GaAs samples doped with low indium and nitrogen contents were investigated by continuous-wave (CWPL) and time-resolved photoluminescence (TRPL) at low temperature (10 K). The simultaneous incorporation of doping amounts of both indium and nitrogen elements in GaAs creates new isoelectronic traps that are not present in indium-free GaAs:N. These traps are built from nitrogen-related defects of GaAs:N now perturbated by one additional indium atom. They are believed to involve a preferential <111> orientation of the In atom and of one N atom in near-neighbor positions. The experiments are consistent with the assumption that the clusters are perturbed by a single In atom but that a definitive assignment cannot be given without addition theoretical modeling. Optical feature characteristics of the exciton bound to these kinds of isoelectronic traps are reported: (i) the exciton fine structure with dipole-forbidden and dipole-allowed states and (ii) the train of phonon replicas that is typical of isoelectronic traps. The observation of the tine structure of the trapped excitons is made possible because of the thermodynamic equilibrium between the two populations of excitons (dipole-allowed and dipole-forbidden states) in the conditions of our measurement. It gives us a direct measurement of the short-range exchange interaction (∼0.7 meV). The TRPL experiments allow us to characterize the exciton recombination dynamics for the different isoelectronic traps and notably the transfer mechanisms of excitons among these various traps.

Room-temperature photoreflectance spectroscopy is performed on a series of GaN-AlGaN quantum wells grown by molecular beam epitaxy. We show that the potentialities of this powerful investigation method previously demonstrated to many low-dimensional systems can be extended to nitride-based quantum wells for accurate large scale characterisation. In particular, this technique allows us to get rid of optical interferences that usually prevent the observation of free-exciton transitions below the band-gap of GaN. Such transitions occur in wide quantum wells, because of large built-in electric fields which also quench the oscillator strength of the transitions. Also, this technique allows us to magnify the features of confined excited states, which are difficult to observe by standard reflectance. (C) 1999 Elsevier Science Ltd. All rights reserved.

AlGaN/GaN quantum well (QWs) were grown on (0001) sapphire substrates by molecular beam epitaxy (MBE) using ammonia as nitrogen precursor. The Al composition in the barriers was varied between 8 and 27 % and the well thickness from 4 to 17 monolayers (MLs, 1ML = 2.59 Angstrom). X-ray diffraction (XRD) experiments are used to investigate the strain state of both the well and the barriers. The QW transition energy are measured by low temperature photoluminescence (PL). A large quantum confined Stark effect is observed leading to QW luminescence much lower than the emission line of the GaN buffer layer for well width above a certain critical thickness. The built-in electric field responsible for such a phenomenon is deduced from fit of the PL data. Its magnitude is of several hundred kV/cm and increases linearly with the Al composition.

We analyze the low-temperature photoluminescence decay times, for a series of MBE-grown samples embedding GaN-AlGaN quantum wells. We investigate a variety of configurations in terms of well widths, barrier widths and overall strain states. We find that not only the wells but also the barriers are submitted to large built-in electric fields. In the case of narrow barriers (5 nm), these fields favor the nonradiative escape of carriers from narrow wells into wider wells. When all wells have the same width, the field in such narrow barriers allow us to observe the recombination of long-lived "inter-well" excitons at energies close to those of the short-lived "intra-well" excitons. Our results also prove that the energies and the dynamics of excitonic recombinations depend on the parameters of the heterostructures in a complicated way, due to the interplay of piezoelectric and spontaneous polarizations.

The 2-K recombination dynamics of coupled GaN-AlGaN multiple quantum wells reveals a composite nature in terms of the joint contributions of short-lived direct excitons and long-lived oblique ones. The possibility to observe these oblique excitons, which produce a unusual blueshift of the photoluminescence at long decay times is found to be in straightforward correlation with the presence of internal electric fields together with the existence of a slight disorder at the monolayer scale. [S0163-1829(99)09815-X].

Room temperature photoreflectance investigations have been performed on a series of AlGaN layers grown both by metalorganic vapor phase epitaxy and molecular beam epitaxy on c-plane sapphire substrates. The aluminum composition was ranging between 0% and 20%, and was determined independently in the different growth laboratories, by various methods. It is found that within the experimental uncertainty, there is no detectable bowing parameter in these alloys. This contradicts some previous experimental investigations and confirms other ones. (C) 1999 American Institute of Physics. [S0003-6951(99)01122-5].

Microcalorimetric measurements of small absorption coefficients have been performed on thin GaN-AlGaN quantum wells grown by Reactive Molecular Beam Epitaxy on Al2O3 substrates. In addition to strong absorption at the energy of the GaN buffer and at the energy of the thick AlGaN barrier layers, we could also readily detect transitions associated to the quantum well. These measurements which pave the way to a precise determination of the gap mismatch between the well and the barrier layers are combined with self consistent excitonic and envelope function calculations in the context of a model for the band line-ups which includes the piezoelectric effect. (C) 1999 Elsevier Science S.A. All rights reserved.