Excitonic spectra and band structure of CdGa2Se4 birefractive crystals

We report on the intersection of spectral dependences of refractive indices no and ne at the wavelengths 546 nm (λ0) and 450 nm (λ01) in CdGa2Se4 single crystals. The value of difference Δn=ne−no is equal to zero at the wavelengths involved. When placed between two crossed polarizers, the crystals of CdGa2Se4 exhibit a transmission band at the wavelength of λ0=546 nm (300 K). The ground and excited states of three excitonic series (A, B and C) were found out at 13 K in CdGa2Se4 crystals, and other parameters of excitons and bands were determined. In the Г point of Brillouin zone the effective mass of electrons mc is equal to 0.14m0, and the effective masses of holes mv2 and mv3 are equal to 0.76m0 and 0.94m0, respectively. The hole mass mv1 depends upon the direction of wave vector k: at polarization E∥c, k∥a the mass mv1=1.15m0, and at polarization E∥c, k∥b mv1=0.84m0. The values of valence bands splitting in the center of Brillouin zone by the crystal field (Δcf=49 meV) and spin–orbital interaction (Δso=351 meV) were determined. The optical functions n, k, ε1 and ε2 in polarizations E⊥c and E∥c for the energy diapason from 3 to 6 eV were calculated from the reflectivity spectra by Kramers–Kronig analysis. The evidenced features are discussed on the basis of recent theoretical calculations of the band structure of CdGa2Se4 crystals.