Carbon-doped GaN layers grown by molecular-beam epitaxy are studied with photoluminescence and positron annihilation spectroscopy. Semi-insulating layers doped with >1018 cm−3 carbon show a strong luminescence band centered at ∼2.2 eV (yellow luminescence). The absolute intensity of the 2.2 eV band is compared with the gallium vacancy concentration determined by positron annihilation spectroscopy. The results indicate that a high concentration of gallium vacancies is not necessary for yellow luminescence and that there is in fact a causal relationship between carbon and the 2.2 eV band. Markedly different deep-level ionization energies are found for the high-temperature quenching of the 2.2 eV photoluminescence in carbon-doped and reference samples. We propose that while the model of Neugebauer and Van de Walle [Appl. Phys. Lett. 69, 503 (1996)] applies for GaN of low carbon concentration, a different yellow luminescence mechanism is involved when the interstitial carbon concentration is comparable to or exceeds the gallium vacancy concentration.

The controlled incorporation of excess As into GaAs grown by molecular beam epitaxy at low growth temperatures (LT-GaAs) is explored. The substrate temperature and the As/Ga flux ratio were systematically varied to investigate the influence of growth parameters on the formation of native defects and structural properties. Near infrared absorption, magnetic circular dichroism of absorption, and slow positron annihilation were applied to determine point defect concentrations of As antisites (AsGa) and Ga vacancies (VGa). Structural properties of as-grown and annealed LT-GaAs layers were investigated by x-ray diffraction and transmission electron microscopy. In a well defined parameter range the lattice expansion of the LT-GaAs layers correlates with the amount of AsGa. The VGa acceptor concentration can quantitatively account for the ionization of the AsGa donors.

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Effects of the growth temperature and As/Ga flux ratio on the incorporation of excess As into low temperature grown GaAs