Role of Coulomb interactions in semicore levels Ga d levels of GaX semiconductors: Implication on band offsets

The positions of the semicore Ga d levels in GaX semiconductors (X = N, P, and As) are underestimated in density functional calculations using either the local density approximation LIDA or the generalized gradient approximation GGA for the exchange functional. Correcting for this inaccuracy within LDA + U calculations with an on-site Coulumb interaction U on the semicore d-states results in a modest enhancement of the band gap. We show that this modest enhancement of the band-gap energy comes from the movement of the valence-band maximum alone, thus not affecting the conduction-band states. Further, the localization of the charge on Ga d states with U leads to a regulation of charge on Ga. This yields a shift of 1-2 eV of the core levels on the Ga atom while the anion core levels remain unchanged. (C) 2009 Elsevier Ltd. All rights reserved.