Electronic structure and stability of II--VI semiconductors and their alloys: The role of metal d bands

It has been traditionally accepted in various theoretical approaches to II–VI semiconductors (e.g., tight binding, pseudopotentials) to neglect the effects of the cation d bands, hoping that in some sense they are ‘‘deep,’’ ‘‘localized,’’ and hence, unresponsive to many perturbations of chemical interest. There are, however, two qualitative reasons to think that this is not so: first, d bands in II–VI’s are only 7–11 eV below the valence‐band maximum (VBM) (i.e., inside the main valence band), and second, in tetrahedral (but not octahedral) symmetry, cation d orbitals have the same representation (Γ15) as the anion p orbitals, hence the two can interact. We have considered the effects of the cation d bands in II–VI’s on both the electronic and structural properties of the binary and ternary compounds, treating all electrons on the same footing, in a self‐consistent first‐principles manner. We find that the d orbitals: (i) reverse the direction of charge transfer in the alloy (e.g., relative to s–p tight b...