First-principles calculations of the structural, electronic and optical properties of CuGaS2 and AgGaS2

The structural, electronic and optical properties of two chalcopyrite crystals: CuGaS2 and AgGaS2 are studied using the full potential linearized augmented plane waves method within the local density approximation (LDA). Geometrical optimization of the unit cell (equilibrium volume, c/a ratio, internal parameter u, and bulk modulus) is in good agreement with experimental data. The energy gap is found to be direct for both materials and the nature of the gap crucially depends on the manner in which the Ga 3d and (Ag 4d, Cu 3d) electrons are treated as valence states. Results on band structures and density of states are presented. We report also our results on optical properties like the complex dielectric functions and the refractive index n of the CuGaS2 and AgGaS2 crystals. We analyze in detail the structures of the dielectric function observed in the studied energy region.