Optical properties of CuIn1-xGaxSe2 quaternary alloys for solar-energy conversion

The optical properties of CuIn1-xGaxSe2 epitaxial single-crystal layers were determined by spectroscopic ellipsometry (SE) and complementary photoreflectance spectroscopy (PR) in dependence of composition. Accurate values of refractive index n and extinction coefficient k and values of the fundamental and higher band-gap energies of quaternary selenides were obtained for six different Ga concentrations: x = 0.08, 0.19, 0.22, 0.50, 0.55, 0.82. In addition, for CuIn1-xGaxSe2 with x = 0.08 and 0.55, variable-angle ellipsometric measurements were performed. Only a very small contribution of the extraordinary component to the measured effective dielectric function was found, which implies that, in chalcopyrite single-crystal layers, the extraordinary component cannot be separated by altering the angle of incidence.