Polytypes and Excitons in GaSe1-xSx Mixed Crystals

We have investigated the quantitative relationship between the polytypes and the direct and indirect exciton gap energies in GaSe 1- x S x mixed crystals. The X-ray diffraction analysis shows that GaSe 1- x S x is the ε -type for 0(GaSe)\({≤}x{}0.01\), the γ-type for \(0.05{}x{}0.4\), the β-type for \(0.5{}x{≤}1\)(GaS), and a mixture of the ε - and γ-types for \(0.01{}x{}0.03\). The direct and indirect exciton gap energies are determined from optical absorption and luminescence excitation spectra. The plots of the direct and indirect exciton gap energies ν s the compositional parameter x show kinks in the neighborhood of x =0.4. Furthermore, a jump in the indirect exciton gap energy is found at around x =0.02. These anomalies at x =0.02 and 0.4 are attributed to the ε -γ and γ-β polytype transformations, respectively.