Effect of lattice deformation on electronic and optical properties of CuGaSe₂: Ab-initio calculations

In this study, we have investigated the effect of bi-axial, ϵ_ab, and uni-axial, ϵ_c, strains on the optoelectronic properties of chalcopyrite semiconductor CuGaSe₂ through first-principles full potential linearized augmented plane wave method. These materials have recently attracted much interest within the materials science community. The results are obtained in the framework of Density Functional Theory (DFT), using the Generalized Gradient Approximation based on the minimization of total energy, together with the modified Becke-Johnson exchange-correlation potential, as implemented in the WIEN2k code. Our results show that unstrained CuGaSe₂ is a direct band gap semiconductor with a energy of 1.16 eV, thus improving the results of some previous DFT calculations, but still below the accepted experimental data. The incorporation of biaxial and uniaxial strain results in a monotonous decreasing behavior of the energy band gap when both ϵ_ab and ϵ_c change between -8% and +8%, with unstrained value being, approximately, at the middle of the variation range. It is also found that strain causes modifications in the index of refraction of the material, with modifications of its static value that rank above 10% over the entire range of deformations considered.