Physics of the Ultrafast Dynamics of Excitons in GaN Nanostructures

For more than fifteen years, III-nitrides have become the materials of choice for the realization of optoelectronics devices operating in the visible-UV spectral range. Yet, while nitride-based technology has truly exploded, the structural quality of this material-class is still low, which is a key issue regarding the radiative efficiency of light-emitting devices. A lot of work is thus devoted to improve the crystal quality of GaN and related materials. As current developments, we mention the reduction of threading dislocations, or the growth of III-nitrides heterostructures along non-polar crystal orientation. In parallel to the efforts dedicated to improve the quality of nitride-based epilayers, it is mandatory to grasp the mechanisms ruling the light-matter interaction, and, in particular, to detail the impact of defects on the radiative efficiency of optoelectronic devices. As a part of this research, the present thesis work is dedicated to the understanding of exciton relaxation and recombination phenomena in nitride-based nanostructures. This experimental study is carried out by time-resolved photo- and cathodoluminescence. We first focus on the emission properties of GaN nanocolumns, nanostructures that raise currently a huge interest, as they are free of any extended defect. Still, contrasting with their perfect structural quality, the low-temperature photoluminescence spectra of these nano-objects exhibit broad excitonic emission lines. In addition, one observes an intense emission line centered at 3.45 eV, a line absent from the luminescence spectra of high-quality GaN thick layers. We first demonstrate that these two features arise from the statistical distribution of donor atoms across the nanocolum: when a donor nucleus is located in the vicinity of the surface, its wave function is perturbed and the emission properties of donor bound excitons is profoundly altered. Based on a qualitative model, we estimate to 8 nm the thickness of the surface shell layer where the radiative properties of donor bound excitons are affected by the surface. We then address the capture processes of excitons by extended structural defects. We focus in particular on basal plane stacking faults, planar radiative defects generally encountered in heteroepitaxial non-polar GaN layers. We first describe the diffusion of excitons and their capture by basal stacking faults, and we show that excitons are localized along the stacking fault planes themselves. Supported by envelope function calculations, our experiments demonstrate that donor atoms distributed in the vicinity of the basal stacking faults efficiently localize excitons along the planar defects. Concerning non-polar (Al,Ga)N/GaN quantum wells grown on sapphire, we show that the dynamics of excitons is dominated by their capture by stacking faults. Finally, we attest that the intersection between basal plane stacking faults and a quantum well can be modeled as a quantum wire. In the last part of this work, we investigate the dynamics of excitons in non-polar (Al,Ga)N/GaN heterostructures grown on bulk GaN crystals. Such layers present dislocation densities four orders of magnitude smaller than those deposited on lattice-mismatched susbtrates, making possible the study of intrinsic relaxation and recombination mechanisms of excitons in the weak as well as in the strong confinement regimes. In the latter case, thanks to the drastic reduction in threading dislocation density, it is possible to observe purely radiative recombination of quantum well free excitons up to 200 K. We attribute the drop in internal quantum efficiency observed at higher temperatures to the escape of quantum well excitons towards the disordered (Al,Ga)N barriers and to their subsequent non-radiative recombination.

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