Résumé
Contrary to GaN-based structures grown along [0001], nonpolar heterostructures allow for the growth of thick QWs, while keeping an optimal overlap between electron and hole wave functions. However, the use of lattice-mismatched substrates induces high densities of stacking faults and dislocations. Then, even at low temperature (T), the exciton decay is dominated by the capture on defects [1]. We investigate here the dynamics of excitons in AlGaN/GaN QWs deposited on the a-facet of GaN crystals. We first determine by cathodoluminescence QW dislocation density and exciton diffusion length at 300 K of 2.105 cm-2 and 100 nm, respectively, demonstrating that dislocations do not affect QW excitons at room-temperature [2]. We then study the time-resolved photoluminescence (PL) of excitons in the 10-320 K range for QW samples with various widths and barrier Al-contents. For all samples, the effective lifetime of QW excitons increases with T, indicating the absence of nonradiative phenomena in the low-T range. We observe purely radiative recombination up to 240 K for a 7 nm thick Al0.06Ga0.94N/GaN QW, i.e. a QW with small exciton localization energy (2 meV) [3]. This observation therefore demonstrates the possibility of achieving UV emitters with a rather narrow emission line and a good radiative efficiency at 300 K. In the high-T range, a drop in the QW PL lifetime is accompanied by an increase in the barrier PL lifetime, until both emissions follow the same dynamics. Supported by a model accounting for the equilibrium between excitons and free carriers in the QWs and the barriers, we show that the nonradiative recombination of charge carriers in the AlGaN barriers is the mechanism limiting the PL lifetime of excitons in the QWs [3]. The nonradiative recombinations at 300 K are suppressed by reducing the thermal escape of carriers from the QW. This is achieved by the growth of thick QWs rather than increasing the barrier Al-content, which is important from the defect/strain generation point of view. [1] P. Corfdir et al., J. Appl. Phys. 107, 043524 (2010). [2] P. Corfdir et al., Phys. Rev. B 83, 245326 (2011). [3] P. Corfdir et al., J. Appl. Phys. 111, 033517 (2012).