d-band excitations in II-VI semiconductors: A broken-symmetry approach to the core hole

Local-density approximation (LDA) band-structure calculations place the 3d band of zinc-blende ZnO, ZnS, ZnSe, and ZnTe at 5.4, 6.4, 6.8, and 7.5 eV below the valence-band maximum (VBM), while photoemission measurements place them at 7.8, 9.0, 9.4, and 9.8 eV below the VBM, respectively. We show that this {~}3-eV LDA error can be accounted for using a ``broken symmetry'' band-structure approach. In this approach, a d core hole is placed in an impuritylike splitoff d subband resulting from the creation of the hole on a particular Zn sublattice. Self-consistent solutions to such a constrained LDA problem reveal that the final hole state is sufficiently localized to trigger a self-interaction correction of 3--4 eV, needed to explain the discrepancy with experiment. This 3--4 eV shift is reduced, by screening effects, from the 9.7-eV value in a free Zn atom. Finally, we calculated the binding energy EMn for Mn 3d states in ZnTe:Mn and the effective Coulomb interaction parameter Ueff. Significant improvements over the results of local-spin-density calculations were found. The calculated EMn=EVBM-3.93 eV and Ueff=6.85 eV are in good agreement with experiments.