Dark-level trapping, lateral confinement, and built-in electric field contributions to the carrier dynamics in c-plane GaN/AlN quantum dots emitting in the UV range

Abstract

c-plane GaN/AlN quantum dots (QDs) are promising zero-dimensional quantum nanostructures that exhibit single photon emission properties up to room temperature and even above. In this context, it is of prime interest to gain a deeper insight into the recombination dynamics of photogenerated electron-hole pairs captured by such dots. Hence, in this work, we study the time-resolved photoluminescence (PL) properties in the low injection regime and at cryogenic temperatures of c-plane GaN/AlN QD ensembles emitting above the bulk GaN bandgap in order to properly understand the nature of the recombination channels behind the observed non-exponential decay time profiles. Such decays reveal the existence of a relaxation channel competing with the radiative recombination one. It is thus observed that for the former process the dynamics is independent of the dot height, which is attributed to a reversible nonradiative transfer that could be mediated by a spin-flip process to a dark-level state. The radiative recombination process is recognizable thanks to the characteristic dependence of its lifetime with the emission energy, which is well accounted for by the built-in electric field inherent to quantum nanostructures grown along the c-axis and the variations in the lateral confinement at play in such QDs. Those conclusions are drawn from the analysis of the time-evolution of the PL spectra by means of a simple analytical model that enables to exclude any screening of the built-in electric field. III. RESULTS

Single photon emission and recombination dynamics in self-assembled GaN/AlN quantum dots

“…A contribution from the WL can, once again, be discarded as all the dots are pumped quasi-resonantly. Unlike previous observations [65], the decay rate at short delays is strongly impacted by the QD size. In fact, the initial decay rate is enhanced for high energy emitters, hinting at an increase in the multi-carrier recombination rate due to the larger electron-hole wave function overlap.…”

“…While robust RT single-photon emission has been demonstrated for these SA GaN/AlN QDs, the long exciton decay time of (16 ± 4) ns extracted from secondorder correlation function measurements recorded at 5 K on QD A , whereas it is emitting near 4.5 eV, is not in line with previous experimental results [62][63][64][65]. There- E n e r g y ( e V )…”

“…3 with a fixed exciton decay time τ decay = γ −1 = (16 ± 4) ns deduced by fitting all traces with a shared recombination rate γ. This value exceeds by about one to two orders of magnitude the decay times previously reported for GaN QDs emitting above the bulk GaN bandgap [62][63][64][65]. First, this suggests a very low nonradiative recombination rate, as the QD of interest remains bright despite such a long-lived recombination time for the excitons.…”

“…To complete our analysis of the exciton recombination dynamics, we compare the long-lived decay times τ L extracted at 5 K with experimental and theoretical exciton lifetimes published in the literature on SA GaN/AlN QDs [62][63][64][65]. In addition, the few short-lived decay times τ S extracted from power-dependent TRPL experiments have also been added to the picture and all the results are shown in Fig.…”

“…GaN/AlN QDs at cryogenic temperature. Experimental results [62][63][64][65] are highlighted with filled markers and the substrate employed by each group is specified in the legend. Computed lifetimes (empty up and down triangles) [76,80] were obtained for various aspect ratios and QD heights.…”