Dielectric functions and fundamental band gaps of Cu2In4Se7, CuGa3Se5 and CuGa5Se8 crystals

Room temperature pseudodielectric function spectra epsilon(omega) = epsilon(1)(omega) + i epsilon(2)(omega) of the ordered defects compounds Cu2In4Se7, CuGa3Se5 and CuGa5Se8 have been measured by spectroscopic ellipsometry. The values of refractive index n and extinction coefficient k are given. The structures observed in e(omega) spectra have been analysed using different methods, including fitting the numerically differentiated experimental spectrum (second derivative) to analytical line shapes. As a result, the energies corresponding to the fundamental gap (E-0) and higher critical points have been determined. A linear correlation of the fundamental gap values with Ga/Cu atomic ratio contents in CuGax Se-y samples is deduced.