Shallow donors in GaN epilayers grown by metalorganic chemical vapour deposition (MOCVD) and by molecular beam epitaxy (MBE) on sapphire, SiC and GaN substrates have been studied by selectively excited photoluminescence (SPL) and far-infrared (FIR) absorption. The spectroscopic studies of neutral donor bound excitons allow us to determine the (DX)-X-0 rotational excited state spectra in a large domain of tensile and compressive strain fields depending on the substrates used for the growth. The energy transitions between the ground state and the n = 2 states of residual donors are determined from the spectra of the two-electron replica of (DX)-X-0 lines in accordance with resonant electronic Raman scattering results. The comparison between FIR results and the two-electron spectroscopy shows, that silicon is the main residual donor (E-d = 30.4 meV in strain-free samples) in both MOVPE and MBE layers. A second unknown donor (E-d = 31.8 meV) is frequently found in GaN/SiC grown by MOCVD. A third deeper level (E-d = 33.1 meV) is rarely detected by two-electron spectroscopy but could be associated with a weak neutral-donor bound exciton shifted down by 0.9 meV from the main Si-Ga related (DX)-X-0 line in MBE hornoepitaxial layers
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