Metastability of Mn$^{3+}$ in ZnO driven by strong $d$(Mn) intrashell Coulomb repulsion: experiment and theory

TL;DR

This study combines experiment and theory to reveal the metastability of Mn$^{3+}$ in ZnO, driven by strong intra-shell Coulomb repulsion, with implications for photoinduced charge state transitions.

Contribution

It provides new insights into the metastability of Mn$^{3+}$ in ZnO driven by Coulomb repulsion, supported by combined experimental and theoretical analysis.

Findings

Depopulation of Mn$^{2+}$ observed below 80 K upon illumination.

Energy barrier for electron recapture estimated around 1 meV.

Theoretical calculations show lattice relaxation increases $d$(Mn) levels.

Abstract

Depopulation of the Mn$^{2+}$ 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$^{2+}$ $\to$ 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.