The oxidation states of Mn in epitaxial GaN films grown by plasma induced molecular beam epitaxy were investigated by electron spin resonance (ESR), elastic recoil detection, superconducting quantum interference device magnetization, and photothermal deflection spectroscopy measurements. Comparison of the measured Mn2+ spin densities with the total Mn concentrations in GaN of about 10(20) cm(-3) indicates that the majority of Mn is present in the neutral Mn3+ acceptor state. In samples codoped with Si, electrons are transferred to the Mn acceptors, which is observed by a strong increase of the Mn2+ spin densities in ESR. Simultaneously, this charge transfer reduces the characteristic optical absorption features at 1.5 and above 1.8 eV, suggesting their assignment to a Mn3+-related gap state. (C) 2003 American Institute of Physics. References: 28
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