Disorder effects on free excitons in CdS_1−<i>x</i>Se_<i>x</i> mixed crystals

Abstract

Abstract In CdS 1− x Se x mixed crystals the energies and transition probability ratios for free A, B, and C excitons are obtained as a function of the composition x by reflection and emission measurements at 77 K. The observation of the disorder‐allowed A(Γ 6 ) exciton transition, the bowing of the energy difference between A and C exciton, and the broadening of the exciton lines clearly demonstrate the importance of disorder effects in these mixed crystals. This conclusion is further supported by the stress‐induced enhancement of the A(Γ 6 ) exciton transition probability by uniaxial stress parallel to the c ‐axis of the mixed crystals. The experimental results are discussed on the basis of an effective exciton Hamiltonian consisting of a quasi‐cubic VCA‐Hamiltonian for wurtzite‐type mixed crystals and an additional lower‐symmetric term which describes the disorder effects phenomenologically by two fluctuating crystal fields parallel and perpendicular to the c ‐axis, respectively.