A detailed understanding of the nitride refractive indices is essential for the modeling and design of III–N laser structures. In this article, we report on the assessment of the refractive index data available for the nitride alloys and present formulas for evaluating the refractive indices for variations in both composition and photon energy. For AlxGa1−xN, an expression is given which fits well to experimental data below x<0.38, sufficient for the molefractions found in the cladding layers of III–N lasers. Due to the almost complete lack of experimental refractive index data for InyGa1−yN, we propose an expression to give a first-order approximation for the refractive index.

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.

Low temperature photoluminescence spectra of Be-doped layers grown on Si (111) by molecular beam epitaxy have been analyzed. Emissions at 3.466 eV and 3.384 eV, and a broad band centered at 2.4-2.5 eV are observed. Their evolution with temperature and excitation power, and time resolved PL measurements ascribe an excitonic character for the luminescence at 3.466 eV, whereas the emission at 3.384 eV is associated with a donor-acceptor pair transition. This recombination involves residual donors and Be-related acceptors, which are located around 90meV above the valence band, confirming Be as the shallowest acceptor reported in GaN. The intensity of the band at 2.4-2.5 eV increases with the Be content. This emission involves a band of deep acceptors generated by Be complex defects, as suggested by the parameter g = 2.008 ± 0.003 obtained by photoluminescence-detected electron paramagnetic resonance.

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