Origin of the large band-gap bowing in highly mismatched semiconductor alloys

Photomodulated reflection, optical absorption, and photoluminescence spectroscopies have been used to measure the composition dependence of interband optical transitions in ZnSe1-xTex and ZnS1-xTex alloys. The results reveal entirely different origins of the large band-gap bowing for small and large Te content. On the Te-rich side, the reduction of the band gap is well explained by the band anticrossing interaction between the Se or S localized states and the ZnTe conduction-band states. On the Se- or S-rich side, an interaction between the localized Te states and the degenerate Gamma valence bands of ZnSe or ZnS is responsible for the band-gap reduction and the rapid increase of the spin-orbit splitting with increasing Te concentration. Results of the soft-x-ray emission experiment provide direct proof of the valence-band anticrossing interaction. The band-gap bowing in the entire composition range is accounted for by a linear interpolation between the conduction-band anticrossing and valence-band anticrossing models.