Résumé
Currently, a growing interest is paid to the study of nonpolar nitride-based heterostructures, as they allow for the growth of thick QWs, while keeping an optimal overlap between electron and hole wave functions [1]. The growth of wide QWs is indeed a key issue to produce high-power nitride-based optoelectronic devices, as they allow reducing the carrier density in the QW, lessening the efficiency of Auger-like mechanisms. However, non-lattice matched foreign substrates are generally used to grow nonpolar GaN, inducing strain in the heteroepitaxial layers. Even when processing techniques such as epitaxial lateral overgrowth are used, strain relaxation through the generation of dislocations or basal stacking faults leads to a drastic reduction of exciton lifetime [2]. In this work, we therefore investigate the dynamics of excitons in single (Al,Ga)N/GaN QWs deposited directly on the a-facet of GaN crystals. We first extract by cathodoluminescence experiments dislocation density and exciton diffusion length at 300 K of 2.105 cm-2 and 100 nm, respectively, demonstrating that dislocations should not play any significant role in the recombination of excitons at room-temperature [3]. We then study by time-resolved photoluminescence the dynamics of excitons in the 10-320 K range for QW samples with various width and barrier Al-content. We first show that, for all samples, the effective lifetime of QW excitons increases with temperature, evidencing the absence of nonradiative phenomena in the low-temperature range. The largest temperature range of purely radiative recombination (up to 240 K) has been observed for a 7 nm thick Al0.06Ga0.94N/GaN QW, i.e. a QW with small exciton localization energy (2 meV) [4]. This observation therefore evidences the possibility of achieving nonpolar room-temperature UV emitters combining a rather narrow emission line with a good radiative efficiency at 300 K. In the high temperature range, a drop in the QW photoluminescence lifetime is always accompanied by an increase in the barrier emission lifetime, until both emissions follow the same dynamics. Supported by a model accounting for the thermodynamic equilibrium between excitons and free carriers in the QWs and the (Al,Ga)N barriers, we demonstrate that at high temperatures, the nonradiative recombination of charge carriers in the (Al,Ga)N barriers is the mechanism limiting the photoluminescence lifetime of excitons confined in the QWs [4]. We finally propose to tackle the thermal escape of carriers 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. Waltereit et al., Nature 406, 865 (2000). [2] P. Corfdir et al., J. Appl. Phys. 107, 043524 (2010); T. J. Badcock et al., Appl. Phys. Lett. 93, 101901 (2008). [3] P. Corfdir et al., Phys. Rev. B 83, 245326 (2011). [4] P. Corfdir et al., J. Appl. Phys. 111, 033517 (2012).