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High-resolution, variable-temperature photoluminescence studies of recombination processes associated with excitons bound to donors in hydride-vapor-phase epitaxial GaN are presented. Detailed analyses of the two-electron satellite ͑2ES͒ region identify transitions associated with ground and excited states of both the donor-bound exciton complexes and of the donor itself. All of the 2ES transitions observed in this work can be accounted for by the recombination of excitons bound to Si and O substitutional impurities and the line positions are in excellent agreement with the energies of donor intraimpurity transitions measured previously by infrared absorption. Conflicting aspects of donor identification and the binding energies of impurities and excitons are clarified.

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Shallow and deep centers were studied by means of temperature dependent Hall effect and photoluminescence (PL) measurements in two sets of undoped n-AlGaN samples grown by organometallic vapor phase epitaxy. The samples of these two series were grown under different conditions and had, as a result, electron concentrations differing by several orders of magnitude. The composition dependence of ionization energies of dominant donors in these two sets of samples is very different indicating that different types of centers are involved, but in both cases they are most probably related to some native defects. These defects behave as hydrogen-like donors for low Al compositions and become increasingly deeper with increasing Al content. The shallow-deep transition occurs at about x=0.2 in the low conductivity AlxGa1−xN series and at about x=0.5 for the high conductivity series. Several PL bands were detected in AlGaN and it is shown that the band at 3.05 eV is due to a radiative transition between deep donors in the upper part of the band gap and holes in the valence band or on shallow acceptors. For the yellow luminescence band at 2.25 eV it is demonstrated that this band consists of two overlapping bands and that the dominant band is due to a transition between the native donors and a carbon-related deep center.

Recently several research groups, including ours, have reported on the deposition of extremely high quality single crystal GaN layers over sapphire substrates. One of the keys to obtaining the high quality was the use of a thin AlN or GaN buffer layer between the sapphire substrate and the grown film. In this communication, we discuss the crystallinity and the influence of the buffer layer in controlling the crystalline, optical, and the electrical properties of the GaN depositions. We also compare the use of GaN and AlN as the buffer layer material. Our results indicate that the buffer layer thickness and the total film thickness are the key factors controlling the electrical, optical, and crystalline properties of the GaN depositions over sapphire substrates.

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Optically detected magnetic resonance has been observed from GaN. Two magnetic resonances have been detected on the 2.2 eV-deep photoluminescence band. The first resonance is sharp [full width at half-maximum (FWHM) ∼2.2 mT] with g∥=1.9515±0.0002 and g⊥=1.9485±0.0002 and is assigned to conduction electrons, in agreement with recent electron paramagnetic resonance (EPR) studies of similar samples. The second feature, which has not been seen by EPR, is much broader (FWHM∼13 mT) with g∥=1.989±0.001 and g⊥=1.992±0.001. These parameters indicate a deep state. A tentative assignment is made to a deep state associated with the N vacancy.