Search

The origin of efficiency droop in state-of-the-art quality In GaN/GaN and GaN/AlGaN quantum wells (QWs) grown on various crystal planes is studied by means of time-resolved photoluminescence spectroscopy associated with a precise determination of the QW carrier density. In a first set of experiments, it is shown that a polar InGaN/GaN QW under nonresonant high optical excitation shows clear signatures of Auger loss mechanism and thus behaves quite differently compared to its binary based GaN/AlGaN QW counterpart, where no Auger signature is observed. In order to get rid of the impact of the built-in polarization field and illustrate the dominant role of carrier localization, similar experiments have been conducted on two m-plane In GaN/GaN QWs with similar In composition but a different degree of disorder. We demonstrate that carrier localization strongly enhances the Auger recombination process in nonpolar In GaN/GaN QWs. We also show that this effect may be further amplified by the presence of polarization fields on polar QWs. The relaxation of the k-selection rule during the Auger recombination process, resulting from QW potential disorder, can account for the enhancement of the efficiency droop in In GaN/GaN QWs.

We present a comprehensive study of the anisotropic optical properties of nonpolar GaN/AlGaN multiple quantum wells intentionally designed to act as an active region of a planar microcavity operating in the strong-coupling regime. The strain induced by the underlying AlGaN-based Bragg reflector leads to a redistribution of exciton oscillator strength as revealed by photoluminescence and reflectivity measurements. Complementary k . p calculations show an excellent agreement with experiments and emphasize the opportunity to tune the nature of the light-matter coupling in a microcavity by means of strain engineering. Finally, the validity of the developed model is proven by angle-resolved photoluminescence studies carried out on the complete microcavity structure. The recorded eigenmode spectra reveal the coexistence of the weak- and the strong-coupling regime along the two orthogonal polarization planes.

International audience; Both weakly and strongly confined excitons are studied by time-resolved photoluminescence in a nonpolar nitride-based heterostructure grown by molecular beam epitaxy on the a-facet of a bulk GaN crystal, with an ultralow dislocation density of 2 × 105 cm−2. Strong confinement is obtained in a 4 nm thick Al0.06Ga0.94N/GaN quantum well (QW), whereas weakly confined exciton-polaritons are observed in a 200 nm thick GaN epilayer. Thanks to the low dislocation density, the effective lifetime of strongly confined excitons increases between 10 and 150 K, proving the domination of radiative recombination processes. Above 150 K theQWemission lifetime diminishes, whereas the decay time of excitons in the barriers increases, until both barrier and QW exciton populations become fully thermalized at 300 K. We conclude that the radiative efficiency of our GaN QW at 300 K is limited by nonradiative recombinations in the barriers. The increase of exciton-polariton coherence lengths caused by low dislocation densities allows us to observe and model the quantized emission modes in the 200 nm nonpolar GaN layer. Finally, the low-temperature phonon-assisted relaxation mechanisms of such center-of-mass quantized exciton-polaritons are described.

Wurtzite GaN/AlN quantum dots (QDs) are studied by time-resolved photoluminescence. Careful measurements allow us to reach the regime of a single electron-hole pair per dot, evidenced by a monoexponential decay of the luminescence and a stop of the time-dependent shift of the emission energy. The transition energy and the radiative lifetime of the QD ground state are measured in the absence of any electric field screening effect, for a wide range of QD heights. These results are analyzed within an envelope function model for the electron and hole confinement. The effective electric field, averaged over the strain distribution in the dots, is determined to be of 9.0 +/- 0.5 MV/cm.

The pronounced optical in-plane anisotropy of a suitably designed nonpolar GaN-based microcavity, mainly inherited from the valence-band complexity of wurtzite semiconductors, allows the coexistence of two different light-matter coupling regimes along orthogonal polarization planes. When increasing the excitation power density, a transition to a nonlinear coherent emission is observed under nonresonant optical pumping for both polarization directions at carrier densities remaining well below the exciton-saturation level. Their differing nature is experimentally revealed by means of polarization-resolved Fourier space and near-field imaging. While one polarization direction exhibits all specificities of the strong light-matter coupling regime and features polariton lasing at high injection, the other one lacks a straightforward interpretation due to its complex coupling regime. Possible mechanisms are addressed and evaluated.