GaN:Fe and (Ga,Fe)N:Mg are promising candidates as ferromagnetic diluted magnetic semiconductors (DMS) for spintronics applications . To gain insight into the ordering mechanisms of this novel DMS, temperature dependent photoluminescence (PL) and Hall measurements are essential.
All samples were fabricated by metallorganic chemical vapor deposition (MOCVD) on (0001) sapphire substrates. Several series of samples were grown with iron and magnesium incorporated both homogeneously and as delta-layers. High-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed the device-quality of the films. The PL spectra of GaN:Fe at 6 K exhibit an excitonic region between 3.3eV and 3.5eV dominated by a optical transition whose intensity scales with the Fe content. By evaluating the activation energies from temperature dependent PL, we attribute this feature to a bound exciton. The activation energies and results from secondary ion mass spectroscopy (SIMS) of different GaN:Fe samples point most likely to oxygen as the binding site for the excitons. The O-concentration, due to the high affinity of oxygen to iron, increases with increasing magnetic ions content. Hall measurements confirm the GaN:Fe layers to be n-type. The carrier concentrations increase with increasing Fe flux and lie between 1016 and 1017cm^-3 at 300 K. Their temperature dependence suggests O as donor as well.
Depending on sequence and thickness of the single layers and the dopants’ concentrations in the delta-codoped (Ga,Fe)N:Mg samples, we measure strong excitonic features in the near band gap region, a shallow donor-to-acceptor transition at 3,25 eV and a yellow luminescence band which centers around 2,23 eV.
Furthermore, we found that by delta-codoping with Mg we achieve an optimized concentration of about 10^20 acceptors/cm^-3.
 T.Dietl, H.Ohno, F.Matsukura, J.Cibert, D.Ferrand, Science 287, 1019 (2000)