Electronic structure, optical properties, and lattice dynamics of orthorhombic Cu2 CdGeS4 and Cu2 CdSiS4 semiconductors

Abstract

The dielectric function of orthorhombic Cu2CdGeS4 and Cu2CdSiS4 single crystals (space group Pmn21) is measured experimentally in the spectral range of 0.7-5.2 eV, and the results are interpreted via comparison with ab initio calculations. Polarized Raman-scattering measurements of both semiconductors are reported. All observed vibrational modes are assigned to specific lattice eigenmodes of the wurtz-stannite structure. Good agreement between theoretical and experimental mode frequencies is established. The differences in the experimental Raman spectra of the two compounds are well correlated with the effect of exchanging Si and Ge predicted by the theory. The strongest lines in the Raman-scattering spectra of both semiconductors are identified as being due to the "breathing" motion of GeS4 and SiS4 octahedra.