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The reflection spectroscopy of chalcopyrite CuGaS2 and CuInS2 single crystals has been applied for light polarized perpendicular () and parallel () to the optical axis in the photon energy range between 1.5 and 6 eV at 77 K. By using the Kramers–Kronig relations, the spectral dependences of the real ε1 and imaginary ε2 components of the complex dielectric function ε(E) = ε1(E)+iε2(E) have been calculated for the investigated materials. As a result, the energy band structure of CuGaS2 and CuInS2 at photon energies higher than the fundamental band gap is derived from the analysis of the structures observed in ε(ω) spectra. Additionally, the spectral dependences of the complex refractive index, extinction coefficient and absorption coefficient s of CuGaS2 and CuInS2 single crystals are determined in the 1.5–6 eV photon energy range.

Photoreflectivity, wavelength modulation spectroscopy and photoluminescence measurements of CuGaSe2 are used to determine the exciton and band parameters as well as the energy band structure of CuGaSe2 at photon energies higher than the fundamental band gap. The spectral dependences of the real ε1 and imaginary ε2 components of the complex dielectric function ε(E) = ε1 (E) + iε2(E) are calculated using the Kramers–Kronig relations. As a result, the energy band structure of CuGaSe2 at photon energies higher than the fundamental band gap is derived from the analysis of the structures observed both in the ε (ω) and in the wavelength modulated reflectivity spectra.

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