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Photoluminescence (PL) and selective photoluminescence (SPL) experiments have been carried out to analyse the near band edge emission (NBE) spectra of unintentionally doped ZnO epilayers. It is shown that the different NBE transitions involve donor centres only. The lines can be divided into two groups. Those lying on the low energy side consist of recombinations of neutral-donor bound exciton complexes, while those lying just below the A free excitonic emission correspond to donor-to-valence band transitions. One of the donors is identified as indium. The emission energy of the In-donor bound exciton, the spectra of the bound exciton excited states, the In-related D 0 h transition energy, and the electronic spectrum of the In-donor are established. The binding energies of another three donor centres are evaluated. The occurrence of two of these centres is shown to depend on the stoechiometry, as revealed by annealing experiments in an O 2 atmosphere.

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 D 0 X 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 D Introduction Convergent and reliable results on shallow donor spectra in GaN are nowadays available from far infrared (FIR) magneto-optical studies [1 −4]. The FIR Zeeman spectroscopy gives evidence of well resolved transmission lines identified as due to 1s−2p ± transitions in undoped GaN grown on sapphire by hydride vapor phase epitaxy (HVPE) [1,4] and metalorganic chemical vapor deposition (MOCVD) [2,3]. Recently, the comparison of donor spectra studied on the same sample via FIR transmittance and selectively excited recombination spectroscopy showed, that very close and complementary results can be obtained by the use of the two techniques [5]. In particular, as the FIR and SPL experiments are detecting 1s−2p ± and 1s−2s transitions, respectively, the small splitting between the 2s and 2p ± donor states can be evidenced [5]. However, the accurate experimental determination of the binding energy of the donor is still being hindered by i) the large variety of substrates used to grow GaN complicating the comparison of data obtained on layers experiencing various biaxial strain fields, ii) the lack of