  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 4T2(F)-4T1(F) of Mn4+ ions. PLE spectrum of the
Mn4+ [4T2(F)-4T1(F)] luminescence reveals intra-center excitation processes via
the excited states of Mn4+ ions. PLE peaks observed at 1.79 and 2.33 eV are
attributed to the intra d-shell 4T1(P)-4T1(F) and 4A2(F)-4T1(F) transitions of
Mn4+, respectively. In addition to the intra-shell excitation processes, a
broad PLE band involving charge-transfer transition of the Mn4+/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 Mn4+/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.
