Point defects as a test ground for the local density approximation +U theory: Mn, Fe, and VGa in GaN

Abstract

Electronic structure of the Mn and Fe ions and of the gallium vacancy V(Ga) in GaN was analysed within the GGA + U approach. First, the +U term was treated as a free parameter, and applied to p(N), d(Mn), and d(Fe). The band gap of GaN is reproduced for U(N) ≈ 4 eV. The electronic structure of defect states was found to be more sensitive to the value of U than that of the bulk states. Both the magnitude and the sign of the U-induced energy shifts of levels depend on occupancies, and thus on the defect charge state. The energy shifts also depend on the hybridization between defect and host states, and thus are different for different level symmetries. In the case of V(Ga), these effects lead to stabilization of spin polarization and the "negative-U(eff)" behavior. The values of Us were also calculated using the linear response approach, which gives U(Fe) ≈ U(Mn) ≈ 4 eV. This reproduces well the results of previous hybrid functionals calculations. However, the best agreement with the experimental data is obtained for vanishing or even negative U(Fe) and U(Mn).

Magnetic properties of isolated Re ion and Re-Re complex in ZnO studied by GGA+U approach

“…In fact, the LDA/GGA approach shows smaller E gap [30] and the deep valence d(Zn) level higher when comparing to the experimental data [31]. Our analysis finds that U Zn Our approach of correcting the d(Zn) and p(O) states is justified by the fact that deficiencies of the LDA/GGA are of general character, and the underestimation of the gap follows from the sublinear dependence of the LDA/GGA total energy on the occupation [20,21,38].…”

Spin Polarization and Magnetic Properties of VGaON and VGaONInGa in GaN: GGA+U Approach

“…However, there is not unequivocal agreement on the structure of V Ga and its complexes-both experimental [12,13,16,17,19,21,22] and theoretical [14][15][16][17][18][20][21][22][23][24][25][26][27][28][29][30][31][32] results are divergent and sometimes contradictory. For example, the prediction of multiband luminescence due to the difference in geometries of V Ga O N was demonstrated in Ref.…”

“…In fact, the calculated defect structure strongly depends upon the used exchange-correlation functional [14][15][16][17][18][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. V Ga O N in III-N nitrides were theoretically investigated by local density approximation (LDA)/generalized gradient approximation (GGA) [14,15,20] or the hybrid functional (HyF) calculations [16-18, 21, 26, 32, 33].…”

“…V Ga O N in III-N nitrides were theoretically investigated by local density approximation (LDA)/generalized gradient approximation (GGA) [14,15,20] or the hybrid functional (HyF) calculations [16-18, 21, 26, 32, 33]. Finally, GGA+U approach with the +U term imposed on p(N) in an examination of V Ga was also tried [30,31]. Both the GGA+U [30,31] and the HyF [16-18, 21, 22, 26-28, 32-34] methods enhance spin polarization V Ga or its complexes and push defect levels deeper into the band gap in comparison with LDA/GGA [14,15,20,29].…”

Enhancement of spin polarization induced by Coulomb on-site repulsion between localized pz electrons in graphene embedded with line defects

“…It has been extensively applied for systems containing d and/or f electrons. [39][40][41][42] For the system we studied here, we calculated the values of U eff for localized p electrons. As a comparison, we firstly calculated the value of U eff for π electrons in graphene.…”