Charge transfer at the Mn acceptor level in GaN

MBE-grown GaN : Mn layers with Mn doping concentrations around 10(20) cm(-3) were investigated by photoconductivity measurements. From electron spin resonance (ESR), Mn is known to be mostly present in the neutral Mn3+ or Mn2+ h(+) state, which leads to a reassignment of the known optical absorption features to charge transfer from Mn3+, either by direct photoionization at about 1.8 eV or by a photothermal ionization process via an excited state ( Mn3+)(+) at 1.42 V higher internal energy than the Mn3+ ground state. It is proposed that the Mn3+/Mn2+ acceptor level is located about 1.8 eV above the valence band edge of GaN so that the nature of the acceptor wavefunction is very different from an effective-mass-like state such as the Mn2++h(+) complex in GaAs : Mn. According to these experimental results, the realization of carrier-mediated ferromagnetism becomes rather unlikely in not co-doped GaN : Mn.