Large band-gap bowing in Cu1-xAgxGaS2 chalcopyrite semiconductors and its effect on optical parameters

The band-gap bowing in the Cu1-xAgxGaS2 chalcopyrite alloy is studied using the full-potential linearized muffin-tin orbital method combined with various structural models, including so-called special quasirandom structures. The calculations confirm a large band-gap bowing (similar to 0.7 eV) in agreement with recent experimental results. It is found that the large bowing in part arises from a nonlinear behavior of the c/a ratio with concentration. Layered structures are found to have similar bowing to special quasirandom structures. The nonlinear band-gap behavior also leads to a nonlinear behavior of the index of refraction and the second-order nonlinear optical susceptibility with concentration. The maximum chi((2)) is found for a 50% alloy and is calculated to be about 27 pm/V.