Electronic states in Mn4+ ions in p-type GaN

There is interest in Mn doped GaN for high Tc ferromagnetic semiconductors. However, Mn in GaN forms deep levels in the band gap. In this study GaN was doped with both Mn and Mg to increase its p-type conductivity. The electronic states of manganese in $p$-type GaN were investigated using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. A series of sharp PL lines at 1.0 eV is observed in codoped GaN and attributed to the intra $d$-shell transition $^{4}T_{2}$(F)-$^{4}T_{1}$(F) of Mn$^{4+}$ ions$.$ PLE spectrum of the Mn$^{4+}$ [$^{4}T_{2}$(F)-$^{4}T_{1}$(F)] luminescence reveals intra center excitation processes via the excited states of Mn$^{4+}$ ions. PLE peaks observed at 1.79 and 2.33 eV are attributed to the intra-d-shell $^{4}T_{1}$(P)-$^{4}T_{1}$(F) and $^{4}A_{2}$(F)-$^{4}T_{1}$(F) transitions of Mn$^{4+}$, respectively. In addition to the intra shell excitation processes, a broad PLE band involving charge-transfer transition of the Mn$^{4+/3+}$ deep level is observed, which is well described by the Lucovsky model. As determined from the onset of this PLE band, the position of the Mn$^{4+/3+}$ deep level is 1.11 eV above the valence band maximum, which is consistent with prior theory using \textit{ab initio} calculations. Our work indicates 4+ is the predominant oxidation state of Mn ions in $p$-type GaN:Mn when the Fermi energy is less than 1.11 eV above the valence band maximum.