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 (Mn ³⁺)* 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.