Optical study of GaN nanowires and GaN/AlN microcavities

This work focuses on the optical study of GaN nanowires and AlN microcavities containingGaN quantum dots. The 1-meV linewidth of the neutral donor-bound exciton line in thephotoluminescence spectrum of MBE-grown GaN nanowires evidences that the strain ishomogeneous in the material. These nanowires do not exhibit any excitonic confinement, butthe efficient strain relaxation allows to grow strain-free zinc-blende GaN nanowires and then toconduct fine spectroscopy on cubic GaN near band edge. Beside, we show that the tentativeattribution of the recombination line at 3.45 eV in the spectrum of wurtzite GaN nanowires toa surface-enhanced two-electron satellite does not hold. Indeed, its dipole polarization selectionrules and its evolution with intense applied magnetic field do not match that of a two-electronsatellite.We also performed the spectroscopy of GaN/AlN quantum dot microdisks. Record qualityfactors for AlN cavities were measured around 3 eV. GaN/AlN quantum dot nanocavitiesembedded in photonic crystal waveguides were also investigated. The attribution of each modeeither to the waveguide or to the cavity, predicted by calculations, is experimentally confirmedby a different light localization. These structures allow excellent quality factors to be reached,from 2300 at 3.45 eV, up to 4400 at 3.14 eV. Although the expected Purcell factor is very high(around 100), we did not manage to observe the Purcell effect. This originates either from anenhancement of non-radiative recombination channels or from an instability of both the cavitymodes and the quantum dot emission under intense exposure. Finally, it appears that the mainlimiting factor to achieve lasing in these structures is the strong built-in electric field, whichslows up the spontaneous emission rate of the quantum dots.