Metastability of Mn3+ in ZnO driven by strong d (Mn) intrashell Coulomb repulsion: Experiment and theory

Abstract

Depopulation of the Mn2+ state in ZnO:Mn upon illumination, monitored by quenching of the Mn 2+ EPR signal intensity, was observed at temperatures below 80 K. Mn 2+ photoquenching is shown to result from the Mn 2+ → Mn 3+ ionization transition, promoting one electron to the conduction band. Temperature dependence of this process indicates the existence of an energy barrier for electron recapture of the order of 1 meV. GGA+U calculations show that after ionization of Mn 2+ a moderate breathing lattice relaxation in the 3+ charge state occurs, which increases energies of d(Mn) levels. At its equilibrium atomic configuration, Mn 3+ is metastable since the direct capture of photo-electron is not possible. The metastability is mainly driven by the strong intra-shell Coulomb repulsion between d(Mn) electrons. Both the estimated barrier for electron capture and the photoionization energy are in good agreement with the experimental values.

Fe dopant in ZnO: 2+ versus 3+ valency and ion-carrier s,p−d exchange interaction

“…44 It is worth to note that despite the almost vanishing U for Mn and Fe found in the corresponding case of GaN, 60,61 a recent study 67 indicates that also for Mn in ZnO, the U (Mn) term is about 1.5 eV, i.e., larger than in GaN.…”

Transition metal ions in ZnO: Effects of intrashell coulomb repulsion on electronic properties

“…Typically, agreement with experiment is obtained for U (TM) of about 2 − 3 eV [16,17,27]. A non-vanishing U (TM) implies occupation dependent corrections to the TM levels [16,17]. Since a comparison with the existing experimental data is beyond the scope of this work, we keep the discussion more transparent and highlight the impact of the Coulomb coupling by neglecting U (TM).…”

“…While the inclusion of U (TM) improves the agreement with experiment, it turns out that the fitted U values are relatively small, and so are the energy changes. Typically, agreement with experiment is obtained for U (TM) of about 2 − 3 eV [16,17,27]. A non-vanishing U (TM) implies occupation dependent corrections to the TM levels [16,17].…”

“…As a consequence, wrong optical transition energies, metallic rather than insulating conductivity, and a non-correct type of magnetic coupling between TM atoms are obtained [14,15]. Here, a correct band gap is obtained by employing the +U corrections to the GGA calculations [16,17]. Application of +U correction only to d(Zn) [18,19] partially improves the situation.…”

“…The thermal ionization energy can be obtained directly from the position of the (+/0) level relative to CBM. The calculation of absorptionrecombination transitions for Mn ion are performed with fixed occupation matrices at the Γ point [16]. In this case, the finite size corrections to the total energy are not necessary.…”