AbstractThe influence of the material composition on the optical properties of the quaternary compound semiconductor system ZnGe(As_xP_1-x)₂ was studied for the first time. The crystals have been synthesized by direct solidification from a stoichiometric melt. X-ray diffraction measurements revealed a chalcopyrite type lattice for the quaternary compound. Closer examination by energy dispersive X-ray microprobe analysis yielded the exact element concentrations for the As and P components. Then electrolyte electroreflectance (EER) spectroscopy was used to determine the energies of the fundamental optical transitions as a function of the composition x. The analysis shows an approximately linear increase of the direct band gap with decreasing As content.Furthermore, the influence of the composition on the splitting and ordering of the three highest valence bands was investigated by polarization dependent EER measurements. The spectra for two samples with x = 0.42 and 0.77 allowed the determination of the spin orbit parameter Aso and the crystal field parameter Δ;_cf by using the quasicubic model. The values were found close to calculated ones, obtained by the same model under the assumption of a precise linear dependence of the band gap on the composition x.