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We analyze the evolution of the exciton recombination energy ${E}_{\mathrm{PL}}$ and its pressure coefficient $d{E}_{\mathrm{PL}}/dp$, with the laser power density (LPD) exciting excitonic photoluminescence. Two $\mathrm{I}{\mathrm{n}}_{0.17}\mathrm{G}{\mathrm{a}}_{0.83}\mathrm{N}/\mathrm{GaN}$ quantum well (QW) structures are used: sample A consisting of a 5.2-nm-wide single QW and sample B consisting of two 2.6-nm-wide QWs separated by a 0.78-nm-thick barrier. Changes of ${E}_{\mathrm{PL}}$ and $d{E}_{\mathrm{PL}}/dp$ with LPD in both structures cover a wide range of magnitude. They switch character in the vicinity of the indirect to direct exciton induced by increasing LPD. In sample B, at low excitation, a negative pressure coefficient of ${E}_{\mathrm{PL}}$ has been found. Moreover, pressure dependences of ${E}_{\mathrm{PL}}$ are different for the two samples under study. We examined pressure dependence of the threshold LPD for indirect-direct exciton switching in both samples. The reported observations originate to a high extent from screening of a built-in electric field by carriers externally introduced into the quantum wells. We also note that the universal relation between transition energies and their pressure coefficients (observed previously in InGaN/GaN quantum wells) does not hold for samples modified by screening.

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.