Optical properties of CuIn5Se8 and CuGa5Se8 from ellipsometric measurements

Spectroscopic ellipsometry measurements were done at room temperature on samples of CuIn5Se8 and CuGa5Se8. This allowed determining the real and imaginary parts of the complex refractive index N, the real and imaginary parts of the complex dielectric function ɛ, the absorption coefficient α and the reflectivity R. The energy gap values, Eg, were obtained from fitting the numerically obtained second derivative, d2ɛ(ω)/d(ω)2, of the experimental data, ɛ(ω), to analytic critical-point line shapes, and are in good agreement with those reported using other techniques. The value of the high frequency dielectric constant ɛ∞ for each compound was determined fitting the real refractive index n to the Sellmeier dispersion formula.