Optical and magnetic properties of Mn in bulk GaN

We report results of electron paramagnetic resonance, magnetization, and optical absorption studies of bulk GaN crystals doped with Mn and, for some samples, codoped with Mg acceptor. The experiments performed show that the charge state of the Mn ion in GaN depends on the Fermi level position in the crystal. In $n$-type samples Mn stays as an ionized acceptor $A^{-}$ center of ${\mathrm{Mn}}^{2+}{(d}^5),$ while in the investigated highly resistive samples with lowered Fermi level, in a different charge state, most probably corresponding to neutral configuration $A^0.$ Optical absorption spectra of GaN:Mn and GaN:Mn,Mg show characteristic absorption bands related to Mn, which may be interpreted as resulting from photoionization of ${\mathrm{Mn}}^{2+}{(d}^5)$ to GaN conduction band (in $n$-type samples) and photoionization of neutral Mn $A^0$ to GaN valence band (in highly resistive samples). The absorption spectra are discussed in terms of a configuration coordinate model. The location of the Mn acceptor level is derived as 1.8 eV below the bottom of the GaN conduction band. The magnetization experiments performed reveal Brillouin-type magnetization with Mn spin $S=5/2\mathrm{}$ in $n$-type crystals, while the investigated highly resistive samples show magnetic anisotropy characteristic for nonspherical transition-metal configurations. The nature of the Mn ion in GaN is discussed.