Relativistic band structure of ternary II-VI semiconductor alloys containing Cd, Zn, Se and Te

The relativistic band structure and band-gap bowing factors of ternary II-VI semiconductor alloys containing Cd, Zn, Se and Te are calculated by the ab initio self-consistent pseudopotential method with spin-orbit correction. The disorder in these alloys is modelled by the virtual crystal approximation (VCA). The numerical results thus obtained are compared with experimental data from photoluminescence spectroscopy. The experimental band-gap bowings for this family of alloys are always concave upwards. However, CdSe_xTe_1-x and ZnSe_xTe_1-x in general show much larger bowing factors than Cd_1-xZn_xSe and Cd_1-xZn_xTe. The former two alloys show a minimum in the band gap versus composition curve near x=0.35 while the latter two have it at x=0. Most of these qualitative trends are well explained by VCA. However, in order to explain the large bowing in the first two alloys, some kind of local order has to be incorporated in the calculation. It was found that a local cluster with AuCu-I-like structure, when relaxed, has quite a significant contribution to the bowing factors. The splittings in the band structure due to both spin-orbit and structural relaxation effects are also discussed.