Optical and transport properties of wurtzite GaN layers, grown by plasma-assisted molecular-beam epitaxy on Si͑111͒ substrates, have been investigated. An emission at 3.455 eV, analyzed by continuous-wave and time-resolved luminescence in undoped and Si-doped GaN layers, is assigned to excitons bound to Si donors with an optical binding energy of 50 meV. A common origin of this peak, for undoped and Si-doped GaN, is backed by secondary-ion-mass spectroscopy that evidences a Si diffusion from the substrate into the GaN layer for growth temperatures above 660°C. Simultaneously, Ga diffusion into the Si substrate generates a highly p-type conductive layer at the GaN/Si interface, leading to unreliable Hall data in undoped and lightly doped layers. Positron annihilation reveals a concomitant vacancy cluster generation at the GaN/Si interface in samples grown above 660°C. No traces of the ''yellow band'' are detected either in undoped or in Si-doped samples. ͓S0163-1829͑98͒00827-3͔

Be-doped GaN layers have been grown on Si(111) by molecular beam epitaxy. The relative Be concentration was measured by secondary ion mass spectroscopy analysis. Photoluminescence spectra have been taken under continuous wave and time-resolved conditions. A new emission at 3.384 eV, which is probably related to substitutional Be, is reported, together with its first and second order phonon replica. Clear blue-shifts are observed when increasing temperature and excitation power, suggesting that this emission is associated with a transition from a residual donor to the Be acceptor. From time-resolved spectra, a very slow and strongly non-exponential decay, as well as a red-shift of the peak energy position with time, confirm the donor-acceptor character of the Be-related emission. The estimated ionization energy of the acceptor is around 90 meV, so Be is the shallowest p-dopant ever reported in GaN.

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Optical thresholds, that correspond to a level located at 1 eV above the valence band, are observed by photocapacitance techniques in n-type Mg-doped GaN. In undoped GaN, this level has been previously related to the yellow emission detected by photoluminescence. In Mg-doped GaN, this yellow luminescence is only observed for excitation energies below the Mg-related band (2.9 - 3 eV). This result evidences that Mg-doping may reduce but not avoid the formation of the yellow band related defects in n-type and semiinsulating Mg-doped samples. The fact that the yellow luminescence is not observed for excitation energies above the bandgap may be justified by a higher efficiency of the Mg-related recombination path.

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