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Present growth techniques using mismatched substrates cannot prevent the creation of dislocations and defects near the interface nor the incorporation of the intrinsic shallow donor 35 meV below the conduction band edge. It is therefore necessary to assess the influence of impurities and dislocations on the excitonic recombination processes and their dynamics in GaN. The present paper will show that the dominating recombination processes near the interface between GaN and its lattice-mismatched substrate differ strongly from those far away from the interface, i.e., near the surface of the epilayer. We will show that in structurally perturbed regions such as in the vicinity of the interface, the radiative recombination of free and shallow bound excitons is quenched and deep dislocation excitons appear in the spectrum. A detailed report on the recombination dynamics both near and far away from the interface will be given. EXPERIMENTAL To separate the influence of the dislocations having a high density near the substrate interface from the properties of relaxed single-crystalline GaN we used in the main an epitaxial GaN layer of 400pm thickness grown by hydride vapor phase epitaxy without a buffer layer [3]. Using band-to-band excitation from either the epilayer side, or the substrate side we sampled without any overlap either high-quality, relaxed GaN near the surface or strongly perturbed GaN near the interface. Due to the high absorption coefficient above the gap, the estimated depth of excitation within the sample is less than 5 pm. We therefore use the terms 'bulk' and 'interface' 589 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society

luminescence to distinguish between excitation from either the epilayer or the substrate side. The sapphire substrate is transparent at the energies of excitation and detection and does not influence the spectrum taken from the interface region. For continuous-wave (cw-) measurements a HeCd laser was employed. Time-resolved measurements were performed at various temperatures using a frequency-doubled dye laser synchronously pumped by an actively mode-locked and frequencydoubled Nd:YAG laser. The overall time resolution employing convolution techniques was 15 ps.

RESULTS 'Bulk' and 'Interface' Photoluminescence Comparative cw low-temperature photoluminescence spectra taken from the 400 pm GaN epilayer are shown in Fig. 1. The spectrum in Fig. I (a) was obtained after excitation of the sample on the surface of the epilayer. The donor-bound-exciton line 12 at 3.4782 eV is most prominent. Emission from the free A-exciton is seen at 3.4800 eV. The localization energy of the exciton at the donor thus amounts to 7.18 meV. High resolution spectra reveal the presence of a weak second donor-bound-exciton line with a localization energy of 3.6 meV. The energy positions of the free exciton and 12 agree precisely with calorimetric reflection and absorption data taken from the same sample [4] and thus unambiguously determine the low temperature energy positions of the free an