Interdash Coupling within Dense Ensembles of Quantum Dashes: Comparison of InAs /( In , Al , Ga ) As / InP and InAs /( In , Al ) As / InP Systems

Lasers, light-emitting diodes, and other optoelectronic devices employing $\mathrm{In}\mathrm{As}$/$\mathrm{In}\mathrm{P}$ quantum dots (QDs) instead of quantum wells (QWs) as their active parts benefit from the quasi-zero-dimensional (0D) density of states while maintaining the emission at the communication-relevant range of $1.55\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$. However, for certain application purposes, the substitution of QWs with QDs is advantageous only if QDs can either be treated as isolated objects or exhibit quantum-mechanical coupling between deeply confined states. Here, we compare two material systems of $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$/$\mathrm{In}\mathrm{P}$ and $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$/$\mathrm{In}\mathrm{P}$ elongated QDs (quantum dashes, QDashes) of comparable surface densities and interdash distances. We investigate the presence and type of coupling between QDashes, focusing on the direct tunnel coupling for both types of carriers manifested already at low temperature. In the time-resolved photoluminescence (PL) experiment, we observe a significant dispersion of the PL decay time for $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ QDashes, resulting from the migration of carriers from high- to low-energy QDashes due to substantial tunnel coupling. In the case of $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$ QDashes, the dispersionless time dependence points toward the absence of such coupling. We confirm this interpretation with multiband $\mathbit{k}\ensuremath{\cdot}\mathbit{p}$ calculations. We check that the proposed coupling scenario is possible and show its much higher probability for $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ than for $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$ QDashes. Finally, in temperature-dependent PL, we observe the redistribution of holes among QDashes via thermal excitation to wetting-layer states for both systems, constituting an additional interdash coupling channel at elevated temperatures. Our results indicate that the $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$ QDash system is preferential for optoelectronic applications where QD isolation is highly sought after, whereas $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ QDashes exhibit quantum-mechanical coupling between deeply confined states.