Birefringence and exciton spectra of CuAlSe2 and CuAlS2 crystals

Abstract

The excitonic reflection spectra of CuAlS2 and CuAlSe2 crystals were measured at the temperature of 10 K for polarizations E||c and E⊥c. Ground and excited states of the excitons were found out in the investigated spectra. Symmetries of the observed excitons were determined. The shapes of ground states lines of Г4 and Г5 excitons were calculated by means of the Kramers-Kronig relations. Exciton parameters and values of energetic gaps (Г7-Г6, Г6-Г6, and Г7-Г6) were determined. Optical reflection spectra in the depth of absorption band (Еg - 6 eV) were measured at 80 K for E||c and E⊥c polarizations. Optical constants were calculated from measured reflection spectra by Kramers-Kronig analysis. The phase in the excitonic region was determined. Transmission spectra of CuAlSe2 single crystals deposit in crossed polarizers demonstrate a birefractive effect. In the case of parallel polarizers interference due to birefraction was observed. The isotropic point was determined.