The vibrational spectra of the chalcopyrite compounds CuInSe2 and CuGaSe2 have been studied by Raman microspectrometry. This technique is very useful in the present case where large single crystals are not generally available. The results have been used to resolve discrepancies in the reported data on the vibrational spectrum of CuInSe2.

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X-ray powder diffraction by p-type Cu2SnSe3, prepared by the vertical Bridgman–Stockbarger technique, shows that this material crystallizes in a monoclinic structure, space group Cc, with unit cell parameters a=6.5936(1) Å, b=12.1593(4) Å, c=6.6084(3) Å, and β=108.56(2)°. The temperature variation of the hole concentration p obtained from the Hall effect and electrical resistivity measurements from about 160 to 300 K, is explained as due to the thermal activation of an acceptor level with an ionization energy of 0.067 eV, whereas below 100 K, the conduction in the impurity band dominates the electrical transport process. From the analysis of the p vs T data, the density-of-states effective mass of the holes is estimated to be nearly of the same magnitude as the free electron mass. In the valence band, the temperature variation of the hole mobility is analyzed by taking into account the scattering of charge carriers by ionized and neutral impurities, and acoustic phonons. In the impurity band, the mobility is explained as due to the thermally activated hopping transport. From the analysis of the optical absorption spectra at room temperature, the fundamental energy gap was determined to be 0.843 eV. The photoconductivity spectra show the presence of a narrow band gap whose main peak is observed at 0.771 eV. This band is attributed to a free-to-bound transition from the defect acceptor level to the conduction band. The origin of this acceptor state, consistent with the chemical composition of the samples and screening effects, is tentatively attributed to selenium interstitials.

From the analysis of the Urbach tail in CuInSe 2 and CuInTe 2 , it is found that the energy h p involved in the electron-exciton-phonon interaction is not that of the ordered compound longitudinal or transverse optical modes. It is established that this energy depends on the structural and cations disorders. An expression of thewhere P and N are adjustable structural and orderdisorder parameters and ⌰ the Einstein characteristic temperature, accurately explains the temperature dependence of the Urbach energy. From extrapolation of the linear variation of N and P with h p , the phonon energy for a completely ordered and disordered systems can be estimated. We discuss the physical meaning of the parameter N as due to the contribution of localized modes produced by substitutional disorder of low energy of formation. We find an intriguing relation between the temperature of the chalcopyrite-sphalerite transition and the phonons contributing to the formation of Urbach tails in the strong-disorder limit.

The optical vibrational modes of the ordered vacancy compounds CuIn3Se5 and CuGa3Se5 have been obtained by Raman spectra at various temperatures. The totally symmetric A1 mode appears around 155 and 167 cm−1 in CuIn3Se5 and CuGa3Se5, respectively. All the lines observed, except the B1 modes, present a reduction in their frequencies by about 10% as compared to the corresponding values in the CuInSe2 and CuGaSe2 chalcopyrites. This is due to the presence in these compounds of an array of vacancies occupying particular sites in the cation sublattice. These tend to relax the stretching forces thus reducing the vibrational frequencies.