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-VI 2 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 .

Mixed chalcopyrite semiconductors like Cu(In,Ga)S2 and Cu(In,Ga)Se2 are characterized by the coexistence of different local atomic arrangements around the S or Se anion. The resulting anion displacement strongly influences the material bandgap. We studied the atomic-scale structure of Cu(In,Ga)S2 as a function of composition using x-ray absorption spectroscopy and valence force field simulations. Applying a specially developed model for not fully random cation distributions, we find that structural relaxation of the anion with respect to In and Ga contributes significantly more to the bandgap bowing observed for Cu(In,Ga)S2 and Cu(In,Ga)Se2 than relaxation with respect to Cu and group-III atoms.

Direct writing of single-mode waveguides into crystalline silicon using ps laser pulses is presented. The embedded structures were fabricated by moving the focal position along the beam axis with the help of a long distance microscope objective. In situ monitoring during inscription was performed to analyze the processing dynamics. The waveguide generation is based on pronounced multi-pulse interaction at moderate pulse energies around 100 nJ. All samples were characterized in terms of mode field distribution and damping losses. Calculations indicate an induced refractive index change in the range of 10 to 10. Moreover, a Y-splitter was realized to demonstrate the potential of this process.

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