zinc-blende site and one third of the cation sites are occupied by the vacancies. Since the vacancies form an array along the [110] direction, the characteristic electronic properties are expected compared to the familiar zinc-blende and chalcopyrite semiconductors. In fact, Okamoto et al. found that both the photo-absorption and luminescence around the fundamental band-gap energy are extremely anisotropic; they are large for the light polarization parallel to the vacancy array. 7, 8) Nakayama and Ishikawa calculated the electronic structures and the absorption spectra of this system, 9) and clarified that the dangling bonds of twocoordinated Se produce one-dimensional band along the [110] direction at the valence band top and the optical transitions from this band become the origin of such anisotropy.On the other hand, to clarify the crystal quality of MBE-grown orthorhombic Ga 2 Se 3 , Yamada et al. carried out the polarized Raman spectrum experiment in a back-scattering geometry. 6, 10) They found that there appears a large peak around 155 cm −1 and its intensity is large for the light polarization perpendicular to the vacancy array, in contrast to the cases of photoabsorption and luminescence. 7, 8) Using a simple vibration model with the interatomic force constants, they assigned that this peak originates from the A 1 -symmetry vibration mode and supposed that such mode is closely related to the vacancy array. However, the origins of the observed large intensity and the different anisotropy from that in the photo-absorption and emission have not been clarified yet. The purpose of this paper is to clarify 3860 these origins by calculating polarized Raman spectra of orthorhombic Ga 2 Se 3 , using the simple vibration model for phonons and the tight-binding method for electronic structures. §2. Geometry and Calculational Method Figure 1 shows the schematic picture of orthorhombic Ga 2 Se 3 viewed from the [001] direction. The unit cell denoted by the arrows is three times large along y compared to x, where x and y denote the [110] and [110] directions, respectively. Vacancies are arranged along x and this system has C 20 2v symmetry. I...

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