Positron annihilation measurements show that negative Ga vacancies are the dominant acceptors in n-type gallium nitride grown by hydride vapor phase epitaxy. The concentration of Ga vacancies decreases, from more than 1019 to below 1016 cm−3, as the distance from the interface region increases from 1 to 300 μm. These concentrations are the same as the total acceptor densities determined in Hall experiments. The depth profile of O is similar to that of VGa, suggesting that the Ga vacancies are complexed with the oxygen impurities.

Photoluminescence of the dominant deep-level acceptor in high-purity freestanding GaN is studied over a wide range of excitation intensities. A yellow luminescence ͑YL͒ band at about 2.2 eV saturates with increasing excitation intensity, whereas a green luminescence ͑GL͒ band at about 2.5 eV increases as a square of the excitation intensity. The YL and GL bands are attributed to two charge states of the same defect, presumably a gallium vacancy-oxygen complex.

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.

The neutral donor bound exciton recombination processes in freestanding GaN have been studied. The photoluminescence spectrum shows emission lines related to silicon and oxygen donors. Time-resolved luminescence allows us to correlate the principal donor bound exciton lines with their two-electron satellites. The magnetic field splitting of the two-electron lines is well described by the theory of the hydrogen atom in a magnetic field. For the oxygen donor a 1.5 meV chemical shift and a 30.8 meV effective Rydberg have been evaluated. Two-electron satellites involving excitations to the 2p and 2s donor states are separated by an energy of 1.0 and 1.3 meV for O and Si impurity, respectively. The temperature dependence of the two-electron emission clearly shows that this separation arises from a splitting of the ground state of the neutral donor bound exciton complex. The nature of this splitting is discussed and it is suggested that it is due to rotational states of donor bound excitons.

Optical properties of GaN epilayers and GaN/AlGaN quantum wells grown by molecular beam epitaxy on GaN(0001) single crystal substrate A series of sharp intense peaks was observed in the low-temperature photoluminescence spectrum of unintentionally doped GaN in the photon energy range between 3.0 and 3.46 eV. We attributed the majority of these peaks to excitons bound to unidentified structural and surface defects. Most of the structural-and surface-related peaks ͑at 3.21, 3.32, 3.34, 3.35, 3.38, and 3.42 eV͒ were observed in Ga polar films. In N polar GaN, we often observed the 3.45 eV peak attributed to excitons bound to the inversion domain interfaces.