Molecular-beam-epitaxy grown GaN:Mn and AlN:Mn layers with Mn concentrations around 1020 cm−3 were investigated by optical absorption and photoconductivity measurements. From electron spin resonance Mn is known to be mostly present in the neutral acceptor state in GaN without codoping. This leads to a reassignment of the optical absorption features to a charge transfer from the neutral Mn3+ oxidation state, either by direct photoionization at 1.8 eV or through a photothermal ionization process via an excited state at 1.42 eV above the Mn3+ ground state by spin-allowed Mn3+ 5E→5T internal absorption. The position of the Mn3+/2+ acceptor level at 1.8 eV above the valence-band edge of GaN makes the realization of carrier-mediated ferromagnetism rather unlikely in GaN:Mn.

We have characterized the structural and magnetic properties of low-temperature molecular-beam epitaxy grown Ge: Mn by means of high-resolution transmission electron microscopy ͑HR-TEM͒, energy dispersive x-ray spectroscopy, and superconducting quantum interference device ͑SQUID͒ magnetometry. We find a coherent incorporation of Mn 5 Ge 3 clusters in an epitaxially grown Ge: Mn matrix, which shows the characteristics of a diluted magnetic semiconductor phase of Mn-doped Ge. The clusters are preferentially oriented with the hexagonal ͓0001͔ direction parallel to the ͓001͔ growth direction of the Ge: Mn matrix, as determined from both HR-TEM and SQUID measurements.

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