I n CdSl -%Se, mixed crystals the energies and transition probability ratios for free A, B, and C' excitons are obtained as a function of the composition 5 by reflection and emission measurements at 77 K. The observation of the disorder-allowed A(r,) 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( I?,) exciton transition probability by uniaxial stress parallel t o the c-axis of the mixed crystals. The experimental results are discussed on the basis of a n 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 t o the c-axis, respectively.
Die Energien und die
IntroductionFor the explanation of the optical properties of the pseudo-binary 111-V and 11-VI semiconducting mixed crystals the virtual-crystal approximation (VCA) is well established, which assumes a perfect lattice and a compositionally averaged crystal potential neglecting completely effects due to compositional and positional disorder. The relevance of disorder-induced corrections to the VCA in these mixed crystals has been discussedcontroversially. Following [l to 51 the observed bowing of the energy gap and, especially, the nonlinear behaviour of the spin-orbit splitting of the valence band as a function of the composition must be considered as disorder effects whereas in [6] the same effects were suggested to be explainable within the framework of the VCA.It is the aim of the present paper to give further evidence for the importance of disorder effects in semiconductor mixed crystals and, in particular, to demonstrate the disorder-induced breaking of the crystal symmetry by the observation of a cancellation of symmetry-caused optical selection rules. I n a preliminary work [7] the disorder-induced broadening of free exciton lines in the reflection spectra of CdSl &3eZ was studied as a function of the composition and shown to be caused by the fluctuations of the composition inside the various exciton volumes of the order of 4za$/3