Magnetic resonance studies of Mg-doped GaN epitaxial layers grown by organometallic chemical vapor deposition
Electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) experiments have been performed on a set of GaN epitaxial layers doped with Mg from 2.5×1018 to 5.0×1019 cm-3. The samples were also characterized by secondary-ion-mass spectroscopy (SIMS), temperature-dependent Hall effect, and low-temperature photoluminescence (PL) measurements. EPR at 9 GHz on the conductive films reveals a single line with g∥~2.1 and g⊥~2 and is assigned to shallow Mg acceptors based on the similarity of the spin density with that found for the number of uncompensated Mg shallow acceptors from Hall effect and the total Mg concentration by SIMS. PL bands of different character are observed from these layers, including shallow-donor-shallow-acceptor recombination at 3.27 eV from the lowest doped sample and, in most cases, broad emission bands with peak energy between 2.8 and 3.2 eV from the more heavily doped films. In addition, several of the films exhibit a weak, broad emission band between 1.4 and 1.9 eV. ODMR at 24 GHz on the ``blue'' PL bands reveals two dominant features. The first is characterized by g∥,g⊥~1.95-1.96 and is assigned to shallow effective-mass donors. The second line is described by similar g tensors as found by the EPR experiments and, thus, is also attributed to shallow Mg acceptors. Although several groups have related the 2.8 eV PL in heavily Mg-doped GaN with the formation of deep donors, no clear evidence was found from the ODMR on this emission for such centers. However, based on the near-midgap PL energy and the observation of the feature assigned to shallow Mg acceptors, the strongest case from magnetic resonance for the existence of deep donors in these films is the isotropic ODMR signal with g=2.003 found on emission <1.9 eV. Possible recombination mechanisms to account for the ODMR on these ``blue'' and near-IR PL bands are discussed.