The optical properties of n-type GaN are investigated for Si doping concentrations ranging from 5ϫ10 16 to 7ϫ10 18 cm Ϫ3. The photoluminescence linewidth of the near-band gap optical transition increases from 47 to 78 meV as the doping concentration is increased. The broadening is modeled in terms of potential fluctuations caused by the random distribution of donor impurities. Good agreement is found between experimental and theoretical results. The intensity of the near-band-gap transition increases monotonically as the doping concentration is increased indicating that nonradiative transitions dominate at a low doping density. The comparison of absorption, luminescence, reflectance, and photoreflectance measurements reveals the absence of a Stokes shift at room temperature demonstrating the intrinsic nature of the near-band edge transition.

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The competition between band gap and the 2.2 eV ͑yellow͒ luminescence of epitaxial GaN is studied for excitation densities ranging from 5ϫ10 Ϫ6 to 50 W/cm 2. The ratio of the peak intensities of the band gap-to-yellow luminescence changes from 4:1 to 3000:1 as the excitation density is increased by 7 orders of magnitude. At room temperature, the band gap luminescence linewidth is 2.3kT, close to the theoretical minimum of 1.8kT. A model is developed describing the intensity of the two radiative transitions as a function of the excitation density. This model is based on bimolecular rate equations and takes into account shallow impurities, deep levels, and continuum states. The theoretically predicted dependences of the two different luminescence channels follow power laws with exponents of 1 2 , 1 and 3 2. Thus the intensity of the yellow luminescence does not saturate at high excitation densities. These dependences are in excellent agreement with experimental results. The relevance of the results for optoelectronic GaN devices is discussed. It is shown that the peak intensity of the yellow luminescence line is negligibly small at typical injection currents of light-emitting diodes and lasers.

The dependence of the near-band edge and the yellow luminescence in n-type GaN grown by organometallic vapor-phase epitaxy is investigated as a function of doping concentration. The band edge and yellow luminescence intensity increase as the doping concentration is increased. However, the band-edge-to-yellow luminescence ratio does not change significantly as the doping concentration is increased by two orders of magnitude. A theoretical model based on rate equations is developed for the band-edge-to-yellow intensity ratio. Analysis of the experimental data in terms of the model reveals that the concentration of the level causing the yellow luminescence increases linearly with doping concentration. This dependence shows that the yellow luminescence is due to a compensating center. © 1997 American Institute of Physics. ͓S0003-6951͑97͒00248-9͔The dominant optically active defect in GaN causes an optical transition at 2.2 eV which is in the yellow part of the visible spectrum. The yellow luminescence is commonly found in GaN grown by organometallic vapor-phase epitaxy ͑OMVPE͒. 1-4 Although, luminescence measurements reveal a distinct optical signature of the defect, the microstructure and chemical nature of the defect has not been identified.Recently, Neugebauer and Van de Walle 5 proposed that a native defect, namely, the Ga vacancy, is the microstructural origin of the yellow luminescence. Their calculations suggest that the Ga vacancy is a triply-charged acceptor level whose abundance increases with the third power of the doping concentration. Thus, the calculations suggest that the center is a compensating native defect which occurs predominantly in n-type GaN. Other models 6,7 propose that the yellow luminescence is due to C acceptors, double donors, iron impurities, and intrinsic defects related to dislocations.In this publication, the UV and yellow luminescence transitions in Si-doped GaN epitaxial films are investigated as a function of the doping concentration. The comparison of a theoretical model with experimental results yields the dependence of the defect concentration on the doping concentration. This dependence can reveal the critical signature of compensating defects which are known to increase with doping concentration according to a power law. [8][9][10] The Si-doped GaN samples were grown by OMVPE on the c plane of a sapphire substrate at a temperature of 1100°C. A growth rate of 2 m/h was employed. The flow of the diluted silane (SiH 4 ) doping precursor was systematically varied to achieve doping densities in the range 5 ϫ10 16 to 7ϫ10 18 cm Ϫ3 . The room-temperature photoluminescence measurements were performed using an 80 mW HeCd laser emitting at 325 nm. The excitation density was adjusted to 3 W/cm Ϫ2 . The luminescence was dispersed in a 0.75 m spectrometer, detected by a GaAs photomultiplier using low-noise phase-sensitive ''lock-in'' amplification.The room-temperature photoluminescence spectrum of an n-type GaN sample grown by OMVPE is shown in Fig. 1.The spectrum displays two features na...

Experimental and theoretical results of Mg-doped superlattices consisting of uniformly doped AlxGa1−xN, and GaN layers are presented. Acceptor activation energies of 70 and 58 meV are obtained for superlattice structures with an Al mole fraction of x=0.10 and 0.20 in the barrier layers, respectively. These energies are significantly lower than the activation energy measured for Mg-doped bulk GaN. At room temperature, the doped superlattices have free-hole concentrations of 2×1018 cm−3 and 4×1018 cm−3 for x=0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conductivity measured for the superlattice structures is 0.27 S/cm and 0.64 S/cm for an Al mole fraction of x=0.10 and 0.20, respectively. X-ray rocking curve data indicate excellent structural properties of the superlattices. We discuss the origin of the enhanced doping, including the role of the superlattice and piezoelectric effects. The transport properties of the superlattice normal and parallel to the superlattice planes are analyzed. In particular, the transition from a nonuniform to a uniform current distribution (current crowding) occurring in the vicinity of contacts is presented. This analysis provides a transition length of a few microns required to obtain a uniform current distribution within the superlattice structure.