Atomic-scale structure and band-gap bowing in Cu(In,Ga)Se2

Mixed systems such as the Cu(In,Ga)Se-2 chalcopyrite semiconductor consist of different local atomic arrangements, that is, of different combinations of first-nearest-neighbor cations surrounding the Se anions. The anion position of Cu-III-VI2 compounds is predicted to strongly influence the material band gap. We therefore used extended x-ray absorption fine structure spectroscopy to study the atomic-scale structure of Cu(In,Ga)Se-2 as a function of composition. Based on these results, the anion position was modeled for all first-nearest-neighbor configurations using a valence force-field approach. We show that the atomic-scale structure strongly depends on the kind of first-nearest-neighbor atoms. Structural relaxation of the anion occurs with respect to both (i) Cu and group III atoms and (ii) In and Ga atoms. In both cases, the average anion displacement exhibits a nonlinear behavior with changing composition and thus results in two separate but significant contributions to the band gap bowing observed in Cu(In,Ga)Se-2.