The band structure of birefractive CdGa2S4 crystals

Abstract

In this paper, we report on the spectral dependence of Δn=no−ne for CdGa2S4 single crystals for shorter and longer wavelengths than the isotropic wavelength λ0=485.7nm (300K). It was established that Δn is positive at λ>λ0 and it is negative in the spectral range λ<λ0. The isotropic wavelength λ0 exhibits blue spectral shift with temperature decreasing. The ground and excited states of three excitonic series A, B and C with binding energies of 53meV, 52meV and 46meV, respectively, were found out at 10K. The effective masses of electrons for k=0 were derived from the calculation of excitonic spectra: mc∥(Е∥с)=0.21m0 and mc⊥(Е⊥с)=0.19m0. The holes masses are equal to 0.59m0 and 0.71m0 for Е∥с and Е⊥с, respectively. The value of valence bands splitting, V1−V2, by crystalline field equals 24meV, and V2−V3 splitting due to the spin–orbital interaction equals to 130meV. The optical functions n, k, ε1 and ε2 for Е⊥с and Е∥с polarizations were calculated by means of Kramers–Kronig analyses in the energy interval 3–6eV. The evidenced features are discussed taking into account the results of new theoretical calculations of CdGa2S4 band structure.