EFFECT OF D-ELECTRONS IN TRANSITION-METAL IONS ON BAND-GAP ENERGIES OF DILUTED MAGNETIC SEMICONDUCTORS

We present a theoretical study of the effect of partially filled d electrons in transition-metal ions on the E(1) and E(1) + Delta(1) band-gap energies of the diluted magnetic semiconductors. Semiempirical tight-binding calculations were performed to study the effects of interactions arising from the hybridization between the localized d orbitals of a transition-metal impurity and the bulk band states of II-VI semiconductors on the Gamma- and L-point band gaps. Our results account for the concentration dependence of the L-point band gaps in the Zn-1-x,(Mn,Fe,Co)(x)Se system in the range of small x. The effect of this hybridization strongly depends on the location of the d states relative to the valence- and conduction-band levels, giving a decrease of the L-point band gaps in Zn1-xMnxSe, an increase in Zn1-xFexSe, and very little change in Zn1-xCoxSe. The exchange interaction has a much smaller effect on the band-gap energies.