Optical investigation of electronic states of Mn ⁴⁺ ions in p-type GaN

The electronic states of manganese in p-type GaN are 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 ⁴T ₂(F)- ⁴T ₁(F) of Mn ⁴⁺ ions. PLE spectrum of the Mn ⁴⁺ [ ⁴T ₂(F)- ⁴T ₁(F)] luminescence reveals intracenter 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 ⁴T ₁(P)- ⁴T ₁(F) and ⁴A ₂(F)- ⁴T ₁ transitions of Mn ⁴⁺, respectively. In addition to the intrashell 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 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 lower than 1.11 eV above the valence band maximum.